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HAMSTRING STRAIN INJURY: Effects of high-speed running, kicking and
concentric versus eccentric strength
training on injury risk and running
recovery
Steven J. Duhig B. Ex. Mvt. Sci.
Doctor of Philosophy
(Thesis by publication)
School of Exercise and Nutrition Sciences
Faculty of Health
Queensland University of Technology
2017
ii
Table of Contents ABSTRACT ........................................................................................................................... iv List of Figures ......................................................................................................................... xii List of Tables ......................................................................................................................... xiv List of Abbreviations .............................................................................................................. xv Statement of Original Authorship .......................................................................................... xvii Acknowledgements.............................................................................................................. xviii
Chapter 1: Introduction .............................................................................................. 19
Chapter 2: Literature Review ...................................................................................... 23 2.1. Hamstring anatomy ..................................................................................................... 23 2.2. Hamstring strain injury definition ................................................................................. 25 2.3. Incidence of hamstring strain injury in sport ................................................................. 26 2.4. Recurrence of hamstring strain injury in sport .............................................................. 27 2.5. Proposed risk factors for hamstring strain injury ........................................................... 28
2.5.1. Non-modifiable risk factors ............................................................................................ 29 2.5.2. Modifiable risk factors .................................................................................................... 32
2.6. Attempts to reduce hamstring strains in sport .............................................................. 44 2.7. Hamstring strain injury mechanisms ............................................................................. 46
2.7.1. Hamstring function during high-speed running ............................................................. 47 2.7.2. Hamstring function during kicking .................................................................................. 48
2.8. The relationship between training load and injury ........................................................ 49
Chapter 3: Program of Research ................................................................................. 52
Chapter 4: Study 1 – The effect of high-speed running on hamstring strain injury risk . 55 4.1. Abstract ....................................................................................................................... 57 4.2. Introduction ................................................................................................................ 58 4.3. Methods ...................................................................................................................... 59 4.4. Results......................................................................................................................... 63 4.5. Discussion.................................................................................................................... 67
Chapter 5: Study 2 – Drop punt kicking induces eccentric knee flexor weakness associated with reductions in hamstring electromyographic activity ............................... 71
5.1. Linking Paragraph ........................................................................................................ 73 5.2. Abstract ....................................................................................................................... 75 5.3. Introduction ................................................................................................................ 76 5.4. Methods ...................................................................................................................... 77 5.5. Results......................................................................................................................... 81 5.6. Discussion.................................................................................................................... 84
Chapter 6: Study 3 – The effect of concentric and eccentric knee flexor strength training on recovery after repetitive sprint sessions ..................................................................... 90
6.1. Linking Paragraph ........................................................................................................ 92 6.2. Abstract ....................................................................................................................... 95 6.3. Introduction ................................................................................................................ 96 6.4. Methods ...................................................................................................................... 98 6.5. Results....................................................................................................................... 106 6.6. Discussion.................................................................................................................. 114
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Chapter 7: General Discussion .................................................................................. 121
References ................................................................................................................... 124
Appendices .................................................................................................................. 137
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ABSTRACT
The hamstring strain injury is a common and problematic injury across sports involving high-
speed running. Within the Australian Football League (AFL) hamstring injuries are an
ongoing issue, with an average of six new injuries sustained and 21 games missed per club
per season. This multifaceted injury has been reported to occur as a result of numerous
mechanisms (e.g. high-speed running, kicking, cutting, wrestling, tackling) with insults most
often affecting the lateral portion, biceps femoris long head, and less often the medial
hamstring muscles, semitendinosus and semimembranosus. However, the high rates of injury
recurrence (16-54%) suggest that there is still a great deal more to be discovered regarding
the factors that determine the risk of recurring hamstring strains. The aims of this program of
research were to: 1) explore the effect of running volumes within elite Australian Rules
football players on the risk of hamstring strain injury; 2) investigate the acute effects of drop
punt kicking on hamstring neuromuscular function in Australian Rules football players; and
3) compare the effects of concentric and eccentric knee flexor strength training on the
recovery of running performance and indices of hamstring muscle damage after intense sprint
training sessions.
The aim of Study 1 was to explore possible relationships between running volumes, session
ratings of perceived exertion (s-RPE) and hamstring strain injuries within elite football
players in the Australian Football League (AFL). Global positioning systems (GPS) derived
running distances (total kilometres covered, distances run at >24 km.h-1
(termed high-speed
running (HSR) in this study), acceleration (>3 m.s-2
) and deceleration (<3 m.s-2
) distances)
and s-RPE for all matches and training sessions over two AFL seasons (2013 and 2014) were
obtained from one AFL team (n=51). All hamstring strain injuries were documented and each
v
player’s running distances and s-RPE were standardised to their 2-yearly session average,
then compared between injured and uninjured players in the four weeks (week -1, -2, -3, -4)
preceding each injury.
The four weeks before each injury was examined and comparisons made between the
absolute distances run and each individual’s distances run relative to their 2-year average.
Hamstring strain injuries were associated with greater than ‘typical’ standardised (Z>0) high-
speed running distances in the four weeks prior to injury. The risk of hamstring strain
increased 6.44 fold in the week prior to injury and 1.96 fold in the four weeks prior to injury
for every Z-score increase of 1. Only trivial differences were observed between injured and
uninjured players in regards to the standardised total running distances, acceleration and
deceleration distances, and training session ratings of perceived exertion. Increasing AFL
experience was associated with a decreased hamstring strain injury risk (OR = 0.77; 95%CI =
0.57 – 0.97; p=0.02). Furthermore, using logistic regression for the high-speed running data
indicated that modification of high-speed running distances in the final week of a four week
training period may influence the probability of hamstring injury As a consequence, sudden
increases in the volume of high-speed running are less likely to result in injury if the final
week of four weeks of training involves a compensatory reduction in high-speed running.
Study 2 examined the effect of 100-drop punt kicks on peak knee flexor strength and muscle
activation, recorded using surface electromyography (sEMG), of biceps femoris (BF) and
medial hamstrings (MH). Thirty-six recreational football players were randomly assigned to
kicking or control groups. Dynamometry was conducted immediately before and after the
kicking or 10 min sitting (control). Eccentric strength declined more in the kicking than the
control group (p < 0.001; d = 1.60), with greater reductions in eccentric than concentric
vi
strength after kicking (p = 0.001; d = 0.92). No significant between group differences in
concentric strength change were observed (p = 0.089; d = 0.60). The decline in eccentric
hamstring sEMG (BF and MH combined) was greater in the kicking than the control group (p
< 0.001; d =1.78), while changes in concentric hamstring sEMG did not differ between
groups (p = 0.863; d = 0.04). Post-kicking reductions in sEMG were greater in eccentric than
concentric actions for both BF (p = 0.008; d = 0.77) and MH (p < 0.001; d = 1.11). In
contrast, the control group exhibited smaller reductions in eccentric than concentric
hamstring sEMG for BF (p = 0.026; d = 0.64) and MH (p = 0.032; d = 0.53). Reductions in
BF sEMG were correlated with eccentric strength decline (R = 0.645; p = 0.007).
Study 3 aimed to investigate whether knee flexor adaptations induced by 5-weeks of
concentric or eccentric strength training influenced sprint performance and markers of muscle
damage. Thirty males were allocated into either a concentric-only (CON) or eccentric-only
(ECC) group, each of which performed nine sessions of resistance training. Prior to and
immediately after the 5-week intervention, each participant’s bicep femoris long head (BFLH)
fascicle length (FL), pennation angle (PA), muscle thickness (MT), peak isometric knee
flexor (KF) torque and Nordic eccentric strength were assessed. Post-intervention,
participants performed two-sprint sessions (10 x 80 m) 48 h apart. With reference to the first
sprint session, blood samples and passive KF torque were collected pre-, post-, 24 h post-,
and 48 h post-sprint exercise. The results from this study showed no significant interactions
for sprint performance decrements within (p=0.595; d=0.24) and between sprint sessions (p =
0.910; d=0.05), peak isometric strength (p=0.480; d=0.12), passive KF torques (p=0.807;
d=0.16), Nordic eccentric strength (p=0.065; d=0.19) and creatine kinase (p=0.818; d=0.06).
Fascicles lengthened in the eccentric group (p<0.001; d = 2.0) and shortened in the
concentric group (p<0.001; d =0.92). Pennation angle decreased for the eccentric group
vii
(p=0.001; d = 0.52) and increased in the concentric group (p<0.001; d = 1.69). Nordic
eccentric strength improvements occurred both the eccentric (p<0.001; d =1.49) and
concentric (p<0.001; d = 0.95) groups.
The new findings from this program of research suggest a relationship between hamstring
strain injuries and elevated high-speed running distances over the course of four weeks prior
to injury. This study suggests that hamstring strain injuries may, at least sometimes, be
overuse injuries rather than isolated events associated with a single injurious contraction or
sudden stretch. These findings indicate that the monitoring of high-speed running volumes
may occasionally allow hamstring injuries to be prevented because coaches and conditioning
staff should be able to prevent athletes from experiencing sudden increases in training loads.
The findings suggest that monitoring of total, acceleration and deceleration distances has
little or no impact on hamstring injury risk.
The second study described here showed that high volumes of drop punt kicking could reduce
eccentric knee flexor strength and hamstring muscle activation in maximal eccentric actions.
Low levels of eccentric strength are associated with an increased risk of hamstring strain
injuries in Australian Rules football and Soccer players and low levels of muscle activation
are associated with a greater propensity for isolated muscles to rupture when they are
forcefully lengthened. The results suggest the need to include the number of kicks and their
intensity in the weekly training load of Australian Rules football players.
The third study showed, contrary to expectations, that concentric and eccentric knee flexor
strength training did not have different effects on the recovery from a high-intensity sprint
running session. This was despite significant differences in hamstring muscle fascicle lengths
viii
and pennation angles in the eccentrically and concentrically trained muscles at the time of the
sprint sessions. The results suggest that the recovery in running performance after a high
intensity and moderate volume sprint session is not influenced significantly by muscle
architecture or the contraction mode employed in strength training.
ix
List of publications related to thesis
Duhig S., Shield A., Opar D., Gabbett T., Ferguson C. and Williams M. The effect of high-speed
running on hamstring strain injury risk. British Journal of Sports Medicine, 50(24), 1436-1540.
IF=6.72
Duhig, SJ., Williams, MD., Minett, GM., Opar, DA., & Shield, AJ. Drop punt kicking
induces eccentric knee flexor weakness associated with reductions in electromyographic
activity. Journal of Science and Medicine in Sport, In Press. IF=3.75
Manuscripts currently under peer review
Duhig, SJ., Buhmann, RL., Bourne, MN., Williams, MD., Roberts, LA., Minett, GM.,
Timmins, RG., Simms, CKE & Shield, AJ. (2016). The effect of concentric and eccentric
conditioning on recovery after repetitive sprint sessions. Submitted October 2016.
Other relevant publications
Bourne MN., Duhig, SJ., Timmins, RG., Williams, MD., Opar, DA., Najjar, AA., Kerr, GK.
& Shield, AJ. Impact of the Nordic hamstring and hip extension exercises on hamstring
architecture and morphology: implications for injury prevention. British Journal of Sports
Medicine. doi:10.1136/bjsports-2016-096130 , 2016. IF=6.72
Opar DA., Williams MD., Timmins RG., Hickey J., Duhig SJ. & Shield AJ. (2015).
Eccentric Hamstring Strength and Hamstring Injury Risk in Australian Footballers. Medicine
and Science in Sports and Exercise. 47(4), 857-865. IF =3.98
x
Opar DA., Williams MD., Timmins RG., Hickey J., Duhig SJ. & Shield AJ. (2015). The
Effect of Previous Hamstring Strain Injuries on the Change in Eccentric Hamstring Strength
During Preseason Training in Elite Australian Footballers. The American Journal of Sports
Medicine, 25(1), 81-85. IF = 4.36
Grants awarded during candidature
Gold Coast Suns Australian Football Club
Duhig S., Shield AJ (2014). The effect of eccentric strength and high-speed running on
hamstring strain injury risk within elite Australian footballers.
$5000
List of conference presentations
Duhig, S. Risk factors associated with hamstring strain injuries. Australian Strength & Conditioning
Association. Gold Coast, 2016. Oral presentation.
Duhig S., Shield A., Opar D., Gabbett T., Ferguson C. and Williams M. The effect of high-speed
running on hamstring strain injury risk. Sports Medicine Australia Conference. Gold Coast, 2015.
Oral presentation.
Duhig S., Shield A., Opar D., Gabbett T., Ferguson C. and Williams M. The effect of high-speed
running on hamstring strain injury risk. Copenhagen’s 1st hamstring injury symposium. Copenhagen,
2015. Oral presentation.
Duhig, S, Bourne, MN, Opar, DA, Williams, MD, Shield, AJ. Is eccentric knee flexor strength
prioritized at elite levels of football? World Congress on Science and Football. Copenhagen, 2015.
Oral presentation.
xi
Duhig S., Shield A., Opar D., Gabbett T., Ferguson C. and Williams M. A new model to monitor
hamstring strain injury risk in elite Australian Rules football. IHBI Inspires. Brisbane, 2015. Oral
presentation.
Duhig S. Effective warm-ups and recovery processes. Queensland Australian Football League Player
Management Seminar. Brisbane, 2015. Oral presentation.
Duhig S. Hamstring strain injuries in Australian Rules football. National AFL Academy Symposium.
Melbourne, 2015 Oral presentation.
Duhig S. Comparison of Eccentric Hamstring Strength and Injury Rates between Professional and
Semi-Professional Australian Footballers. International Olympic Committee World Congress for the
Prevention of Injury and Illness in Sport. Monaco, 2014. Poster presentation.
xii
LIST OF FIGURES
Figure 1 - A new hamstring strain injury model providing insight into risk factors
associated with first-time and recurring injuries. .........................................20
Figure 2 - The average proportion of new and recurrent hamstring strain injuries
sustained across one season of rugby matches .............................................40
Figure 3 - Running gait phases. .................................................................................47
Figure 4 - The phases of ball kicking used in Australian Football. ...........................49
Figure 5 - Standardized weekly session loads ...........................................................65
Figure 6 - The influence of summed four-week standardized mean high-speed running
(HSR) session distances on the probability of hamstring strain injury ........66
Figure 7 - Pre to post mean changes in knee flexor torque. .......................................82
Figure 8 - Changes in mean normalized hamstring sEMG in peak eccentric and
concentric isokinetic contractions .................................................................84
Figure 9 – Pre and post intervention individual and mean changes in bicep femoris
long head fascicle lengths. ..........................................................................108
Figure 10 – Pre and post intervention individual and mean changes in bicep femoris
long head pennation angles. ........................................................................108
Figure 11 - Pre and post intervention individual and mean changes in bicep femoris
long head thickness. ....................................................................................109
xiii
Figure 12 - Pre and post intervention individual and mean changes in Nordic eccentric
strength. ......................................................................................................110
Figure 13 - Group averages for subjective ratings of delayed onset muscle soreness.
Error bars represent SD. .............................................................................114
xiv
LIST OF TABLES
Table 1 - Training program variables......................................................................104
Table 2 - Participant characteristics .......................................................................107
xv
LIST OF ABBREVIATIONS
ACC acceleration
AFL Australian Football League
ARF Australian Rules football
BF biceps femoris
CI confidence interval
CON concentric
DEC deceleration
DIST total distance covered
FL fascicle length
INJ injured
K kicking
KF knee flexor
Kg kilograms of body mass
HE hip extension
HSI hamstring strain injury
HSR high-speed running
GPS global positioning system
MT muscle thickness
N newtons of force
NHE Nordic hamstring exercise
NK no kicking
PA pennation angle
xvi
PT peak torque
SD standard deviation
sEMG surface electromyography
s-RPE session rating of perceived exertion
UNINJ uninjured
xvii
STATEMENT OF ORIGINAL AUTHORSHIP
The work contained in this thesis has not been previously submitted to meet requirements for
an award at this or any other higher education institution. To the best of my knowledge and
belief, the thesis contains no material previously published or written by another person
except where due reference is made.
Signature:
Date: 10th
August 2017
QUT Verified Signature
xviii
ACKNOWLEDGEMENTS
There are a number of people I would like to thank for their support and guidance during my
PhD candidature. First and foremost, my partner Michelle Platz, for without her care and
understanding I would not have been able to complete my studies (both undergraduate and
postgraduate). To my supervisory team, Dr Anthony Shield, Dr Morgan Williams and Dr
Geoffrey Minett, your knowledge and assistance ensured my candidature was as ‘smooth’ as
possible and I cannot thank you enough for the valuable information and expertise you have
passed on which has nurtured the development of my research skills. To my fellow research
team members and O-block postgraduates, it has been a pleasure sharing the ‘journey’ with
you all and I appreciate your support over the last few years. Finally, I would like to thank the
Queensland University of Technology for giving me the opportunity to complete my doctoral
studies.
19
Chapter 1: Introduction
The hamstring strain injury is common and problematic across a wide variety of sports,
particularly those requiring high-speed running (Brooks, Fuller, Kemp, & Reddin, 2005a;
Drezner, Ulager, & Sennet, 2005; Ekstrand, Hägglund, & Waldén, 2011b; Orchard, James, &
Portus, 2006; Woods et al., 2004). These injuries occur frequently and exhibit high
recurrence rates. Recurrent hamstring injuries are typically more severe than primary insults
(Brooks, Fuller, Kemp, & Reddin, 2006; Koulouris, Connell, Brukner, & Schneider-Kolsky,
2007) and involve longer rehabilitation periods (Koulouris et al., 2007). Hamstring injury
results in a significant financial burden on sporting teams (Hickey, Shield, Williams, & Opar,
2013). In an attempt to understand this complex injury our research group has developed a
hamstring injury model that is based on identified risk factors and how they interact with one
another (Figure 1).
20
Figure 1 - A new hamstring strain injury model providing insight into risk factors associated with first-time and
recurring injuries. (Adapted from Fyfe et al., 2013)
Longitudinal reports from professional rugby union (Brooks et al., 2005a; Brooks, Fuller,
Kemp, & Reddin, 2005b; Brooks et al., 2006), athletics (Opar et al., 2012), the Australian
Football League (AFL) (Orchard, Seward, & Orchard, 2013) and elite level soccer (Ekstrand,
Waldén, & Hägglund, 2016; Hagglund, Walden, & Ekstrand, 2009; Woods et al., 2004)
suggest either no change or a slight increase (4%) in hamstring strain injury prevalence. This
is of particular concern given recent research findings that prophylactic exercises (e.g. Nordic
hamstring exercise) can reduce hamstring strains (Arnason, Andersen, Holme, Engebretsen,
& Bahr, 2008; Petersen, Thorborg, Nielsen, Budtz-Jørgensen, & Hölmich, 2011; van der
Horst, Smits, Petersen, Goedhart, & Backx, 2015). The reason for little success in reducing
injury rates may be explained by a generalised reluctance to implement these interventions
within elite sporting programs (McCall et al., 2015) and perhaps further information is
required in order for sporting clubs to adopt these findings.
21
As a result of hamstring strain injury, time lost from training and competition not only
challenges the athlete physically and mentally (Raysmith & Drew, 2016), but also provides
significant financial burdens for professional sporting clubs. Within the 2009 AFL season,
hamstring injuries were estimated to cost clubs AUD $1.5 million in ‘lost wages’ (Opar,
Williams, & Shield, 2012) and between 2002 and 2012 the yearly cost of hamstring strain
injuries per club increased on average by 71% (Hickey et al., 2013). Moreover, the average
financial cost of a single hamstring injury increased by 56% from AUD $25 603 in 2003 to
AUD $40 021 in 2012, despite little change in the incidence rates during that period (Hickey
et al., 2013). Similarly, English premier league clubs reported a staggering £74.4 million in
wages paid to unavailable players during the 1999-2000 seasons (Woods, Hawkins, Hulse, &
Hodson, 2002).
Our group has recently proposed a model that helps explain factors that may contribute to
hamstring strain injury (Figure 1.). The model represents the groups’ stance on important risk
factors for injury, whereby we currently propose three major risk factors for hamstring injury:
(1) low eccentric strength (Opar et al., 2015; Timmins, Bourne, et al., 2015), or a between-
limb imbalance that reveals weak hamstrings in one limb (Bourne, Opar, Williams, & Shield,
2015), (2) short bicep femoris fascicle lengths (Timmins, Bourne, et al., 2015) and (3) poor
load management (i.e. amount of high-speed running and kicking performed). Given the
complex multifaceted nature of the hamstring injury, we also recognise the role that other
factors may play (please refer to section 2.5).
Therefore, based on the negative performance (Hägglund et al., 2013) and economic
implications arising from hamstring strains (Hickey et al., 2013), further investigation into
the mechanisms and risk factors associated with this problematic injury is warranted. The aim
22
of this review is to provide an overview of the hamstring strain injury literature (i.e.
hamstring anatomy and morphology, injury incidence and prevalence within sport, associated
risk factors, and injury mechanisms).
23
Chapter 2: Literature Review
2.1. Hamstring anatomy
The three muscles situated on the posterior thigh are collectively described as the hamstring
muscle group. This group can be further compartmentalised into the medial hamstrings (MH),
consisting of the semimembranosus and semitendinosus, and the lateral hamstring, comprised
of the short and long head of the biceps femoris (BF) muscle. With the exception of the
monoarticular short head of biceps femoris, the hamstrings are biarticular, crossing both the
knee and hip joints. This arrangement allows for the prime movements of knee flexion and
hip extension when actively shortening. Collectively these muscles share common actions,
although individually there are significant morphological, architectural and functional
differences (Marieb & Hoehn, 2007; Markee et al., 1955). Therefore, it is vital to recognize
these differences in order to understand the potential impact hamstring structure and function
have on hamstring strain injury occurrence.
Medial hamstring compartment
The medial hamstring compartment consists of the semitendinosus and semimembranosus
(Marieb & Hoehn, 2007; Markee et al., 1955). The superficial semitendinosus is classified as
a fusiform muscle. This muscle originates from three locations; the inferomedial aspect of the
ischial tuberosity, the medial aspect of the long head biceps femoris tendon and an
aponeurosis arising from the proximal tendon of biceps femoris long head, and inserts via a
long tendon onto the superomedial surface of the tibia, or pes anserinus (Marieb & Hoehn,
2007; Markee et al., 1955). Two branches from the tibial portion of the sciatic nerve
innervate the superior and inferior portions of semitendinosus; a nerve branch dividing from
the sciatic nerve close to the ischial tuberosity innervates motor units superior to the
24
tendinous intersection. A nerve diverging from the sciatic nerve proximal to the upper and
middle third of the thigh innervates the inferior motor units (Marieb & Hoehn, 2007; Markee
et al., 1955).
The deeper of the two medial hamstrings, the semimembranosus, is classified as a bipennate
muscle (Marieb & Hoehn, 2007; Markee et al., 1955). Originating from the superomedial
ischial tuberosity, it inserts via numerous locations; the posteromedial aspect of the medial
tibial condyle, the knee joint capsule and tibial collateral ligament (Marieb & Hoehn, 2007;
Markee et al., 1955). Innervation of semimembranosus is via the same nerve that supplies the
distal section of semitendinosus. However, a further five divisions of this nerve branch out to
the proximal origin through to the distal aponeurosis (Marieb & Hoehn, 2007; Markee et al.,
1955). Active shortening of the medial hamstring produces knee flexion, hip extension,
medial tibial rotation (when the knee is flexed) and internal rotation of the hip (when the hip
is extended) (Marieb & Hoehn, 2007; Markee et al., 1955).
Lateral hamstring compartment
The lateral hamstring is comprised of the biceps femoris muscle, which as its name suggests
has two heads, termed short and long. Biceps femoris short head arises from the lateral lip of
the linear aspera, the upper two-thirds of the lateral supracondylar line and lateral
intermuscular septum (Marieb & Hoehn, 2007). The distal attachment is via a common
tendon with the long head, inserting onto the fibular head and femoral lateral condyle
(Marieb & Hoehn, 2007). As this monoarticular muscle crosses only the knee joint and
causes knee flexion when actively shortening (Marieb & Hoehn, 2007). The short head is a
composite muscle innervated by the common fibular branch of the sciatic nerve (Marieb &
Hoehn, 2007; Markee et al., 1955).
25
Bipennate in appearance, the multiple origins of the biceps femoris long head (Marieb &
Hoehn, 2007) extend from the lateral aspect of the medial facet of the ischial tuberosity, a
shared tendon with semitendinosus and the inferior part of the sacrotuberous ligament
(Marieb & Hoehn, 2007). The distal fibres of biceps femoris long head converge into an
aponeurosis, which receives the short head fibres, with the distal tendon dividing around the
lateral collateral ligament and inserting onto the anterolateral aspect of the fibular head and
the tibial plateau (Marieb & Hoehn, 2007). Innervation of the long head is received via two or
three nerves divided from the tibial portion of the sciatic nerve (Marieb & Hoehn, 2007).
Active shortening of the long head results in knee flexion, hip extension, lateral tibial rotation
(when the knee is flexed) and external rotation of the hip (when the hip is extended) (Marieb
& Hoehn, 2007).
The anatomical characteristics of the hamstring muscle tendon may also influence risk of
strain. In particular, individuals who possess a disproportionately small biceps femoris long
head proximal aponeurosis may experience and increased likelihood of injury (Evangelidis,
Massey, Pain, & Folland, 2015) due to increased mechanical strain during high-speed
running (Fiorentino & Blemker, 2014). However, large prospective studies are required to
ascertain relationships between aponeurosis size and risk of future hamstring strain injury.
2.2. Hamstring strain injury definition
Hamstring muscle injury type is dependent upon the trauma mechanism (i.e. lacerations,
contusions, and strains). However, the majority of hamstring injuries in sport and recreational
activity are muscle strains (Bennell & Crossley, 1996; Ekstrand et al., 2011b; Feeley et al.,
2008; Orchard et al., 2013). Hamstring strain injuries are typically characterised by the
sudden onset of proximal posterior thigh pain, occasionally accompanied by an audible ‘pop’,
26
which is the result of partial to complete disruption of muscle fibres (Heiderscheit, Sherry,
Silder, Chumanov, & Thelen, 2010). The specific lesion site can usually be ascertained by the
action being performed at the time of injury, with proximal regions affected more so than
distal (Heiderscheit et al., 2010). For example, running-related insults will mostly disrupt the
intramuscular tendon or aponeurosis and adjacent muscle fibres of the biceps femoris long
head (Askling, Tengvar, Saartok, & Thorstensson, 2007) whereas hamstring strain injuries as
a consequence of relatively slow or static stretching more commonly present in the proximal
free tendon and/or muscle tendon junction of the semimembranosus (Askling, Tengvar,
Saartok, & Thorstensson, 2007). The severity of these injuries depends on numerous factors,
including: the action being performed (kicking type injuries more severe than running
(Heiderscheit et al., 2010); the location and magnitude of stress and strain experienced; and
the physical state (i.e. fatigued) of the muscle at that point in time (Agre, 1985). Injury
severity is often graded on a 3-level system: Grade 1 strains involve minimal muscle fibre
tears with minor swelling and discomfort, minimal to no loss of function; Grade 2 strains
involve greater muscle damage with obvious loss of strength and function; and Grade 3
strains involve a complete tear extending across the whole muscular cross-section resulting in
complete loss of strength and function.
2.3. Incidence of hamstring strain injury in sport
There is a high incidence of hamstring strain injuries in sports that involve high–speed
running such as athletics (Bennell & Crossley, 1996; D'Souza, 1994; Drezner et al., 2005;
Opar et al., 2012), American football (Elliott, Zarins, Powell, & Kenyon, 2011; Feeley et al.,
2008); Australian football (Gabbe, Finch, Wajswelner, & Bennell, 2002; Orchard et al.,
2013); rugby union (Brooks et al., 2005a, 2005b) and soccer (Ekstrand, Hägglund, &
Waldén, 2011a; Ekstrand et al., 2011b; Woods et al., 2002; Woods et al., 2004). In athletics,
27
hamstring strains have been reported to represent a quarter of all injuries and three-quarters of
all lower limb muscle strains (Opar et al., 2012). While hamstring lesions only account for
11.6% of all injuries in elite American football, they are typically among the most severe
(mean days lost per injury = 8.3) (Feeley et al., 2008). Over the last two decades of injury
surveillance within the elite Australian Football League (AFL), hamstring injuries have
remained, on average, at six new injuries per club per season (Orchard et al., 2013). These
rates account for 16% of all injuries sustained (Orchard & Seward, 2009) which resulted in
28% of all AFL games missed in the 2009 season, exceeding strains occurring in the groin
(17.9%) and quadriceps (7.8%) (Orchard & Seward, 2009). A similar trend is observed
within professional rugby with one large-scale study reporting hamstring injuries represent
15% of all injuries sustained during matches (Brooks et al., 2005a), and are the cause of the
greatest time lost from training and competing (mean days lost per injury; hamstring strain =
17, quadriceps strain = 12, groin strain = 10) (Brooks et al., 2005a, 2005b). The high
occurrence of hamstring strain injuries within professional European soccer results in seven
out of twenty-five players from each team incurring an injury each season (Ekstrand et al.,
2011b). Furthermore, hamstring strains are the most common severe (>28 days of time lost
from training and competing) injuries in soccer and account for 12% of all injuries sustained,
which once again surpasses strains in the quadriceps (7%) groin (9%) and ankle sprains (7%)
(Ekstrand et al., 2011b).
2.4. Recurrence of hamstring strain injury in sport
First-time hamstring strain injuries may incur several adverse effects on athletes; however,
perhaps the most concerning is the increased propensity for recurrence (Brooks et al., 2006;
Croisier, 2004; Orchard et al., 2013; Woods et al., 2004) as subsequent injuries are generally
more severe than the initial insult (Brooks et al., 2006; Ekstrand et al., 2011a). Early studies
28
reported hamstring strain recurrence rates of 32% in American football players (Heiser,
Weber, Sullivan, Clare, & Jacobs, 1984) and 34% in AFL players (Seward, Orchard, Hazard,
& Collinson, 1993). Recent epidemiological studies show recurrence rates dropping by 10%
in Rugby (Brooks et al., 2006) and by 7% across the last 20 years in the AFL (Orchard &
Seward, 2010), although these reductions have been proposed to be the product of more
conservative decisions regarding return to play rather than improved rehabilitation practices
(Orchard & Seward, 2010). Nevertheless, in contrast to other strain injuries sustained in the
AFL, hamstrings still exhibit higher recurrence rates than groin (23%) and quadriceps
muscles (17%) (Orchard & Seward, 2010). These rates would suggest the need for further
investigations aimed at reducing both first-time and recurring hamstring injuries.
2.5. Proposed risk factors for hamstring strain injury
The multifaceted nature of the hamstring strain injury has led to numerous risk factors being
reported within the literature (Mendiguchía, Alentorn-Geli, Samuelsson, & Karlsson, 2015;
Opar et al., 2012). These risk factors can be categorised into either non-modifiable or
modifiable:
Non-modifiable
Age (Arnason et al., 2004; Edouard, Branco, & Alonso, 2016; Verrall, Slavotinek, Barnes, Fon,
& Spriggins, 2001)
Previous injury (Arnason et al., 2004; Gabbe, Bennell, Finch, Wajswelner, & Orchard, 2006;
Hagglund, Walden, & Ekstrand, 2006; Opar et al., 2015; Timmins, Bourne, et al., 2015)
29
Modifiable:
Muscular strength (Arnason et al., 2008; Askling, Karlsson, & Thorstensson, 2003; Croisier,
Forthomme, Namurois, Vanderthommen, & Crielaard, 2002; Croisier, Ganteaume, Binet,
Genty, & Ferret, 2008; Gabbe, Bennell, et al., 2006; Opar et al., 2015; Orchard, Marsden,
Lord, & Garlick, 1997; Sugiura, Saito, Sakuraba, Sakuma, & Suzuki, 2008; Timmins, Bourne, et
al., 2015; Yeung, Suen, & Yeung, 2009)
Neuromuscular fatigue (Brooks et al., 2006; Ekstrand et al., 2011b; Heiser et al., 1984; Mair,
Seaber, Glisson, & Garrett, 1996; Opar et al., 2012; Schuermans, Van Tiggelen, Danneels, &
Witvrouw, 2016; Woods et al., 2004)
Biceps femoris long head fascicle lengths (Timmins, Bourne, et al., 2015)
Flexibility (Bradley & Portas, 2007; Henderson, Barnes, & Portas, 2009; Witvrouw, Danneels,
Asselman, D'Have, & Cambier, 2003)
However, for the purpose of this review, the primary focus will be on load management,
muscular (eccentric) strength, and biceps femoris long head fascicle lengths, and secondarily
on the other risk factors. Recognising and considering each of these risk factors should assist
in developing individual risk profiles in the attempt to reduce first-time and recurring
hamstring strain injuries.
2.5.1. Non-modifiable risk factors
Non-modifiable risks factors are unable to be altered as a result of an intervention. Due to the
focus of this program of research, this review will primarily deliver evidence supporting the
association between hamstring strain injuries and age and gender. However, the hamstring
strain injury risk relationships between previous injury and race will also be discussed.
30
2.5.1.1. Age
The majority of the hamstring strain injury risk literature supports increasing age as a risk
factor. Older athletes have been shown to have a greater risk of future hamstring injury in
Australian football (Gabbe, Bennell, & Finch, 2006; Opar et al., 2015), soccer (Timmins,
Bourne, et al., 2015; Verrall et al., 2001) and athletics (Edouard et al., 2016). Elite AFL
players older than 25 years have a four-fold risk of injury in contrast to players less than 20
years (Gabbe et al., 2006). Similarly, the odds of a community-level AFL player older than
23 years were four-fold compared to players under the age of 23 years (Gabbe, Finch,
Bennell, & Wajswelner, 2005). Within elite Icelandic soccer, with each chronological year
there was a risk increase of 10% (Arnason et al., 2004). In athletics, 26, 30 and 35-year-old
male international level sprinters exhibit relative risks of 1.6, 1.9 and 2.1 times those of their
20-year-old counterparts respectively (Edouard et al., 2016). However, findings from a recent
large-scale prospective study conducted within Australian rugby union did not show a
relationship between age and risk of hamstring strain injury (Bourne et al., 2015).
A range of factors have been proposed to contribute to the age-related rise in hamstring
strains, including: poor hip flexor flexibility (Gabbe, Bennell, et al., 2006), increased body
mass index (Gabbe, Bennell, et al., 2006), inability to recovery from and/or handle high
training and competing loads (Gabbe, Bennell, et al., 2006; Pizzari, Wilde, & Coburn, 2010)
and hypertrophy of the lumbosacral ligament (Orchard, Farhart, & Leopold, 2004). However,
only recently have studies conducted within elite Australian Rules (Opar et al., 2015) and
soccer players (Timmins, Bourne, et al., 2015) found modifiable risk factors (i.e. eccentric
hamstring strength and biceps femoris long head fascicle lengths) to at least partly counter the
effects of non-modifiable risk factors. These reports show players older than 23 years can
mitigate their injury risk with increased strength levels (Opar et al., 2015). Moreover,
31
professional Australian soccer players exhibiting relatively high levels of eccentric hamstring
strength coupled with longer biceps femoris long head fascicles can offset the effects of age
on the risk of hamstring strains (Timmins, Bourne, et al., 2015). These recent advances
provide a better understanding of the age-hamstring injury relationship and should encourage
future work on possible interactions between modifiable and non-modifiable risk factors.
2.5.1.2. Previous hamstring strain injury
From all known hamstring strain injury risk factors, the literature suggests that previous
injury has the greatest influence on subsequent risk (Bennell et al., 1998; Hagglund et al.,
2006; Orchard, 2001; Verrall et al., 2001). Prospective studies show elite soccer players who
were injured in the previous season increased their chance of future injury 12 fold (Arnason
et al., 2004; Hagglund et al., 2006). These reports are reaffirmed with similar findings from
elite (Gabbe et al., 2006; Orchard, 2001) and community-level (Verrall et al., 2001)
Australian Rules football players. The underlying factor(s) associated with previous strain
injury are uncertain, however, there are presumed to be a number of maladaptations arising
from the insult (Opar et al., 2012) or the persistence of pre-existing risk factors that are not
addressed adequately in rehabilitation (Bradley & Portas, 2007; Brockett, Morgan, & Proske,
2004; Croisier et al., 2002; Jonhagen, Nemeth, & Eriksson, 1994; Silder, Heiderscheit,
Thelen, Enright, & Tuite, 2008; Silder, Reeder, & Thelen, 2010; Witvrouw et al., 2003).
Our group has proposed that persistent neuromuscular inhibition (Fyfe, Opar, Williams, &
Shield, 2013) of the previously injured hamstring muscles may account for observations of
eccentric weakness (Croisier & Crielaard, 2000; Croisier, 2004; Croisier et al., 2002; Dauty,
Potiron-Josse, & Rochcongar, 2003; Jonhagen et al., 1994; Opar, Williams, Timmins, Dear,
& Shield, 2013; Opar et al., 2014), muscle atrophy (Silder et al., 2008) and shortened muscle
32
fascicles (Timmins, Porter, Williams, Shield, & Opar, 2014; Timmins, Shield, Williams,
Lorenzen, & Opar, 2015), all of which have been observed to persist after return to sport and
some of which may be risk factors for future injury (Opar et al., 2015; Silder et al., 2008;
Timmins, Bourne, et al., 2015; Timmins, Shield, Williams, Lorenzen, et al., 2015). These
factors should be trainable but the failure to recognise their proposed cause (inhibition) may
at least partially explain their persistence and the relatively high hamstring injury recurrence
rates that have been reported (Croisier et al., 2002; Hawkins, Hulse, Wilkinson, Hodson, &
Gibson, 2001; Orchard et al., 1997; Orchard et al., 2013). It should be noted that conclusive
evidence for this proposal has not been produced.
2.5.2. Modifiable risk factors
Risk factors that can be altered as the result of an intervention are defined as modifiable. For
the purpose of this program of research this review will focus mainly upon neuromuscular
fatigue (Brooks et al., 2006; Heiser et al., 1984; Mair et al., 1996; Woods et al., 2004),
strength (Aagaard, Simonsen, Magnusson, Larsson, & Dyhre-Poulsen, 1998; Arnason et al.,
2008; Askling et al., 2003; Brockett, Morgan, & Proske, 2001; Burkett, 1970; Croisier et al.,
2002; Croisier et al., 2008; Friden & Lieber, 1992; Gabbe, Bennell, et al., 2006; Garrett,
Safran, Seaber, Glisson, & Ribbeck, 1987; Lee, Reid, Elliott, & Lloyd, 2009; Opar et al.,
2015; Timmins, Bourne, et al., 2015) and bicep femoris long head fascicle lengths (Timmins,
Bourne, et al., 2015). Other modifiable risk factors such as flexibility (Bradley & Portas,
2007; Henderson et al., 2009; Witvrouw et al., 2003) and lumbopelvic control (Chumanov et
al., 2007; Sherry & Best, 2004) will also be briefly discussed.
33
2.5.2.1. Muscle Weakness
The role of strength as a risk factor for muscle strain injury can be traced back to early in situ
studies that found maximally stimulated muscles, able to produce the greatest force, to endure
more stress than sub-maximally stimulated muscles (Garrett et al., 1987; Mair et al., 1996).
Should these in situ results replicate in vivo function, then stronger muscles would be more
resilient to strain injury than weaker muscles (Garrett et al., 1987). Therefore, to further
understand the influence of knee flexor strength on hamstring strain injury this section will
cover: eccentric knee flexor strength, knee flexor torque-joint angle relationships, between-
limb knee flexor strength asymmetries, and the hamstring to quadriceps (H: Q) ratio (i.e. ratio
of knee flexor to knee extensor strength).
It should be noted that the literature often refers to ‘hamstring strength’ despite the fact that
there is a contribution from other knee flexors (gastrocnemii, gracilis and sartorius) although
the hamstrings account for approximately 50% of the physiological cross-sectional areas of
all muscles crossing the back of the knee (Handsfield, Meyer, Hart, Abel, & Blemker, 2014).
This greater physiological cross-sectional area and significantly longer moment arm for the
hamstrings suggests this muscle has the ability to generate the majority of knee flexor torque
and that any changes in strength must be largely influenced by changes in hamstring force
generation (Handsfield et al., 2014).
2.5.2.2. Eccentric knee flexor strength
The link between eccentric knee flexor strength, or lack thereof, and hamstring strain injury
risk seems logical given the role of the hamstring muscle during high-speed running (see
section 2.7.1.). Nevertheless, there is disparity within the literature with reports either
supporting (Croisier et al., 2008; Opar et al., 2015; Orchard et al., 1997; Timmins, Bourne, et
34
al., 2015; van Dyk et al., 2016) or refuting (Bennell et al., 1998; Bourne et al., 2015; Zvijac,
Toriscelli, Merrick, & Kiebzak, 2013) such an association. While it is uncertain as to why
this contraindication exists, an explanation may lie in varying experimental methodology (i.e.
skill level of dynamometer operator, season time-point where strength was measured,
familiarisation with the protocol and equipment). A notable difference in the aforementioned
studies was that some used the Nordbord (Opar et al., 2015; Timmins, Bourne, et al., 2015)
and the others isokinetic dynamometry (Croisier et al., 2008; Orchard et al., 1997; van Dyk et
al., 2016; Bennell et al., 1998; Zvijac, et al., 2013). While the current gold standard
measurement of eccentric knee flexor strength is isokinetic dynamometry (Bennell et al.,
1998; Croisier et al., 2008; Orchard et al., 1997; Zvijac et al., 2013), the device is expensive,
bulky, requires a high level of operator skill and requires a lengthy assessment protocol and
familiarisation of participants. These limitations were the catalyst for the development of a
device (i.e. Nordbord) that allows for the quantification of the forces produced by the knee
flexors during the Nordic hamstring exercise (Opar, Piatkowski, Williams, & Shield, 2013).
The effectiveness of the Nordbord has been strengthened by large-scale prospective studies
associating low levels of eccentric knee flexor strength (Opar et al., 2015; Timmins, Bourne,
et al., 2015) or between-limb strength differences (Bourne et al., 2015) with future risk of
hamstring injury. However, a recent study by van Dyk and colleagues is the largest of its type
(n=614) to have prospectively investigated the role of hamstring strength and hamstring
injury risk (van Dyk et al., 2016). These authors observed only a weak relationship between
eccentric hamstring strength and hamstring strain injury rates
2.5.2.3. Knee flexor torque-joint angle relationships
Dynamometry allows for the collection of torque throughout a range of joint angles and when
plotted together these measurements represent the torque joint-angle relationship. The
35
dynamometer derived knee flexor torque-joint angle relationship is assessed via slow
concentric or isometric muscle contractions (Brockett et al., 2001). This relationship is a
representation of changes in muscle force and moment arm length throughout a range of
motion and is essentially a substitute for in vivo force-length relationships (Brockett et al.,
2001). One aim of this assessment is to estimate hamstring strain injury risk based on the
belief that the character of the torque joint-angle relationship is indicative of hamstring
fascicle length (i.e. peak torque produced at a greater or lesser degree of knee flexion
symbolizes shorter or longer fascicle lengths, respectively)(Brockett et al. 2004). It is argued
that muscles with shorter fascicles are more susceptible to muscle strain (Morgan, 1990).
Morgan (1990) hypothesised that short fascicles (those with fewer in-series sarcomeres) are
unable to endure forceful eccentric contractions and eventually reach a point of elongation at
which the longest and weakest sarcomeres will overextend or ‘pop’. Continued exposure to
repeated eccentric contractions (e.g. high-speed running) increases strain on neighbouring
sarcomeres and their likelihood of ‘popping’. It has been argued that these ‘popped’
sarcomeres represent microscopic myofibril damage (Morgan & Allen, 1999; Morgan, 1990)
and that if a large enough number of sarcomeres have overextended that the injury then
becomes macroscopic at the time when a strain injury is reported (Garrett, Nikolaou,
Ribbeck, Glisson, & Seaber, 1988). However, should only a relatively small number of
sarcomeres be affected and the muscle given sufficient time to recover, the repair process
ensures the restoration of damaged myofibrils, possibly with extra sarcomeres added in-series
to those that existed before the exercise stimulus (Morgan & Allen, 1999). The longer muscle
fascicles (which have more in-series sarcomeres) are suggested to have less sarcomere
lengthening per unit of in-series strain (Morgan, 1990) and therefore a reduced susceptibility
to eccentrically-induced muscle damage and injury risk (Lieber & Fridén, 1999; Proske,
Morgan, Brockett, & Percival, 2004).
36
Brockett and colleagues (2004) have reported that injured hamstrings generate peak torque at
significantly more flexed knee joint angles compared with the contralateral uninjured limbs.
Based on this observation, these authors suggest that a deficit in in-series sarcomeres (short
muscle fascicles) may be the initial cause of injury and also increase the risk of reinjury,
although, the retrospective nature of this study prevents establishing a causal link. In contrast,
a subsequent prospective study has refuted the relationship between knee flexor torque joint-
angle relationships and risk of hamstring strain (Yeung et al., 2009). Future studies should
aim to identify whether a relationship exists between muscle fascicle lengths and the joint
angle of peak torque.
2.5.2.4. Between-limb strength asymmetries
On the basis that strength is a risk factor for hamstring strain injury, the assessment of
strength differences between limbs seems logical in the attempt to identify a weaker
hamstring, which may exhibit elevated risk of injury. As mentioned previously, the testing of
eccentric hamstring strength is conducted on either an isokinetic dynamometer or Nordbord.
Isokinetic dynamometry derived between-limb asymmetries in knee flexor strength suggest
an association between the weaker limb and increased risk of hamstring strain in soccer
(Croisier et al., 2002; Croisier et al., 2008), track and field (Sugiura et al., 2008; Yamamoto,
1993), Australian football (Orchard et al., 1997) and American football (Burkett, 1970).
Although smaller and underpowered (Bahr & Holme, 2003) prospective studies on sprinters
(Yeung et al., 2009) and Australian Rules football players (Bennell et al., 1998) have not
shown any such relationship. However, a recent extremely large study (n=462) on
professional soccer players, suggested that those exhibiting strength imbalances during the
37
preseason were four times more likely to sustain a hamstring injury than athletes without
imbalances or those who received an intervention to rectify imbalances (Croisier et al., 2008),
although this study did not focus purely on between limb imbalances making the results
harder to interpret. Furthermore, prospective studies carried out on elite-level sprinters
(Sugiura et al., 2008; Yamamoto, 1993) and Australian Rules football players (Orchard et al.,
1997) found that athletes with unilateral hamstring muscle weakness had a significantly
increased likelihood of subsequent hamstring strain injury. The degree of between-limb
imbalances required before it impacts upon hamstring injury risk differs amongst cohorts;
early studies found >10% to influence risk in American football players (Burkett, 1970) and
track and field athletes (Heiser et al., 1984), >8% in Australian football players and >15% in
soccer players (Croisier et al., 2008).
Nordbord derived strength assessments suggest that Australian Rugby players with a
between-limb imbalance greater than 15% or 20% increase their future risk of injury by 2.4-
fold (95%CI, 1.1 to 5.5) and 3.4-fold (95%CI, 1.5 to 7.6), respectively (Bourne et al., 2015).
However, two studies using identical methodology found no association between asymmetry
and injury in elite Australian Rules football players (Opar et al., 2015) or professional
Australian soccer players (Timmins, Bourne, et al., 2015). Therefore, based on the current
research it seems that depending on the device used to assess between-limb knee flexor
strength imbalances there are differing ‘at risk’ thresholds across different sports.
The underpinning mechanism(s) that explain the risk associated with between-limb
asymmetries and hamstring strain injury are not yet clear. However, Croisier and colleagues
(2008) propose biomechanical alterations during running may arise from these asymmetries
that increase hamstring lengths (strain) in the late swing phase of running gait and thereby
38
increase injury risk to the weaker hamstring. Finally, given the proposed effect on sprinting
technique and therefore biomechanical alternations (Croisier et al., 2008), future
investigations should also be directed towards strength imbalances in other muscles involved
in the terminal swing phase of running (e.g. hip flexors (Lee et al., 2009)).
2.5.2.5. Hamstring to quadriceps ratio
A strength comparison between the knee flexor and extensor torques seems rational
considering the decelerating role of the hamstrings during the terminal swing phase of
running and kicking (see section 2.7.). Given the hamstrings are exposed to moderate degrees
of strain and high levels of stress (Garrett, 1990) during running, should the knee extensors
be disproportionately stronger than flexors the strain and stress experienced by the knee
flexors may be intensified, resulting in a higher risk of injury. This notion led to the
exploration of isokinetic dynometrically derived concentric knee flexor and extensor ratios,
otherwise known as the conventional hamstring to quadricep ratio (H:Q ratio) (Burkett, 1970;
Orchard et al., 1997), on hamstring strain injury risk. Early small-scale studies within
Australian rules (Orchard et al., 1997) and American (Heiser et al., 1984) football found
players with a conventional H:Q ratio of <0.61 and <0.50, respectively, had a greater
likelihood of hamstring injury in the subsequent season. However, the validity of the
conventional H:Q ratio has been questioned and criticised for ignoring the vital eccentric role
of the hamstrings during high-speed running. Consequently, the eccentric knee flexor to
concentric knee extensor or functional H:Q ratio, has become more widely accepted (Aagaard
et al., 1998; Croisier et al., 2008; Sugiura et al., 2008; Yeung et al., 2009).
Two small-scale studies have reported no association between conventional or functional
H:Q ratio and subsequent hamstring injury (Bennell et al., 1998; Yeung et al., 2009).
39
However, it must be acknowledged that such studies, with small numbers of injuries (n<20)
are likely underpowered to identify moderate associations between these variables. A much
larger, multi-centered study of French, Belgian and Brazilian soccer players (n= 462)
suggests that functional H:Q ratios <0.80–0.89 or conventional ratios <0.45–0.47 were
associated with a higher rate of hamstring strain injury (Croisier et al., 2008). More recently,
however, the largest study of this type (n=614) carried out on players in Qatari elite soccer
has reported no association between either H:Q ratio and hamstring injury (van Dyk et al.,
2016).
2.5.2.6. Neuromuscular fatigue
Neuromuscular fatigue has been defined as “a loss in the capacity for developing force and/or
velocity of a muscle, resulting from muscle activity under load and which is reversible by
rest” (Gandevia, 2001). Neuromuscular fatigue can further be divided into peripheral (which
occurs distal to the neuromuscular junction) and central fatigue (failure to voluntarily activate
the muscle) (Gandevia, 2001). The progressive reductions in voluntary activation during
exercise may eventually lead to the cessation of exercise (Gandevia, 2001). Moreover, it has
been proposed that athletes exposed to neuromuscular fatigue have an increased propensity
for injury (Brooks et al., 2006; Ekstrand et al., 2011b; Heiser et al., 1984; Mair et al., 1996;
Opar et al., 2012; Schuermans et al., 2016; Woods et al., 2004).
Recently, relationships between neuromuscular fatigue and hamstring strain injury have been
observed in soccer (Ekstrand et al., 2011a; Woods et al., 2004) and rugby (Brooks et al.,
2006) with the majority of injuries occurring in the later stages of the first and second halves
of matches (Figure 2). These observations have led to investigations aimed at assessing the
effect of fatigue on hamstring function by applying running protocols designed to simulate
40
the physiological demands of soccer (Greig, 2008; Rahnama, Reilly, Lees, & Graham-Smith,
2003; Small, McNaughton, Greig, & Lovell, 2010; Timmins et al., 2014). These studies
found a decline in eccentric knee flexor strength of 15-18% (Greig, 2008; Rahnama et al.,
2003; Small et al., 2010; Timmins et al., 2014), which may be explained via a reduction in
biceps femoris sEMG (Timmins et al., 2014). Further evidence for the role fatigue plays in
hamstring strain injuries has been provided by Schuermans and colleagues (2016), whose
prospective study on soccer players found that those ceasing a leg curl exercise before 256
seconds were more likely to sustain an injury in the subsequent season than those able to
perform over a longer period of time. The lack of knee flexor endurance is a sign of
neuromuscular fatigue and further supports the growing body of evidence that suggests a lack
in eccentric knee flexor strength increases the susceptibility of hamstring strain injury
(Bourne et al., 2015; Croisier et al., 2008; Opar et al., 2015; Sugiura et al., 2008; Timmins,
Bourne, et al., 2015).
Figure 2 - The average proportion of new and recurrent hamstring strain injuries sustained across one season of
rugby matches (Brooks et al., 2006)
From a training and match workload monitoring perspective, the classification of ‘fatigue’ is
slightly different from the aforementioned definition. Gabbett and colleagues (Gabbett,
41
Hulin, Blanch, & Whiteley, 2016) use an acute: chronic workload ratio to estimate the risk of
future injury. This ratio defines acute workload as the most recent week’s training load and
chronic workload as a 4-week rolling average of acute workload (Gabbett et al., 2016). This
concept suggests injury risk is minimal when chronic workload is high (i.e. ‘fitness’ has been
developed) and the acute workload is low (i.e. experiencing relatively minimal ‘fatigue’)
(Gabbett et al., 2016). On the other hand, exposure to rapid increases in acute workload
relative to chronic workload result in ’fatigue’ and likely increases the athlete’s risk of injury
(Gabbett et al., 2016).
The underpinning mechanism(s) that may explain the relationship between neuromuscular
fatigue and muscle strain may be in part be revealed by early in situ animal studies (Garrett et
al., 1987; Mair et al., 1996). These reports suggest neuromuscular fatigue has a debilitating
effect on the muscle’s contractile function thus negating its energy-absorbing capacity
(Garrett et al., 1987; Mair et al., 1996). Additionally, despite fatigued and non-fatigued
muscles failing at the same relative length (Mair et al., 1996), the fatigued muscle was unable
to absorb as much energy before stretch-induced muscular failure which may lead to an
inability to prevent over-lengthening and ultimately causing strain injury. Further evidence to
support the effect of fatigue on human in vivo proprioception can be found in studies
investigating knee joint proprioceptive changes induced by knee flexor neuromuscular fatigue
(Allen, Leung, & Proske, 2010; Brown, Child, Donnelly, Saxton, & Day, 1996; Skinner,
Wyatt, Hodgdon, Conard, & Barrack, 1986). These reports suggest that knee flexor fatigue
results in errors in proprioception, likely due to alterations at the sensorimotor cortex level
(Allen et al., 2010). Moreover, hamstring neuromuscular fatigue, induced by an intermittent
running protocol and knee flexor resistance training session, led to increases in knee
extension range during running with concurrent elevations in hamstring neuromuscular
42
activity during the terminal swing phase (Pinniger, Steele, & Groeller, 2000). This hampered
spatial awareness of knee joint angle during high-speed running may lead to the muscle over
lengthening and demand heightened levels of hamstring muscle activity to cope, therefore
accentuating the onset of fatigue and further increasing the risk of injury.
2.5.2.7. Muscular architecture
Recent work by Timmins and colleagues (Timmins et al., 2014; Timmins, Bourne, et al.,
2015; Timmins, Ruddy, et al., 2015) has further explored Morgan’s (Morgan, 1990) proposed
injury-fascicle length relationship. Initially, Timmins et al. (2015) found athletes with a
previous unilateral hamstring injury exhibited shorter BFLH fascicles and greater pennation
angles than the uninjured contralateral muscle. Although the retrospective nature of this
initial study prohibits any inferences to be made between fascicle length and risk of
hamstring strain, a subsequent large-scale prospective study in professional Australian soccer
players found a four-fold increase in risk in athletes with short bicep femoris long head
fascicles (<10.56 cm) (Timmins, Bourne, et al., 2015). In contrast, longer fascicles seemed to
provide a protective benefit regardless of other risk factors (i.e. increasing age and prior
hamstring injury). These findings suggest that shorter fascicle lengths increase the risk of
injury and should fascicle lengths decrease as a maladaptation caused by injury then this
would suggest a greater chance of re-injury. Although hamstring architecture seems to be
negatively affected by injury this can be counteracted by a strength training intervention that
focuses on eccentric-only contractions (Timmins, Ruddy, et al., 2015). Further work is
needed to better understand the effect shorter bicep femoris long head fascicle lengths have
on hamstring strain injury risk.
43
2.5.2.8. Flexibility
There is a common belief, especially in those with minimal to no exposure to the sports
medicine/ injury preventative literature, that a lack of hamstring flexibility is a risk factor for
injury. This is perhaps based on the belief that greater compliance of passive elastic structures
may increase the energy absorptive capabilities of the muscle tendon unit (Worrell, Smith, &
Winegardner, 1994). The measurement of hamstring flexibility has been recorded via
numerous methods: active (Gabbe, Bennell, et al., 2006; Gabbe et al., 2005; Malliaropoulos,
Isinkaye, Tsitas, & Maffulli, 2011; Rolls & George, 2004; Warren, Gabbe, Schneider-Kolsky,
& Bennell, 2010) or passive (Arnason, Tenga, Engebretsen, & Bahr, 2004; Engebretsen,
Myklebust, Holme, Engebretsen, & Bahr, 2010; Rolls & George, 2004; Witvrouw et al.,
2003; Worrell, Perrin, Gansneder, & Gieck, 1991) knee extension, hamstring
musculotendinous and leg stiffness (Watsford et al., 2010) and maximum static range of
motion (Bradley & Portas, 2007). Given the numerous confounding variables associated with
current hamstring flexibility assessments (i.e. lumbar spine flexibility, upper to lower limb
ratios, neural extensibility, and subjective measurements) it is not surprising that there is
uncertainty surrounding flexibility as a risk factor for a hamstring injury. Future work should
focus on more objective measures of hamstring length, and perhaps fascicle length, which has
been shown to influence the future risk of hamstring strains (Timmins, Bourne, et al., 2015).
A meta-analysis by Freckleton and Pizzari (2012) found two of the studies employing the
active knee extension method (Gabbe, Bennell, et al., 2006; Gabbe et al., 2005) reported no
association between flexibility and injury rates (p = 0.08). Moreover, two of the three studies
excluded from the meta-analysis using the same method suggested no relationships between
poor hamstring flexibility in young soccer players (Rolls & George, 2004) and injury
recurrence (Warren et al., 2010). Whereas the third study excluded found athletes with
44
deficits in active knee extension of 10-19º at 48 h post initial insult were more likely to
experience a recurrent strain than those with lesser deficits (Malliaropoulos et al., 2011).
When two studies measuring hamstring length using the passive knee extension test were
included in the meta-analysis (n= 907) (Freckleton & Pizzari, 2012) no significant differences
were found between the injured and uninjured groups (p = 0.32), nor were bilateral hamstring
length asymmetries found to be associated with strain injuries (Fousekis, Tsepis, Poulmedis,
Athanasopoulos, & Vagenas, 2010). More work is required to definitively explain the
relationship between flexibility and hamstring injury and this work will require better
measures of flexibility than have previously been described.
2.6. Attempts to reduce hamstring strains in sport
The two previous sections have highlighted the incidence and prevalence of hamstring strain
injuries across numerous sports. In recent times there has been an abundance of research
directed towards providing a prophylactic exercise for hamstring strains (Arnason,
Gudmundsson, Dahl, & Johannsson, 1996; Croisier et al., 2008; Petersen et al., 2011; van der
Horst et al., 2015; Verrall, Slavotinek, & Barnes, 2005). One such exercise is the Nordic
hamstring curl which has strong support in the form of two large randomised control trials
(RCTs) (Petersen et al., 2011; van der Horst et al., 2015) along with a number of non-
randomised intervention studies (Arnason et al., 2008; Brooks et al., 2006; Engebretsen,
Myklebust, Holme, Engebretsen, & Bahr, 2008; Gabbe, Branson, & Bennell, 2006; Seagrave
et al., 2014). The combined results from the RCTs show that soccer players who completed a
Nordic hamstring exercise program had a 67.5% reduction in hamstring strains in contrast to
those who did not perform the exercise (Nordic group = 753 players, 25 injuries; Control
group = 768 players, 77 injuries). In addition to the Nordic hamstring exercise, eccentric
hamstring strength training using a flywheel has been shown to decrease subsequent injury in
45
elite Swedish soccer players (Askling, Karlsson & Thorstensson, 2003). This study found that
players in the intervention group had a 3.33 fold reduction in injury risk in comparison to the
control group (Askling et al. 2003). Another successful intervention has reported with the
implementation of a sports specific program in elite Australian Rules football (Verrall et al.,
2005). Verrall and colleagues (2005) found that the implementation of an intervention
program involving stretching whilst fatigued, paddling a football along the ground for five
minutes and placing an emphasis on increasing the amount of high-intensity anaerobic
interval training reduced the two-year incidence of hamstring strains by 70% when compared
to the two-year baseline measurement. Further, a systematic review and meta-analysis
(Goode et al., 2014) aimed at determining the effect of eccentric hamstring strengthening on
the risk of injury suggests that participants who comply with eccentric strength training have
a significant reduction (risk ratio = 0.35 (95% CI 0.23 to 0.55)). Despite these studies proving
to be effective in reducing hamstring strain injuries there seems to be reluctance in adopting
(McCall et al., 2015) or complying with such programs (Bahr, Thorborg, & Ekstrand, 2015),
which is likely a contributor to the 4% rise of hamstring strain injuries in men’s professional
football (Ekstrand et al., 2016). Despite supportive evidence from randomised and non-
randomised intervention studies, some decision makers in elite sport and research (Guex &
Millet, 2013; McCall et al., 2015) refuse to accept some exercises on the basis that they are
“non-functional” given they do not closely mimic the injury mechanism, and therefore
ineffective. However, should an exercise have substantial scientific evidence for its ability to
reduce injury, then from a team success standpoint, it might be worthwhile employing given
greater player availability has been shown to positively affect team performance (Hägglund et
al., 2013).
46
2.7. Hamstring strain injury mechanisms
Numerous mechanisms have been associated with hamstring strain injuries including stretch
manoeuvres in dancing, rapid changes of direction (cutting) and tackling (Askling, Lund,
Saartok, & Thorstensson, 2002; Brooks et al., 2006; Ekstrand & Gillquist, 1983; Verrall,
Slavotinek, Barnes, & Fon, 2003; Woods et al., 2004); however, these injuries primarily
occur during high-speed running (Brooks et al., 2006; Opar et al., 2015; Woods et al., 2004)
and secondarily from kicking (Opar et al., 2015; Timmins, Bourne, et al., 2015). The
mechanism of injury can determine, to some extent, the location of the lesion and tissues
involved, with insults from high-speed running primarily affecting the proximal
musculotendinous junction of the biceps femoris long head (Heiderscheit et al., 2010).
Lesions caused by kicking are thought to more often involve the proximal tendon and/or
muscle-tendinous junction of the semimembranosus (Heiderscheit et al., 2010), although the
total number of magnetic resonance imaging (MRI) confirmed kicking injuries is small.
Whilst each mechanism is distinctively different, the hamstring’s role during both running
and kicking is similar whereby the muscle acts as a brake to decelerate the flexing hip and
extending knee. During this deceleration phase, the hamstrings can experience high levels of
muscular strain (changes in muscle length during motion) and stress (force per unit of cross-
sectional area) (Garrett, Jr., 1990). Whilst the effect of strain on muscle injury has directly
been explored (Garrett et al., 1987), the role of stress has to be assumed based on
observations that suggest hamstring injuries commonly occur during the terminal swing
phase of running (Brooks et al., 2005b; Ekstrand et al., 2011b; Woods et al., 2002), where
stress is high (Schache, Dorn, Blanch, Brown, & Pandy, 2012) and strain is moderate
(Thelen, Chumanov, Best, Swanson, & Heiderscheit, 2005). The question to whether stress or
strain are the primary component for muscle injury appears debateable and there may be an
47
interplay between the two, whereby in the presence of high strain only low-moderate levels
of stress are required and vice-versa (Opar et al., 2012).
2.7.1. Hamstring function during high-speed running
Running gait can be broken down into two major phases, the stance and swing, each further
divided into three sub-categories. The stance phase includes initial contact, mid-stance and
take-off; while the swing phase contains initial swing, mid swing and terminal-swing (Figure
3). It is during the mid and terminal swing phase that the hamstrings are actively lengthening
to brake (decelerate) the concurrent flexing hip and extending knee (Montgomery, Pink, &
Perry, 1994). Increases in running velocity and/or sudden bursts of acceleration require
greater forces produced by the respective agonist muscles which result in a decreased
duration spent in each running phase, in turn requiring the hamstrings to generate more
opposing force at both joints over a shorter period (Agre, 1985).
Figure 3 - Running gait phases. Image adapted from http://www.kintec.net/wp-
content/uploads/2016/02/runninggait-1024x375.png - accessed 25/04/2016.
There is uncertainty as to the exact time-point at which hamstring strain injuries occur.
During the running gait cycle, peak activation of the hamstrings has been observed during
two phases; the early stance, where the hamstrings are actively shortening to extend the hip,
48
and terminal swing, when the hamstrings are actively lengthening to prevent excessive hip
flexion and knee extension (Chumanov, Heiderscheit, & Thelen, 2011; Yu et al., 2008).
Notably, one report also found the hamstrings to contract eccentrically in the late stance
phase (Novacheck, 1998). It is evident that each of the long hamstring muscles reaches its
longest length (~10% greater than that in the anatomical position) during the terminal swing
phase, which therefore exposes them to the highest degrees of potentially injurious strain
during the gait cycle. However, only two separate case studies, each involving serendipitous
motion analysis, have reported the onset of hamstring strain injury to occur during the
terminal swing phase (Heiderscheit et al., 2005; Schache, Wrigley, Baker, & Pandy, 2009).
2.7.2. Hamstring function during kicking
The phases of drop punt kicking can be categorised based on the kicking leg’s backswing,
wind-up and forward swing (Figure 4) (Orchard, McIntosh, & Garlick, 1999). An early study
by Orchard and colleagues (1999) examined the kinematics of the drop punt kick and the
activity of major lower limb muscles involved using surface electromyography. The initial
contraction of the hamstrings is concentric to extend the hip and flex the knee followed by an
eccentric contraction during the follow through phase to decelerate the flexing hip and
extending knee joint (Orchard et al., 1999). Unfortunately, there are limitations that exist
within this study, as surface electromyography does not allow for the complete differentiation
between specific hamstring muscle activities. In a follow-up study, Baczkowski and
colleagues (2006) used functional magnetic resonance imaging, which provides a more
accurate measure of muscle activation, to further investigate the role of the different muscles
involved in kicking (Baczkowski et al., 2006). Interestingly, the findings from this study were
that the semitendinosus displayed the highest relative percentage of hamstring muscle activity
in the kicking leg, followed by biceps femoris and semimembranosus (mean signal intensity
49
change (±SE) = 37.42% (±8.02); 13.27% (±3.46); 5.43% (±2.69), respectively) (Baczkowski
et al., 2006). The aforementioned studies (Baczkowski et al., 2006; Orchard et al., 1999)
provide a foundation for hamstring recruitment patterns as a consequence of kicking, and
despite both suggesting an association between kicking and muscle injury neither study offers
insight as to why injuries occur.
Figure 4 - The phases of ball kicking used in Australian Football. (Adapted from Orchard et al., 1990)
2.8. The relationship between training load and injury
Competing in any sport requires a degree of physical preparedness that can be obtained
through regular training sessions. There are numerous approaches by which coaches progress
athletes in order to reach their peak physical condition, although it is common, in some sports
at least, to apply regular cycles or training blocks over four-week periods, known as
mesocycles (Plisk & Stone, 2003). These cycles typically involve three weeks of
progressively increasing training loads, followed by a single week of reduced loading, which
is thought to safeguard against over training and thereby foster improvements in fitness and
performance over the longer term. Monitoring athlete-training loads may be necessary for
maximising improvements in fitness and for prescribing near-optimal training volumes and
intensities suitable for each individual. Depending on the volume and intensity of the training
program the body will respond either positively (increased physical capacity) or negatively
(with reduced performance or possibly, injury) (Drew & Finch, 2016).
Backswing Wind-up Forward swing
50
The training load-injury risk literature categorises training loads, as either ‘internal’ or
‘external’ (Drew & Finch, 2016). Drew and Finch (2016) define internal workloads as a
measure of effort by the athlete such as rating of perceived exertion or heart rate response to a
stimulus (Drew & Finch, 2016), and external workloads being the quantification of
workloads outside of the athlete, for example running distances covered. A popular method
for measuring internal workloads is the subjective rating of perceived exertion which allows
for the quantification of the athlete’s perception of physiological stress caused by training
(i.e. running loads) by multiplying the athlete’s session rating, on a 10-point scale, by total
minutes of their training session or match (Gallo, Cormack, Gabbett, Williams, & Lorenzen,
2015). The accuracy of measuring external workloads within elite sport has improved with
technological advances including the integration of global position system devices (GPS),
allowing for the recording of running distances covered (Wisbey, Montgomery, Pyne, &
Rattray, 2010). These training loads can then be analysed as either absolute or relative.
Absolute training loads are the sum of all activity performed over a given period, whereas
relative training loads have been expressed as a percentage increase or decrease over a given
period or as a ratio of recent training loads to some measure of an athlete’s historical loads
(e.g. acute to chronic workload ratio) (Hulin, Gabbett, Lawson, Caputi, & Sampson, 2015).
It can been argued that there is a fine line between prescribing the right training load to have
athletes physically prepared for competition whilst also remaining injury free. Early reports
show positive adaptations occur as a consequence of performing greater running distances
(Foster, Daniels, & Yarbrough, 1977) and swimming intensities (Mujika et al., 1995) in
training. However, rapid increases in training and game loads increase the risk of all injuries
in AFL players (Rogalski, Dawson, Heasman & Gabbett, 2013) elite cricketers (Hulin et al.,
2013) and rugby league players (Hulin et al., 2015). Furthermore, GPS-derived data from
51
elite rugby league shows that sharp increases in high-speed running loads increase the
incidence of soft-tissue injuries (Gabbett & Ullah, 2012). Interestingly, despite numerous
studies investigating the effect of GPS-derived running loads on injury risk (Colby, Dawson,
Heasman, Rogalski, & Gabbett, 2014; Ehrmann, 2002; Gabbett & Ullah, 2012; Hulin et al.,
2015; Piggott, 2008) and the high occurrence of hamstring strain injuries across numerous
sports (Opar et al., 2012), there have not been any studies, to date, that have investigated the
effect of running loads on hamstring strain injury.
In conclusion, this review has highlighted the need for further investigation into the risk
factors and mechanisms of hamstring strain injuries. Therefore, as the most common
mechanisms for hamstring strain injury are running and kicking, further research should be
directed towards exploring potential relationships between running distances and risk of
hamstring strains and assessing the effects of kicking on hamstring strength and activation.
Moreover, additional research is required to provide further insight into the effects of
hamstring conditioning on sprint performance and recovery from high-speed running
sessions.
52
Chapter 3: Program of Research
This program of research aimed to: 1) investigate the effect of accumulated training and
match loads on hamstring strain injury risk within elite Australian Rules football players; 2)
assess the effects of drop punt kicking on hamstring strength and muscle activation; and 3)
compare the effects of concentric and eccentric knee flexor conditioning on the recovery of
sprinting performance between consecutive sprint running sessions.
The first study described here had the goal of determining whether GPS-derived running
related variables, such as total distance covered, distances covered at high-speeds (≥24 km.h-
1), acceleration and deceleration distances, and training s-RPE were associated with the
incidence of hamstring strain injury. Previous studies within the AFL (Rogalski, Dawson,
Heasman, & Gabbett, 2013), cricket (Hulin et al., 2013) and rugby league (Hulin et al., 2015)
have associated ‘rapid’ rises in training and game loads with increased ‘injury’ rates with no
attempt to associate load changes specifically with hamstring injury. As a consequence, there
is no evidence linking the common hamstring strain with rapid unaccustomed running loads,
despite the rigorous athlete monitoring systems these AFL teams employ (Ritchie, Hopkins,
Buchheit, Cordy, & Bartle, 2015; Rogalski et al., 2013; Scott, Black, Quinn, & Coutts, 2013).
However, one report has suggested that hamstring injuries may be in some way influenced by
what the player is doing in the weeks leading up to injury (i.e. the number of interchanges of
an individual over a 3-week period) (Orchard, Driscoll, Seward, & Orchard, 2012). The
current general consensus is that hamstring strain injuries are thought to be a ‘sudden onset’
(acute) injury rather than ‘overuse’ (chronic). Therefore, it is of practical importance to
prospectively explore whether a running load-injury relationship can be established for a
53
common hamstring strain. The results of this research will hopefully improve athlete load
monitoring processes in the attempt to reduce such problematic injury rates.
Alterations in neuromuscular function as a consequence of fatigue are of particular interest in
this program of research. Observations in soccer suggest neuromuscular fatigue may increase
the risk of hamstring strain injuries because the majority of insults occur towards the end of
each 45-minute ‘half’ (Woods et al., 2004). As a consequence, previous work has focused on
changes in neuromuscular function resulting from running-induced fatigue (Greig, 2008;
Small et al., 2010). These studies indicate that running results in reductions of eccentric knee
flexor strength with little or no effect on concentric strength (Greig, 2008; Small et al., 2010;
Timmins et al., 2014). Recently, it has also been reported that these declines in strength are
associated with reductions in BF sEMG activity (Timmins et al., 2014). However, despite
hamstring strains occurring during both high-speed running (Askling et al., 2007) and kicking
(Askling et al., 2007), the consequences of kicking-induced fatigue are not understood. These
findings would be of particular interest for sports such as Australian Rules football and soccer
as both involve large volumes of kicking during training sessions and, to a lesser degree,
matches. It may be that neuromuscular fatigue induced by repeated kicking also has
implications for running type injuries. Therefore, it is important to understand the effect of
repeated kicking on neuromuscular function.
Finally, this program of research will investigate the effects of concentric and eccentric knee
flexor strength training on recovery in sprint performance and indices of muscle damage after
an intense and high volume sprint training session. Recently, it has been suggested that
hamstring architectural characteristics (i.e. biceps femoris fascicle length) can influence the
risk of injury in professional Australian soccer players (Timmins, Bourne, et al., 2015).
54
Moreover, it has been found that biceps femoris long head fascicle adaptations can be
induced by performing either concentric-only or eccentric-only muscle contraction resulting
in either shorter or longer fascicle lengths, respectively (Timmins, Ruddy, et al., 2015).
Therefore, as longer fascicles may protect the hamstrings from injury (Timmins, Bourne, et
al., 2015) and from microscopic levels of damage associated with delayed muscle soreness
(Brockett et al., 2004), the primary aim of this study was to assess whether eccentric and
concentric strength training have different effects on the recovery from intense sprinting.
Research questions
This program of research was designed to answer the following research questions:
1. Are GPS-derived measures of total running volume, the volume of high-speed running
and the volume of acceleration and deceleration or athlete perceptions of workload
associated with the risk of hamstring strain injuries in elite Australian Rules football
players?
2. What is the effect of repeated maximal effort drop-punt kicking on knee flexor strength
and hamstring muscle activation?
3. How does concentric and eccentric knee flexor conditioning influence the ability to
recover from an intense sprint running session?
55
Chapter 4: Study 1 – The effect of high-speed
running on hamstring strain injury risk
Publication statement
This chapter is comprised of the following paper published in the British Journal of Sports
Medicine (Impact factor 6.72; Q1):
Duhig, SJ., Shield, AJ., Opar, DA., Gabbett, TJ., Ferguson, C. &. Williams, M. (2016). The
effect of high-speed running on hamstring strain injury risk. British Journal of Sports
Medicine, 50(24), 1436-1540. doi: 10.1136/bjsports-2015-095679.
56
Statement of Contribution of Co-Authors for Thesis by Published Paper
The authors listed below have certified* that:
1. They meet the criteria for authorship in that they have participated in the conception,
execution, or interpretation, of at least that part of the publication in their field of
expertise;
2. They take public responsibility for their part of the publication, except for the
responsible author who accepts overall responsibility for the publication;
3. There are no other authors of the publication according to these criteria;
4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor
or publisher of journals or other publications, and (c) the head of the responsible
academic unit
5. They agree to the use of the publication in the student’s thesis and its publication on
the Australasian Research Online database consistent with any limitations set by
publisher requirements.
Contributor
Statement of contribution*
Steven Duhig
Experimental design, ethical approval, data collection and analysis,
statistical analysis, manuscript preparation
Anthony Shield
Supervised the application for ethics approval, statistical analysis,
experimental design and manuscript preparation
David Opar Aided in experimental design and manuscript preparation
Tim Gabbett Aided in manuscript preparation
Cameron Ferguson Aided in data collection and manuscript preparation
Morgan Williams Assisted with statistical analysis and manuscript preparation
Principal Supervisor Confirmation
I have sighted email from all co-authors confirming their certifying authorship.
Dr Anthony Shield ____________________ ______________
57
4.1. Abstract
Background: Hamstring strain injuries are common within the Australian Football League
with most occurring during high-speed running. Therefore, this study investigated possible
relationships between mean session running distances, session ratings of perceived exertion
(s-RPE) and HSIs within AFL footballers. Methods: Global positioning systems (GPS)
derived running distances and s-RPE for all matches and training sessions over two AFL
seasons were obtained from one AFL team. All hamstring strain injuries were documented
and each player’s running distances and s-RPE were standardised to their 2-yearly session
average, then compared between injured and uninjured players in the four weeks (week -1, -
2, -3, -4) preceding each injury. Results: Higher than ‘typical’ (i.e., Z = 0) HSR session
means were associated with a greater likelihood of hamstring strain injury (week -1 OR =
6.44, 95%CI = 2.99 to 14.41; p<0.001; summed -1 and -2 OR = 3.06, 95%CI = 2.03 – 4.75,
p<0.001; summed -1, -2 and -3 OR = 2.22, 95%CI = 1.66 – 3.04, p<0.001; and summed -1, -
2, -3 and -4 OR = 1.96, 95%CI = 1.54 - 2.51, p<0.001). Increasing AFL experience was
associated with a decreased hamstring strain injury risk (OR = 0.77; 95%CI = 0.57 – 0.97;
p=0.02). Furthermore, high-speed running data modelling indicated that reducing mean
distances every fourth week may decrease the probability of hamstring strain injury.
Conclusion: Exposing players to large and rapid increases in high-speed running distances
above their 2-yearly session average increased the odds of hamstring strain injury. However,
reducing high-speed running distances every four weeks may offset hamstring strain injury
risk.
58
4.2. Introduction
Australian Rules football (ARF) is a challenging contact sport requiring high levels of fitness
and skill. Within Australia, the elite level of ARF is the Australian Football League (AFL).
Each AFL season spans November to September, during which teams complete a preseason
(preparation) phase followed by 22 weekly games, and possibly finals. In the last two
decades, hamstring strain injuries have remained an ongoing problematic issue, constituting a
large proportion of soft tissue injuries sustained in the AFL (Orchard et al., 2013). The
predominant injury mechanism for hamstring strain injuries is sprinting (Opar et al., 2015),
and fatigue may play a role because higher injury rates have been reported during the latter
stages of soccer and rugby matches (Brooks et al., 2006; Woods et al., 2004).
On average, an AFL game lasts 100:01 ± 14:22 min during which players cover a distance of
12.2 ±1.9 km, reach maximum velocities of 30.1 ± 6.7 km h-1
and perform numerous
accelerations (246 ± 47 (>4 kmh-1
in 1 s)) and decelerations (14 ± 5 (over 10 km h-1
in 1 s))
(Wisbey et al., 2010). Unsurprisingly, teams within the AFL implement rigorous monitoring
systems to carefully observe training and competition loads (Ritchie et al., 2015; Rogalski et
al., 2013; Scott et al., 2013), allowing for appropriate programming to ensure optimal
performance (Morton, Fitz-Clarke, & Banister, 1990) and a reduced injury risk (Rogalski et
al., 2013). Two popular monitoring methods include 1) objective running loads collected via
GPS devices (Wisbey et al., 2010), and 2) subjective ratings of perceived exertion, which
together allow for the quantification of physiological stress caused by the application of
external loads (e.g. running loads) (Gallo et al., 2015) and the estimation of injury risk
(Rogalski et al., 2013).
59
Previous studies have found that rapid increases in training and game loads increase the risk
of injuries in AFL footballers (Rogalski et al., 2013), elite cricketers (Hulin et al., 2013) and
rugby league players (Hulin et al., 2015). Furthermore, GPS-derived data from elite rugby
league demonstrates that greater volumes of high-speed running result in more soft-tissue
injuries (Gabbett & Ullah, 2012). Additionally, regular interchanges made during AFL
matches have been suggested to protect players against hamstring strain injuries but increase
the risk for opposition players (Orchard et al., 2012).
The predominant injury mechanism for hamstring strain injuries is high-speed running (Opar
et al., 2015), however, no studies have explored the effect of high-speed running distances on
the risk of hamstring injury. Therefore, the aim of this study was to determine whether
running distances and s-RPE were associated with an increased risk of hamstring strain injury
in elite AFL players. We hypothesised that rapid and large increases in high-speed running
distances over four weeks might influence hamstring strain injury risk.
4.3. Methods
Study Design
This study employed an observational prospective cohort design and was completed over 102
weeks spanning the 2013 and 2014 AFL and the concurrent ‘reserves’ competition (North
East Australian Football League) seasons (Nov 2012 – Aug 2013 and Nov 2013 – Aug 2014).
All participants had their running distances collected via GPS devices (V4 Catapult, South
Melbourne, Australia) and s-RPE collected via SMARTABASE (Fusion sport, Brisbane,
Australia).
60
Participants
Fifty-one elite male footballers (age = 22.2 ± 3.4 y, height = 188.2 ± 7.1 cm, mass = 86.6 ±
8.7 kg with a median of 4 y (range 1-12 y) of AFL playing experience from a single AFL
team were recruited for this study. The university’s human research ethics committee
approved the study and participants gave informed written consent.
GPS and s-RPE Data Collection
GPS measures of athlete movements have previously been reported to be reasonably accurate
and reliable (Castellano, Casamichana, Calleja-González, San Román, & Ostojic, 2011;
Varley, Fairweather, & Aughey, 2012). Each player was fitted with a 10 Hz GPS unit (V4
Catapult, South Melbourne, Australia) contained within their guernsey or undergarment on
the upper back during all running sessions and games throughout the two season
observational period. Uploaded data containing ‘signal drop-out’ errors or players not
involved in the football drills were removed.
SMARTABASE (Fusion Sport, Brisbane, Australia) is a software platform that allows
players to enter their subjective judgments of a training session or match load (a product of
rating of perceived exertion and duration (min)). This measurement is used in the attempt to
assess how the athletes are coping with training loads and previous work has demonstrated
moderate to very large associations between s-RPE and both high-speed running (r = 0.51)
and total distance covered (r = 0.88) (Gallo et al., 2015). Players were required to report
RPE’s within 5 hours of training sessions and 4-6 hours of matches.
61
Hamstring Strain Injury
A hamstring strain injury was defined as acute pain in the posterior thigh that caused
immediate cessation of exercise (Opar et al., 2015). Damage to the muscle and or tendon was
later confirmed by the club’s physiotherapist via clinical assessment or magnetic resonance
imaging examination. All reports were forwarded to the investigators at the conclusion of the
competitive season.
Data Analysis
Once GPS and s-RPE data were entered in a spreadsheet, all match and training sessions were
analysed (number of files = 11457; median, minimum and maximum files collected per
player = 246, 79 and 302, respectively). Playing experience was defined as the time spent
within the AFL system and was included to assess its effect on hamstring strain injury risk.
The derived variables included: the session ratings of perceived exertion, total distance
travelled (km), high-speed running distance (≥24 km h-1
) and distance (m) covered whilst
accelerating (>3m/s/s) and decelerating (<-3m/s/s). For each variable, players had their
weekly session totals summed across the two years. A two-yearly session mean and the
session mean for each of the four weeks (week -1, week -2, week -3 and week -4) leading up
to each injury was also calculated. The four weeks preceding each injury was chosen for three
reasons: (1) hamstring strain injuries occurred randomly throughout the season without any
apparent relationship to absolute running distance, (2) four weeks is generally accepted as an
appropriate mesocycle length (Plisk & Stone, 2003), and (3) previous findings have used this
time period to estimate injury risk (Gabbett & Ullah, 2012; Hulin et al., 2013). To standardise
the variables for each player, high-speed running distances were log transformed and z-scores
calculated using the following formula:
62
Z = (VARWSM – VAR2YSM)/ VAR2YSSD
where VARWSM is a variable’s weekly session mean, for each of the four weeks preceding
each hamstring strain injury, and VAR2YSM and VAR2YSSD represent the variable’s session
mean and standard deviation across the two years, respectively. Standardized scores of zero
then represented a ‘typical’ week for a particular player while positive and negative scores
indicated heavier or lighter than typical training loads respectively. Injured players were
those who sustained a hamstring strain injury at any stage in the two years including the pre-
season training and in-season periods. No players had a current hamstring strain injury at the
start of data collection (November 2012).
Statistical Analysis
All statistical analyses were performed using JMP 10.02 (SAS Institute, Inc., Cary, NC).
Independent t-tests were used to compare total high-speed running distance performed in
each season between injured (INJ) and uninjured (UNINJ) groups. Paired t-tests were used to
compare the high-speed running distances between the first and second season.
Variables for which the 95% confidence intervals (CI) fell below zero in any of the four week
‘blocks’ prior to injury were removed from further analysis. Standardised mean high-speed
running session distance was the only variable for which the 95%CI remained above zero.
Independent sample t-tests were used to compare four-week mean high-speed running
distances between injured and uninjured players in each of the four-week blocks prior to
every hamstring strain injury. Once it was established that the injured group were performing
greater standardized mean high-speed running session distances, two models were produced
63
to assess the likelihood of hamstring strain injury. The first model examined week -1, the sum
of weeks -1 and -2, the sum of weeks -1, -2 and -3, and the sum of weeks -1, -2, -3 and -4.
The second model examined the risk of injury associated with various combinations of load
in weeks -2 to -4 and week -1 prior to injury. Age has previously been reported to be a risk
factor for hamstring strain injury (Opar et al., 2015; Orchard, 2001). Therefore, we assessed
whether a relationship between playing experience and injury existed. This variable was
added to both models. Z scores were reported as means with 95% confidence intervals.
At each injury time-point, Z-scores for the preceding four weeks were calculated for all
players and independent sample t-tests used to compare mean session distances between
injured and uninjured players. Logistic regression was employed to determine the odds ratio
(OR) of injury with increasing or decreasing standardized mean high-speed running session
distances, in the four weeks leading up to injury. Additionally, the effect of standardized
mean high-speed running session distance changes in the week prior to injury on hamstring
strain injury risk were modelled. Two injuries were excluded from analysis due to missing
GPS data. Statistical significance was set at P<0.05.
4.4. Results
Hamstring strain injury incidence and distances covered
Twenty-two hamstring strain injuries were sustained across the 2013 (n=11) and 2014 (n=11)
seasons, all of which occurred after the first 13-weeks of each preseason. Two injuries were
excluded from analysis due to incomplete data. As previously reported (Murphy, Connolly, &
Beynnon, 2003), the majority of hamstring strain injuries were sustained during match-play
(14 out of 20) rather than training. On average, players covered a total distance of 807 ± 95
64
km in the 2013 season and 775.3 ± 166 km in the 2014 season, of which 22.6 ± 8 km and
15.5 ± 5 km were at high-speed (>24km h-1
). No significant differences were found in total
absolute high-speed running distances between the injured and uninjured groups in 2013 (INJ
mean = 22.1 ± 5 km; 95% CI = 16 – 28 km; and UNINJ mean = 22.6 ± 9 km; 95% CI = 20 –
25 km; p = 0.90) or 2014 (INJ mean = 16.6 ± 4 km; 95% CI = 14 – 19 km; range = 13 – 23
km; and UNINJ mean = 15.2 ± 6 km; 95% CI = 13 – 17 km; range = 2 – 30 km; p=0.49).
Furthermore, despite a significant reduction in the absolute distance of high-speed running
between the two seasons (p<0.01), there was no decrease in injury rates. Players with greater
than four years playing experience did not sustain hamstring injury: INJ (median = 4, range =
1 – 4 y) compared to UNINJ (median = 4, range = 1 – 12 y).
Relationships between running distances and hamstring strain injuries
Due to the 95% CIs falling below “0” in both the INJ and UNINJ in the four weeks leading
up to injury, session ratings of perceived exertion, total distance covered, acceleration and
deceleration distances (Figure 5) were excluded from further analysis. However, standardized
high-speed running distances were higher in the INJ than the UNINJ (Figure 5).
65
Figure 5 - Standardized weekly session loads (y-axis) for each of the four weeks prior to each injury (x-axis)
are shown from top to bottom: deceleration (DEC), acceleration (ACC), total distance covered (DIST), session
ratings of perceived exertion (s-RPE) and high-speed running (HSR). Dashed and solid lines represent injured
and uninjured groups, respectively. Errors bars represent 95% CI.
The average summed four week standardised high-speed running distances for INJ and
UNINJ were; z = 2.36±2.76 and z = -0.05±1.63, respectively (p<0.001). Using logistic
regression, the likelihood of hamstring strain injuries increased (OR = 1.96, 95%CI = 1.54 -
2.51, p<0.001) with greater relative high-speed running distances in the four weeks prior to
injury (Figure 6). The largest effect of high-speed running distance on injury risk was
observed in the week prior to injury (OR = 6.44, 95%CI = 2.99 to 14.41; p<0.001), followed
by the sum of weeks -1 and -2 (OR = 3.06, 95%CI = 2.03 – 4.75, p<0.001) and the sum of
weeks -1, -2, and -3 (OR = 2.22, 95%CI = 1.66 – 3.04, p<0.001). When added to the model,
greater playing experience was associated with a reduced likelihood of injury risk (OR =
0.77, 95%CI = 0.57 – 0.97, p=0.021) without confounding the effect of standardized high-
speed running distance (OR = 1.91, 95%CI = 1.51 – 2.47, p=0.022; p<0.001).
66
Figure 6 - The influence of summed four-week standardized mean high-speed running (HSR) session distances
on the probability of hamstring strain injury (HSI). Average high-speed running mean session distance
corresponds to zero on the x-axis.
Figure 7 shows the impact of the final week of the four-week mesocycle on the probability of
hamstring strain injury. Here the association between the summed high-speed running session
distances in weeks -4, -3 and -2 (OR = 1.73, 95% CI = 1.24 - 2.39, p = 0.001) and the week
preceding injury (OR = 3.02, 95% CI = 1.36 - 7.26, p = 0.006) was tested and the resultant
probability of hamstring strain injury determined. According to this model, the probability of
hamstring strain injuries was decreased with reduced standardised high-speed running
distances in week -1. When experience was added to this model, a similar protective effect
was observed (OR = 0.78, 95% CI = 0.57 – 0.96, p = 0.022) and there was no evidence to
suggest it confounded the other variables (summed high-speed running session distances in
weeks -4, -3 and -2 OR = 1.70, 95% CI = 1.22 - 2.36, p = 0.002, and the week preceding
injury OR = 2.98, 95% CI = 1.33 - 7.27, p = 0.007).
67
Figure 7 - Modeling of the impact of standardized mean high-speed running session
distances in the four weeks prior to a hamstring injury. Injury risk is influenced by mean
high-speed running session distances in weeks -4 to -2 (as shown on the x-axis) and in week -
1 (as shown by the curves). The probability of hamstring strain injury (HSI) can be
influenced by modifying either or both summed mean high-speed running session distances of
week 2, 3 and 4 prior to injury (x-axis) and the week prior to injury. The 2-yearly average
high-speed running session distance is represented by 0 on the x-axis. Each curve represents
the standardized mean high-speed running session distance covered in the first week (week -
1) prior to injury; top curve = high to very high (0.94 to 1.82), second curve = moderate to
high (0.08 to 0.94), third curve = low to moderate (-0.82 to 0.08), fourth curve = very low to
low (-1.71 to -0.82) and bottom curve = extremely low to very-low (-2.62 to -1.71), z-score
thresholds within brackets.
4.5. Discussion
This study is the first to investigate relationships between athlete running distances and
hamstring strain injuries. Players who performed significantly more than their two-yearly
average amount of high-speed running (>24 km h-1
) in the four weeks prior to injury had a
greater risk of hamstring strain injury than players who did not. In contrast, hamstring strain
68
injury risk was not influenced by the player’s s-RPE, total distance covered, absolute amount
of high-speed running or by the total distances covered while accelerating or decelerating.
Acute high-speed running loads during week -1 appears to have had a greater impact on
injury risk compared to chronic loads (the sum of -2, -3 and -4). These findings demonstrate
that transiently elevated high-speed running distances increase the likelihood of hamstring
strain injury. A secondary finding was that an increase in playing experience resulted in a
small protective benefit against hamstring strain injury.
Previous studies have reported relationships between suddenly elevated training loads and all
forms of injury (Colby et al., 2014; Gabbett & Ullah, 2012; Rogalski et al., 2013). The results
from this study add to the training-injury literature (Anderson, Triplett-McBride, Foster,
Doberstein, & Brice, 2003; CARL Foster, 1998; Gabbett, 2003; Gabbett, 2004; Gabbett &
Jenkins, 2011; Hulin et al., 2013; Morton et al., 1990; Rogalski et al., 2013) by reaffirming
the injury risk associated with high-speed running (Gabbett & Ullah, 2012). The current
model has been based on performance (Morton et al., 1990) and injury risk models (Hulin et
al., 2013). These models are based on the premise that training load has both positive and
negative influences, with higher chronic loads (i.e. 4-weeks) associated with better fitness
(Morton et al., 1990) and higher acute (i.e. 1-week) loads associated with a greater risk of
injury (Hulin et al., 2013). Moreover, previous investigations suggest that fitness levels
increase when chronic load exceeds acute load (Morton et al., 1990) and injury risk increases
when acute load significantly exceeds chronic load (Hulin et al., 2013; Hulin et al., 2015).
Our major finding was similar to these previous observations, whereby players exposed to
large and rapid increases in high-speed running distances above their 2-yearly average were
more likely to sustain a hamstring strain injury than players who were not. It was beyond the
scope of this descriptive study to determine the optimal load monitoring time period to
69
estimate future risk of a hamstring injury, however, a trend was observed for briefer time
frames (1-week) to have a greater effect on injury risk than longer time frames (2-4 weeks).
From a training-performance perspective, careful consideration should be given when
interpreting and applying the current findings to the high-performance sports setting. In
alignment with earlier reports showing a positive relationship between greater training
distance (Foster et al., 1977) and intensity (Mujika et al., 1995) with improved performance,
Gabbett and Ullah (2012) suggest a fine balance exists between training load restriction, to
prevent injury, and increasing training loads to physically prepare players for competition.
Therefore, taking into account the need for an appropriate stimulus to improve performance,
we used the current data to produce a model, based on a common mesocycle period of four
weeks (Plisk & Stone, 2003). Our model suggests that players will be exposed to greater risk
of hamstring strain injury when high-speed running distances extend beyond a player’s
typical load either acutely or chronically. Planned decreased mean high-speed running
session distances in the fourth week of each mesocycle may offer the ‘balance’ between
reducing injury risk and improving performance (Gabbett & Ullah, 2012). As such, the
execution of three weeks of relatively large volumes of high-speed running followed by a
recovery week, where less distance is covered, may allow the application of overload while
also reducing the risk of hamstring strain injury. Therefore, the current findings provide some
support for monitoring player’s high-speed running and periodising training loads as a means
of reducing hamstring strain injury risk while maintaining a desired chronic load for
improving performance (Fry, Morton, & Keast, 1992; Graham, 2002; Stone et al., 1999). It is
noteworthy to consider that whilst the current model(s) suggest particular time periods can
estimate hamstring strain injury risk, other soft-tissue injuries may be susceptible to different
loading cycles, occurring more rapidly or slowly with changes in training volume.
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Finally, there is evidence to support the association between advanced age and hamstring
strain injury in some (Opar et al., 2015; Orchard, 2001) but not all studies (Bourne et al.,
2015). A survey from the football departments of AFL clubs has revealed a belief amongst
some conditioning staff, that younger and older players have an elevated risk of hamstring
strain injury. The rationale behind this belief was that younger players were unable to tolerate
training loads and older players are unable to sufficiently recover between training sessions
and matches (Pizzari et al., 2010). Interestingly, the current findings show a small protective
benefit against hamstring strain injury with increasing playing experience. However, when
interpreting these findings it is important to consider the fact that the sample only included
one AFL team and the practices performed by this club may differ significantly from other
clubs. Whilst purely speculative, it may be that more experienced players are more robust
having survived the early years of an AFL career and can manage their themselves and their
workloads better or are monitored more closely than less experienced teammates.
In summary, this study highlighted the influence high-speed running distance has on
hamstring strain injury risk in elite AFL players. These results demonstrated the increasing
likelihood of injury when athletes performed more high-speed running than that to which
they were accustomed across a four-week period. Therefore, gradual increases in each
individual’s standardized mean high-speed running session distance should be prescribed
over a period of time, thereby ensuring players have required fitness levels for competition
with a reduced risk of injury. Future work exploring the impact of periodic reductions in
mean high-speed running session distance on hamstring strain injury risk is warranted.
71
Chapter 5: Study 2 – Drop punt kicking induces
eccentric knee flexor weakness associated with
reductions in hamstring electromyographic
activity
Publication statement
This chapter is comprised of the following paper published in the Journal of Science and
Medicine in Sport (Impact factor 3.75; Q1):
Duhig, S. J., Williams, M. D., Minett, G. M., Opar, D. A., & Shield, A. J. (2017) Drop punt
kicking induces eccentric knee flexor weakness associated with reductions in
electromyographic activity. Journal of Science and Medicine in Sport, 20(6), 595-599.
72
Statement of Contribution of Co-Authors for Thesis by Published Paper
The authors listed below have certified* that:
1. They meet the criteria for authorship in that they have participated in the conception,
execution, or interpretation, of at least that part of the publication in their field of
expertise;
2. They take public responsibility for their part of the publication, except for the
responsible author who accepts overall responsibility for the publication;
3. There are no other authors of the publication according to these criteria;
4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor
or publisher of journals or other publications, and (c) the head of the responsible
academic unit
5. They agree to the use of the publication in the student’s thesis and its publication on
the Australasian Research Online database consistent with any limitations set by
publisher requirements.
Contributor
Statement of contribution*
Steven Duhig
Experimental design, ethical approval, data collection and
analysis, statistical analysis, manuscript preparation
Geoffrey Minett Assisted with manuscript preparation
Morgan Williams Assisted with statistical analysis and manuscript preparation
David Opar Assisted with manuscript preparation
Anthony Shield Supervised the application for ethics approval, experimental
design, statistical analysis and manuscript preparation
Principal Supervisor Confirmation
I have sighted email from all co-authors confirming their certifying authorship.
Dr Anthony Shield ____________________ ______________
73
5.1. Linking Paragraph
The study in Chapter 3 demonstrated the increased risk of hamstring strain injury when elite
Australian footballers were exposed to rapid increases of high-speed running across four
weeks that was above their 2-yearly average. The findings suggest the possibility that
hamstring strains may not only be acute incidents as once thought. Whilst hamstring strains
generally occur during high-speed running they are also sustained during kicking (Opar et al.,
2015; Timmins, Bourne, et al., 2015). However, there is a dearth of information related to the
hamstring strains that result from kicking, the second most common mechanism for this
injury. Kicking related hamstring strains also appear to be more severe than those caused by
running and the effects of kicking on hamstring fatigue warrants further investigation
(Askling, Saartok, & Thorstensson, 2006).
Study 1 is presented as published in the British Journal of Sports Medicine (volume 50, issue
24, pages 1436-1540). The following comments are made in response to reviewers’
comments:
1) Rapid and large are defined as: happening in a short time period, and relatively great size,
respectively.
2) There is a simulation of the data provided in the study that suggests reducing the amount
of mean high speed running distances may decrease injury risk.
3) I would like to state that no amount of monitoring or programming can ensure either
optimal performance or a reduced risk of injury as written on page 59, paragraph two,
sentence two.
4) With reference to the 10Hz GPS device used, inter-device reliability yielded a coefficient
of variation of 1.3% and 0.7% for 15m and 30m (Castellano et al. 2011). The validity has
been shown to range from 3.1% to 8.3% depending on starting velocity (Varley et al. 2012).
74
5) The scale used to assess RPE was the modified Borg scale (Foster et al. 2001).
6) The definition for the file: the data collected from each unit on all players for every session
(match or training).
7) The INJ and UNINJ groups are defined as players who injured their hamstring or those
who remained uninjured, respectively. The time (weeks) which were analysed related to
when an injury occurred and then a comparison made between the injured player and all of
those who remained uninjured.
8) Playing experience was chosen, as we were interested in the effect of exposure to training
and match loads at the elite level (AFL).
9) The units for Figure 5, 6 and 7 y-axes are Z scores.
10) The data presented in Figure 6 is based on the INJ group.
75
5.2. Abstract
Objectives: To examine the effect of 100 drop punt kicks on isokinetic knee flexor strength
and surface electromyographic (sEMG) activity of bicep femoris (BF) and medial hamstrings
(MH). Design: Randomised control study. Methods: Thirty-six recreational footballers were
randomly assigned to kicking or control groups. Dynamometry was conducted immediately
before and after the kicking or 10 minutes of sitting (control). Results: Eccentric strength
declined more in the kicking than the control group (p < 0.001; d = 1.60), with greater
reductions in eccentric than concentric strength after kicking (p = 0.001; d = 0.92). No
significant between group differences in concentric strength change were observed (p =
0.089; d = 0.60). The decline in normalized eccentric hamstring sEMG (BF and MH
combined) was greater in the kicking than the control group (p < 0.001; d =1.78), while
changes in concentric hamstring sEMG did not differ between groups (p = 0.863; d = 0.04).
Post-kicking reductions in sEMG were greater in eccentric than concentric actions for both
BF (p = 0.008; d = 0.77) and MH (p < 0.001; d = 1.11). In contrast, the control group
exhibited smaller reductions in eccentric than concentric hamstring sEMG for BF (p = 0.026;
d = 0.64) and MH (p = 0.032; d = 0.53). Reductions in BF sEMG were correlated with
eccentric strength decline (R = 0.645; p = 0.007). Conclusions: Reductions in knee flexor
strength and hamstring sEMG are largely limited to eccentric contractions and this should be
considered when planning training loads in Australian Football.
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5.3. Introduction
Australian Rules football is a popular contact sport which demands that players possess ball-
handling skills, along with high aerobic and anaerobic fitness capacities (Young et al., 2005).
Other than running with the ball, there are two methods by which players can move the ball
around the field; the hand-ball and the drop punt kick. This kick is responsible for a small
proportion of hamstring strain injuries (Opar et al., 2015; Timmins, Bourne, et al., 2015)
which represents the most common cause of lost playing time in the game (Orchard et al.,
2013).
Hamstring strain injuries commonly occur from two mechanisms, high-speed running
(Askling et al., 2007) and stretching at extreme joint positions, which are comparable to those
adopted during the follow through of drop punt kicking (Askling et al., 2007). In both
running and kicking the hamstrings essentially act as brakes to decelerate the flexing hip and
extending the knee. During this action, these muscles experience high-force eccentric
contractions coupled with moderate to high muscle strains (Garlick, Orchard, Walt, &
McIntosh, 1998), the combination of which is likely to cause injury (Schache, Dorn, Blanch,
Brown, & Pandy, 2011). In contrast to high-speed running injuries, which most often affect
the long head of biceps femoris (Askling et al., 2007), injuries consequent to kicking
generally result in greater time-loss, and smaller strength deficits (Askling et al., 2006). Both
injuries, therefore, create a substantial financial burden (Hickey et al., 2013) and have a
negative impact on team performance.
Neuromuscular fatigue is also thought to play a role in hamstring injury with studies of
soccer showing that a majority of such incidents occur towards the end of each 45 minute
‘half’ (Woods et al., 2004). This may be partly explained by significant reductions in
77
eccentric knee flexor strength and biceps femoris muscle activation (Timmins et al., 2014)
that occur as a result of running-induced fatigue. However, it is unknown whether similar
changes occur as a consequence of kicking, which has been reported to involve significantly
greater activity of the semitendinosus than the biceps femoris or semimembranosus muscles
(Baczkowski et al., 2006).
Given the vital role of the drop punt kick in Australian Football and its association with
hamstring strain injury, it is important to further investigate the effect of kicking on
hamstring strength and neuromuscular function. Therefore, the aim of this study was to
determine if reductions in knee flexor strength and hamstring sEMG activity occurred
following repetitive high force kicking. We hypothesised that: 1) kicking would result in a
contraction mode specific decline in eccentric knee flexor strength; and 2) this strength
decline would be associated with a reduction in medial hamstring sEMG.
5.4. Methods
Thirty-six recreational level male footballers (age = 22.2 ± 3.1 years, height = 184.0 ± 6.8
cm, mass = 82.6 ± 6.4 kg) and football playing experience = 12.6 ± 4.7 years) were recruited
for this randomised controlled study. None of the participants had a history of hamstring
strain injury in the past 36 months. Each player was informed of the risks and benefits of the
investigation prior to providing written consent to participate and was then randomly
assigned to either a kicking group (K) (n=18) or a control group that performed no kicking
(NK) (n=18). Each participant was instructed not participate in physical activity more intense
than activities of daily living for twenty-four hours prior to testing. The kicking protocol
consisted of 100 drop punt kicks, as employed previously by others (Baczkowski et al.,
2006), all tests were conducted between 10am and 3pm, with an even distribution of
78
experimental and control participants in morning and afternoon sessions. Each participant
completed a standardised 5 min warm-up that consisted of lower body dynamic stretches and
ten kicks of progressively increasing intensity. Participants were then instructed to kick an
Australian Rules football (SHERRIN, Melbourne, Australia) as quickly and with as much
force as possible into a nylon net (Baczkowski et al., 2006). An assistant collected the balls
and passed them to the kicker as quickly as possible to ensure minimal rest between
repetitions (<5sec) and prevent unwanted muscle activity when retrieving balls from the
floor. Those in the control group were seated for 10 min, which is the approximate time
required to complete the kicking protocol. Ethical approval for this study was granted by the
university’s Human Research Ethics Committee.
Prior to and after completing the kicking or ten minutes of sitting, participant’s knee flexor
strength on their dominant (preferred kicking) limb was assessed via isokinetic dynamometry
(Biodex® System 3, Shirley, USA). All had been familiarized with the dynamometer tests 7
– 9 days before formal testing. Participants were seated on the dynamometer with a hip angle
of approximately 85° from full extension and were restrained by straps around the tested
thigh, waist, and chest to minimise compensatory movements (Timmins et al., 2014). All
seating variables (e.g., seat height, pad position) were recorded to ensure the replication of
positions. Gravity correction for limb weight was also conducted and range of motion (ROM)
was set between 5° and 90° of knee flexion. The warm-up prior to the initial strength test
involved three sets of four concentric knee extension and flexion contractions at an angular
velocity of 240°.s
-1, with progressively increasing intensities of effort (Timmins et al., 2014).
The test protocol began one minute following the final warm-up set and as soon as possible
after completion of kicking (~2.5 minutes) and consisted of one set of three concentric and
eccentric maximum voluntary contractions (MVCs) of the knee flexors at 180°.s
-1 with a
79
between-set rest period of 60 s. The testing speed was chosen based on previous studies that
have investigated the effect of fatigue on knee flexor strength (Timmins et al., 2014). The
investigators loudly exhorted participants to exert maximal effort during all contractions. The
testing order of contraction modes was randomised and counterbalanced across the
participant pool.
We have previously examined the test-retest reliability of this isokinetic protocol. Intraclass
correlations (ICCs) and typical error as a coefficient of variation (%TE) for peak knee flexor
torque under both concentric 180°.s
-1 (ICC = 0.93; TE% = 4.5%) and eccentric 180°
.s
-1 (ICC
= 0.82; TE% = 6.0%) conditions were acquired (Timmins et al., 2014). These results are
comparable to previously published data for concentric contractions (ICC = 0.97 and 0.96)
(Tsiros, Grimshaw, Shield, & Buckley, 2011) and eccentric contractions (ICC = 0.83) (Li,
Wu, Maffulli, Chan, & Chan, 1996).
Neonatal ECG electrodes (10 mm in diameter, 15 mm interelectrode distance) (Ambu®,
Ballerup, Denmark) were used to record medial hamstring and biceps femoris surface
electromyographic (sEMG) activity (Timmins et al., 2014). Following the initial skin
preparation, electrodes were positioned on the posterior thigh half way between the ischial
tuberosity and tibial epicondyles with electrodes oriented parallel to the line between these
two landmarks. The reference electrode was placed on the head of the ipsilateral fibula.
Muscle bellies of the medial and lateral portions of the hamstrings were identified via
palpation during forceful isometric knee flexion, and correct electrode placement confirmed
by observing sEMG activity during active internal and external rotation of the flexed knee,
respectively. Electrodes were then covered by rectangular sections (20cm x 15cm) using
fixomull stretch tape (BSN medical, Luxembourg City, Luxembourg) to ensure adhesion
80
during the kicking protocol. To minimise sEMG movement artefact that may arise from the
dynamometer chair, participants sat on a custom-made foam pad with cut-outs under the sites
of electrode placement.
Dynamometer torque and lever position data and sEMG signals were transferred to a
computer at 1000Hz via a 16-bit PowerLab 26T AD recording unit (ADInstruments, Bella
Vista, Australia) (amplification = 1000; common mode rejection ratio = 110 dB). sEMG data
were filtered using a Bessel filter with a frequency bandwidth of 10–500 Hz and then
smoothed and rectified over 100ms moving windows (Timmins et al., 2014). For each
contraction mode, mean knee flexor torque and sEMG data were taken between the knee
angles of 15° and 35° (full knee extension = 0°). Pilot work in this laboratory has shown that
hamstring sEMG is most sensitive to the effects of fatiguing exercise at these angles. Surface
EMG was averaged, for each contraction mode and at each time point, across the three
contractions. Average eccentric sEMG from before kicking and average concentric and
eccentric sEMG from post-kicking were normalized to the average sEMG signal obtained
during the three concentric knee flexion efforts obtained in the pre-test. All testing was
conducted by a single investigator (SD).
Statistical analysis
Levene’s tests for homogeneity were used to assess the equality of variances for each variable
(p>0.05). A Shapiro-Wilks test was conducted to assess normality. Knee flexor strength
changes were analysed via a contraction mode (eccentric & concentric) by group (K & NK)
repeated measures ANOVA. Changes in normalised sEMG were analysed via a contraction
mode (eccentric & concentric) by group (K & NK) by muscles (BF & MH) repeated
measures ANOVA. When interactions were significant, post-hoc pairwise comparisons were
81
made with Bonferroni corrections for multiple comparisons. Pearson’s correlation was used
to assess the strength of the relationship between torque and sEMG changes. Statistical
significance was set at P ≤ 0.05 and Cohen d effect sizes calculated using the following
thresholds; trivial < 0.20, small = 0.20-0.49, medium = 0.50-0.79 and large > 0.80 (Cohen,
2013). All statistical analyses were carried out using SPSS (version 21; SPSS Inc., Chicago,
USA).
5.5. Results
The observed power for strength change (contraction mode x group) was 0.95. The observed
power for normalized sEMG (contraction mode x group) was 0.99.
The kicking and control groups did not differ in terms of age (K = 22.0 ± 3.5; NK = 22.3 ±
2.5 years, p = 0.71), height (K = 183.9 ± 6.9; NK = 184.0 ± 6.9, p = 0.98) or body mass (K =
82.1 ± 6.2; NK = 82.9 ± 6.6, p = 0.72). The peak eccentric torques measured prior to kicking
or the 10 minute rest were 152 ± 38 and 149 ± 48N for the kicking and control groups,
respectively. Peak concentric torques measured at the same times were 87 ± 19 for the
kicking and 76 ± 23N for the control group. These between group strength differences were
not statistically significant (p = 0.8 for eccentric and p = 0.14 for concentric torque).
A significant interaction was observed for contraction mode by group (p = 0.001).
Furthermore, significant main effects were found for group (p < 0.001) and contraction mode
(p < 0.001). Eccentric strength declined significantly more in the kicking (-19 ± 13%) than
the control group (-1 ± 9%) (mean difference = -28 Nm, 95%CI = -41 to -15; p < 0.001; d =
1.60). In contrast, concentric strength changes in the kicking (-7 ± 13%) and control groups
82
(0 ± 10%) were not significantly different (mean difference = -7 Nm, 95%CI = -15 to 1; p =
0.089; d = 0.60) (Figure 1).
Strength loss after kicking was significantly greater when measured eccentrically (-19 ± 13%)
than concentrically (-7 ± 13%) (mean difference = -24 Nm, 95%CI = -32 to -16; p < 0.001; d
= 0.92). By contrast, the small eccentric (-1 ± 9%) and concentric (0 ± 10%) strength changes
exhibited by the control group were not significantly different (mean difference = -3 Nm,
95%CI = -11 to 5; p = 0.471; d = 0.10).
Figure 7 - Pre to post mean changes in knee flexor torque. Ecc = eccentric contractions, Con = concentric
contractions. Error bars represent SD.
No significant contraction mode (Con v Ecc) by muscle (BF v MH) by group (K v NK)
interaction was found for changes in sEMG (p = 0.56). However, there was a significant
interaction for contraction mode by group (p < 0.001), along with a significant main effect for
group (p = 0.01) and no significant main effect for contraction mode (p = 0.283).
83
Pairwise comparisons, subsequent to the contraction mode by group analysis, revealed that
the decline in normalised eccentric sEMG (for biceps femoris and medial hamstring
combined) was greater in the kicking (-0.35) than the control group (-0.01) (mean difference
= -0.34, 95%CI = -0.45 to -0.23; p < 0.001; d =1.78). By contrast, changes in concentric
hamstring sEMG in the kicking (-0.15) and control (-0.14) groups were not significantly
different from each other (mean difference = -0.01, 95%CI = -0.15 to 0.12; p = 0.863; d =
0.04).
Post kicking reductions in normalised sEMG were significantly greater in eccentric than
concentric actions for both biceps femoris (mean difference = -0.16, 95%CI = -0.28 to -0.05;
p = 0.008, d = 0.77) and medial hamstrings (mean difference = -0.23, 95%CI = -0.34 to -0.12;
p < 0.001; d = 1.11). In contrast, the control group exhibited smaller reductions in eccentric
than concentric normalised sEMG for the biceps femoris (mean difference = 0.14, 95%CI =
0.02 to 0.25; p = 0.026; d = 0.64) and medial hamstrings (mean difference = 0.12, 95%CI =
0.01 to 0.23; p = 0.032; d = 0.53) (Figure 2) No significant between-muscle differences
(biceps femoris v medial hamstrings) were observed for sEMG changes in either contraction
mode in either group (p > 0.05 for all).
84
Figure 8 - Changes in mean normalized hamstring sEMG in peak eccentric and concentric isokinetic
contractions for K and NK. Error bars represent SD.
The reductions in biceps femoris sEMG activity were correlated with the decline in eccentric
torque (R = 0.645; p = 0.004) whereas changes in medial hamstring sEMG activity were not
(R = -0.003; p = 0.990).
5.6. Discussion
As far as we are aware, this is the first study to have investigated the immediate effects of
drop punt kicking on isokinetic concentric and eccentric knee flexor strength and associated
electromyographic signals. The results show, as hypothesised and as reported previously for
repeated sprint running (Small et al., 2010; Timmins et al., 2014), that strength loss after
kicking-induced fatigue is largely specific to the eccentric contraction mode. As
hypothesised, the loss of eccentric knee flexor strength was accompanied by reductions in
85
medial hamstring sEMG, although we also observed significant reductions in biceps femoris
sEMG which were unanticipated.
Low levels of eccentric knee flexor strength, measured in unfatigued athletes in pre-season
tests, may contribute to an elevated risk of subsequent hamstring strain injury (Opar et al.,
2015; Timmins, Bourne, et al., 2015) although there is some disagreement in the literature
(Bennell et al., 1998; Bourne et al., 2015). A recent extremely large (n = 614) study has also
suggested that any association between isokinetic eccentric knee flexor strength and future
hamstring injury is weak (van Dyk et al., 2016). Nevertheless, an association between fatigue
and a hamstring injury has been assumed after reports that these insults are more likely to
occur in the later portions of each playing period in soccer (Ekstrand et al., 2011a) and Rugby
(Brooks et al., 2006).
Furthermore, running-induced fatigue (Small et al., 2010; Timmins et al., 2014), and now
kicking, have been shown to result in preferential losses in eccentric strength and there is
some basic research which suggests that fatigued skeletal muscle (Mair et al., 1996) fails
(exhibits strain injury) at lower stresses than unfatigued muscle. It remains possible then, that
the preferential loss of eccentric strength after kicking and running may contribute to a
hamstring injury in Australian Rules football and other sports with similar demands.
The current study shows that eccentric strength loss after kicking is associated with a
contraction mode-specific decline in sEMG within both the lateral and medial hamstrings and
we have previously reported that reductions in maximal eccentric sEMG after sprint running
were limited to the biceps femoris (Timmins et al., 2014). While not conclusive proof, the
current findings are consistent with the possibility that reduced neural drive contributes to
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strength loss after kicking. Incomplete activation of isolated mammalian skeletal muscle is
known to reduce the stresses associated with tissue failure (Garrett et al., 1987), so this
decline in muscle activation may play a role in hamstring strain injuries.
Most hamstring injuries in Australian Football (Opar et al., 2015), Soccer (Timmins, Bourne,
et al., 2015) and Rugby (Bourne et al., 2015) occur during running and affect the long head of
the biceps femoris. While there is a relative dearth of published information on the site of
hamstring lesions after kicking injuries, the joint positions in the kick’s follow-through
(flexed hip and near fully extended knee) are similar to those observed in slow stretch
manoeuvres that are typically associated with injuries to the semimembranosus (Askling et
al., 2007). It should be noted, however, that functional magnetic resonance imaging has
revealed significantly greater involvement of the semitendinosus than either the biceps
femoris or semimembranosus muscles after 100 drop punt kicks (Baczkowski et al., 2006).
This suggests that the semitendinosus is more likely than the other two-joint hamstrings to
exhibit kicking-induced fatigue, although it is unclear whether this has any bearing on the site
of a hamstring injury. In a recent prospective study of elite Australian footballers, we
observed five hamstring strain injuries as a consequence of kicking, four of which were
located within the bicep femoris long head while one was to the semimembranosus (Opar et
al., 2015).
In the current study, both the biceps femoris and medial hamstring sEMG associated with
maximal eccentric actions declined significantly more after kicking than in the control group.
However, while the changes in biceps femoris sEMG were correlated with the changes in
eccentric knee flexor strength, the same was not true for medial hamstring sEMG. We have
previously reported a similar correlation between reductions in biceps femoris sEMG and
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eccentric strength after sprint running (Timmins et al., 2014), and this unexplained finding
warrants further investigation. Fine wire EMG suggests that the long head of biceps femoris
exhibits progressively greater activation as the knee extends while the other hamstring
muscles exhibit progressively less (Onishi et al., 2002), so perhaps, in the range of motion in
which we have assessed sEMG (15-350 from full extension), the long head of biceps femoris
has a greater role in torque generation than other hamstring muscles.
The number of kicks in Australian football matches averages 16.4 per player (2015 season),
but this varies considerably depending on player position and ability. Nevertheless, the extent
of kicking-induced fatigue observed in this study is likely far greater than that which is likely
to occur in a competitive game. Nevertheless, the exhausting running demands of football
matches sees players covering 12.2 ±1.9 km, reaching maximum velocities of 30.1 ± 6.7 km
h-1
and performing numerous accelerations (246 ± 47 (>4 kmh-1
in 1 s)) and decelerations, (14
± 5 (over 10 km h-1
in 1 s)) (Wisbey et al., 2010) and this, combined with kicking, may have
an additive effect on injury risk. Furthermore, training sessions involve significantly more
kicks (~50-100+) than are performed in a game (unpublished observations). The amount of
high-force kicking in training sessions may, therefore, influence the risk of a hamstring injury
in Australian Rules football.
We chose to assess the knee flexors at a much slower speed than the maximum angular
velocities experienced in kicking. However, there are few commercially available isokinetic
dynamometers that can assess torque at greater than 500°/s-1
. Furthermore, the chosen
velocity of 180°.s
-1 has previously been used (Timmins et al., 2014) to show decreases in
eccentric knee flexor torque as a consequence of running induced fatigue.
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Twitch interpolation is widely considered the most appropriate means of determining the
completeness of voluntary muscle activation (Shield & Zhou, 2004), however, it is difficult to
electrically stimulate the sciatic nerve and this makes it very challenging to obtain accurate
assessments of hamstring activation. Surface electromyography has limitations when
assessing muscle activation because it is influenced by factors other than motor unit
recruitment and firing rates. For example, the sEMG signal is also influenced by the degree
of motor unit synchrony, which is known to change with muscle fatigue. Nevertheless, the
hamstring sEMG signals associated with maximal eccentric actions are smaller than those
associated with concentric ones (Timmins et al., 2014) and this matches the well-established
pattern observed in other muscles assessed via twitch or ‘train’ interpolation (Beltman,
Sargeant, Mechelen, & Haan, 2004). Furthermore, muscular fatigue seems unlikely to
account for a high proportion of the strength loss reported in this study because, in this
circumstance, similar changes in eccentric and concentric strength would be expected. It,
therefore, seems likely that the current sEMG findings are reflective of muscle activation and
the changes in it induced by kicking.
Conclusion
This study showed that a reduction in eccentric knee flexor strength was accompanied by a
decline in biceps femoris and medial hamstring sEMG activity following repeated drop punt
kicking. These reductions may exacerbate hamstring strain injury risk independently or when
coupled with high-speed running (Timmins et al., 2014), and prolonged match and training
session exposure (Woods et al., 2004). Therefore, the amount of kicking and running
performed within a training session should be considered from the perspective of injury
prevention.
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Practical Implications
Reductions in eccentric knee flexor strength and hamstring sEMG activity may
exacerbate hamstring strain injury risk independently or when coupled with high-
speed running, and prolonged match and training session exposure.
Kicking volume and intensity within a single session should be considered from an
injury prevention perspective.
The impaired neuromuscular function of biceps femoris after kicking may provide
further insight as to why this muscle has a greater susceptibility to injury.
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Chapter 6: Study 3 – The effect of concentric
and eccentric knee flexor strength training on
recovery after repetitive sprint sessions
Publication statement
This chapter is comprised of the following paper that is currently under review:
Duhig, SJ., Buhmann, RL., Bourne, MN., Williams, MD., Roberts, LA., Minett, GM.,
Timmins, RG., Simms, CKE & Shield, AJ. (2016). The effect of concentric and eccentric
knee flexor strength training on recovery after repetitive sprint sessions. Submitted October
2016.
91
Statement of Contribution of Co-Authors for Thesis by Published Paper
The authors listed below have certified* that:
1. They meet the criteria for authorship in that they have participated in the conception,
execution, or interpretation, of at least that part of the publication in their field of
expertise;
2. They take public responsibility for their part of the publication, except for the
responsible author who accepts overall responsibility for the publication;
3. There are no other authors of the publication according to these criteria;
4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the editor
or publisher of journals or other publications, and (c) the head of the responsible
academic unit
5. They agree to the use of the publication in the student’s thesis and its publication on
the Australasian Research Online database consistent with any limitations set by
publisher requirements.
Contributor
Statement of contribution*
Steven Duhig
Experimental design, ethical approval, data collection and
analysis, statistical analysis, manuscript preparation
Llion Roberts Assisted with data collection, analysis and manuscript
preparation
Matthew Bourne Assisted with data collection
Robert Buhmann Assisted with data collection
Casey Simms Assisted with data collection
Ryan Timmins Assisted with manuscript preparation and data collection
Geoffrey Minett Assisted with manuscript preparation
Morgan Williams Assisted with statistical analysis and manuscript preparation
Anthony Shield Supervised the application for ethics approval, experimental
design, data collection, statistical analysis and manuscript
preparation
Principal Supervisor Confirmation
I have sighted email from all co-authors confirming their certifying authorship.
Dr Anthony Shield ____________________ ______________
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6.1. Linking Paragraph
The association between neuromuscular fatigue and muscle strain injury was first established
using animal models, finding fatigued skeletal muscle experiences strain injury at lower
stresses than unfatigued muscle (Mair et al., 1996). The link between fatigue and hamstring
strains has also been recognised in soccer (Ekstrand et al., 2011a; Woods et al., 2004) and
Rugby (Brooks et al., 2006), with insults reported to occur in the latter stages of match play.
This match induced neuromuscular fatigue is likely to be influenced by the physical demands
of running and perhaps kicking (dependent on the football code). The current literature shows
the detrimental effect of running on eccentric hamstring strength and activation (Greig, 2008;
Small et al., 2010; Timmins et al., 2014), and now with the contribution of Study 2 there is
evidence showing the debilitating effect of repetitive kicking. Given the influence of running
and kicking on hamstring function, it may be they can act independently or collectively to
exacerbate injury risk in sports with similar physical demands.
There is a growing body of evidence that supports the use of prophylactic eccentric hamstring
strengthening exercises in hamstring injury prevention programs (Arnason et al., 2008;
Petersen et al., 2011; van der Horst et al., 2015). Furthermore, there is a significant and
growing amount of research involving training load monitoring (Gabbett et al., 2016; Gabbett
& Jenkins, 2011; Gabbett & Ullah, 2012; Hulin et al., 2013; Hulin et al., 2015; Rogalski et
al., 2013) and injury. However, incidence rates have either remained fairly constant across
the last two decades in AFL (Orchard et al., 2013) while increasing 4% per annum in elite
European soccer (Ekstrand et al., 2016). Anecdotal evidence suggests some strength and
conditioning coaches are hesitant to employ exercises that they regard as ‘non-functional’ or
not specific to the movement patterns observed on the sporting field. For instance, there
seems to be reluctance for some to employ the Nordic curl, despite strong evidence for its
93
ability to reduce incidence rates (Petersen et al., 2011; van der Horst et al., 2015). Therefore,
the aim of the final study was to investigate the relative effects of concentric-only and
eccentric-only knee flexor strength training on the recovery from a high-speed running
training session. The findings of this study may provide further insight into how exercise
using only one contraction type may further compliment strength and conditioning/ injury
prevention programs.
Study 2 is presented as published in the Journal of Science and Medicine in Sport (volume
20, issue 6, pages 595-599.). The following comments are made in response to reviewers’
comments:
1) Brooks et al. (2006) have also reported the more injuries occur in Rugby during the latter
stages of each half and the match. However, this was not included as there are limitations
with the amount of references used for the journal this paper was published.
2) The range of motion during the kicking protocol was not measured and the authors
recognise that varying leg ranges of motion may have an effect on muscle activity.
3) The knee’s fulcrum was aligned with the dynamometer’s lever axis but was done so
without a moderate contraction. Movement was minimised by using thigh and lap straps.
4) It is acknowledged that despite strapping the thigh to the seat, that the knee and
dynamometer axes are not aligned throughout the full range of motion. As a consequence,
knee angles may not be accurately reflected by the angle of the dynamometer’s lever
arm. This is a significant limitation to inferring that changes have been made or not made in
torque - joint angle relationships.
5) Joint angles were defined in relation to the lever arm. Full knee extension is define as 0°
which is at a 90° angle to the vertical axis.
94
6) The units for the data expressed on the Figure 8 y-axis represents the change in normalised
sEMG. This is expressed as a percentage but may be somewhat confusing to the reader
should (%) be included.
95
6.2. Abstract
The relative effects of eccentric and concentric knee flexor (KF) strength training on the
recovery from sprint running has not been explored. PURPOSE: To investigate whether KF
adaptations induced by 5-weeks of concentric (CON) or eccentric (ECC) strength training
have different effects on sprint running recovery. METHODS: Thirty males (age, 22.8 ±
4.1y; height, 180.1 ± 6.4cm; weight, 85.2 ± 14.6kg) were allocated into either a CON or ECC
group, each of which performed nine sessions of resistance training. Prior to and immediately
after the 5-week intervention, each participant’s bicep femoris long head (BFLH) fascicle
length (FL), pennation angle (PA), muscle thickness (MT), peak isometric KF torque and
Nordic eccentric strength were assessed. Post intervention, participants performed two sprint
sessions (10x80m) 48 hours apart. With reference to the first sprint session, blood samples
and passive KF torque were collected before, after, 24 hours and 48 hours post. RESULTS:
After the 5-week strength-training period fascicles lengthened in the ECC group (p<0.001; d
= 2.0) and shortened in the CON group (p<0.001; d =0.92) while PA decreased for the ECC
group (p=0.001; d = 0.52) and increased in the CON group (p<0.001; d = 1.69). Nordic
eccentric strength improvements occurred in both the ECC (p<0.001; d =1.49) and CON
(p<0.001; d = 0.95) groups. Peak isometric strength (p=0.480; d=0.12), passive KF torques
(p=0.807; d=0.16), sprint performance decrements within (p=0.595; d=0.24) and between
sprint sessions (p = 0.910; d=0.05) and creatine kinase (p=0.818; d=0.06) did not differ
significantly between groups. CONCLUSION: There was no difference between the effects
of ECC and CON knee flexor strength training on sprint performance decrements.
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6.3. Introduction
Eccentric and concentric training programs have unique effects on skeletal muscle adaptation.
For example, eccentric training decreases and concentric training increases the susceptibility
of rat and human skeletal muscles to microscopic damage and soreness consequent to a bout
of eccentric contractions (Lynn, Talbot, & Morgan, 1998; Whitehead, Allen, Morgan, &
Proske, 1998). Eccentric training of the human hamstrings has also recently been reported to
increase fascicle lengths within the long head of the biceps femoris, while concentric training
through the same range of motion was associated with a decrease in fascicle lengths (FL)
(Timmins et al., 2016).
Eccentric strength training interventions employing the Nordic hamstring exercise (NHE)
have been shown to decrease hamstring strain rates in sport (Andersen, Holme, Engebretsen,
& Bahr, 2008; Petersen, Thorborg, Nielsen, Budtz-Jørgensen, & Hölmich, 2011; Seagrave et
al., 2014; van der Horst, Smits, Petersen, Goedhart, & Backx, 2015), while low levels of
Nordic eccentric strength have been reported to be associated with an increased risk of injury
in some (Opar et al., 2015; Timmins, Bourne, et al., 2015), but not all prospective studies
(Bourne, Opar, Williams, & Shield, 2015).While the mechanisms mediating the effectiveness
of eccentric strength training are not entirely understood (McHugh, 2003) it has been
proposed that the addition of in-series sarcomeres may at least partially explain an increased
resistance to microtrauma and muscle strain injury (Brockett, Morgan, & Proske, 2004; Lynn
& Morgan, 1994; Morgan, 1990) Indeed, short biceps femoris long head (BFLH) fascicles
have recently been reported to be associated with a higher risk of hamstring strain injury in
elite Australian soccer players (Timmins, Bourne, et al., 2015). The NHE is also known to be
an effective means of increasing BFLH fascicle lengths (Bourne et al., 2016).
97
We have shown that large and rapid changes in high-speed running loads over a 1-4 week
period are associated with an increased risk of hamstring strain injury (Duhig et al., 2016).
This is consistent with the possibility that hamstring injuries may not always be isolated acute
events caused by a single over-long stride, kick or stretch but instead they may occur as a
consequence of accumulated microtrauma which could eventually become macroscopic
(Duhig et al., 2016). However, such trauma may be mitigated with use of eccentric hamstring
strength training.
The effects of eccentric and concentric strength training of the human hamstrings on running
performance and recovery are not well known. Sprint running involves high knee flexor
forces and powerful eccentric actions during terminal swing which is thought to be a likely
time for hamstring strain injury (Heiderscheit et al., 2005; Schache, Wrigley, Baker, &
Pandy, 2009). These powerful eccentric actions in running are also likely to cause hamstring
microtrauma, soreness and a loss of strength which may result in a performance decrement, in
the absence of sufficient recovery, when two high-speed running sessions are planned within
24-72 hours of each other. Theoretically, eccentric conditioning should better prepare the
hamstrings for repeated bouts of such exercise. Accordingly, this study was designed to
investigate the effects of eccentric and concentric hamstring conditioning on the change in
running performance that occurred between two consecutive running sessions held 48 h apart.
We hypothesised that adaptations induced by eccentric training would, in comparison with
concentric training, result in a better maintenance of sprint performance between sessions
with reduced markers of muscle damage (lesser passive knee flexor torque, delayed onset
muscle soreness (DOMS) and venous creatine kinase levels (CK)). For the purpose of this
study, ‘recovery’ is defined as the ability to perform similarly between two sprint sessions
and exhibit the same levels of muscle damage markers as per the baseline measurement.
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6.4. Methods
Participants and study design
This strength training intervention was conducted between July and September, 2016. Thirty
recreationally active males (age, 22.8 ± 4.1 y; height, 180.1 ± 6.4 cm; weight, 85.2 ± 14.6 kg)
provided written informed consent and completed a cardiovascular screening questionnaire
before participating. All participants were free from soft tissue and orthopaedic injuries to the
lower limbs, hips, and trunk with no prior history of hamstring strain or knee ligament injury.
This study was approved by the university’s research ethics committee.
Each participant had their relaxed bicep femoris long head architecture (fascicle length (FL),
pennation angle (PA) and muscle thickness (MT)), passive knee flexor torque, isometric peak
knee flexor torque and eccentric strength assessed prior to and after the five-week
intervention. Once pre-intervention assessments had been conducted, participants were
ranked and paired in order of fascicle lengths and an individual from each pair was randomly
allocated to either the concentric-only (CON) group or eccentric-only (ECC) group, with the
remaining individual from each pair allocated to the other group. Both groups completed nine
strength training sessions over a five week period (Table 1) and were instructed not to
participate in any additional hamstring strength training, DOMS from each of the hamstring
strength sessions was collected 24 h post-training. Seven to nine days after the final strength
training session participants performed a sprint running session (10 x 80 m) on a grass sports
field. This running session was repeated 48 hours after the first. 24 and 48 hours after the first
sprint training session, dynamometry was used to measure passive knee flexor torque and
perceived muscle soreness (DOMS) was recorded using a 0-10 numeric pain rating scale 24
and 48 hours after the first sprint session. The sequence of tests was the same before and after
99
training. Blood samples were drawn using standard venipuncture techniques at four time
points (prior to the first sprint session, immediately after then 24 h and 48 h post).
Biceps femoris long head architecture assessment
Bicep femoris long head fascicle lengths were collected using ultrasound images taken along
the longitudinal axis of the muscle belly utilising a two-dimensional, B-mode ultrasound
(frequency, 12Mhz; depth, 8 cm; field of view, 14 x 47 mm) (GE Healthcare Vivid-i,
Wauwatosa, U.S.A). Participants were positioned prone on a plinth with their hips in neutral
position and knees fully extended, images were acquired from a point midway between the
ischial tuberosity and the popliteal fold, parallel to the presumed orientation of BFLH
fascicles. After the scanning site was determined, the distance of the site from various
anatomical landmarks was recorded to ensure its reproducibility for post testing. These
landmarks included the ischial tuberosity, head of the fibula and the popliteal fold at the mid-
point between BF and ST tendon. For post testing, the scanning site was determined and
marked on the skin and then confirmed by replicated landmark distance measures. Images
were obtained from both limbs following at least five minutes of inactivity. To gather
ultrasound images, the linear array ultrasound probe, with a layer of conductive gel was
placed on the skin over the scanning site, aligned longitudinally and perpendicular to the
posterior thigh. Care was taken to ensure minimal pressure was placed on the skin by the
probe as this may affect the accuracy of the measures (Klimstra, Dowling, Durkin, &
MacDonald, 2007). The orientation of the probe was manipulated slightly by the
ultrasonographer if the superficial and intermediate aponeuroses were not parallel.
Ultrasound images were analysed using MicroDicom software (Version 0.7.8, Bulgaria). For
each image, 6 points were digitised as described by Blazevich and colleagues (Blazevich,
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Gill, & Zhou, 2006). Following the digitising process, muscle thickness was defined as the
distance between the superficial and intermediate aponeuroses of the BFLH. A fascicle of
interest was outlined and marked on the image. Fascicle length was determined as the length
of the outlined fascicle between aponeuroses and was reported in absolute terms (cm). As the
entire fascicles were not visible in the probe’s field of view, their lengths were estimated
using the following equation:
FL=sin (AA+90°) x MT/sin(180°-(AA+180°-PA)).
Where FL= fascicle length, AA= aponeurosis angle, MT= muscle thickness and PA=
pennation angle (Blazevich et al., 2006).
All images were collected and analysed by the same ultrasonographer who was blinded to
participant identity and training group allocation. The assessment of BFLH architecture using
the aforementioned procedures by this ultrasonographer is highly reliable (intraclass
correlations >0.90; minimal detectable change at a 95% confidence interval, MT=0.18,
PA=0.96, FL=0.74) (Timmins, Shield, Williams, Lorenzen, & Opar, 2015).
Strength assessments
Nordic eccentric strength test (NeST)
The assessment of eccentric knee flexor force using the NHE has been reported previously
(Bourne et al., 2016; Opar et al., 2014). Participants knelt on a padded board, with the ankles
secured by individual ankle straps which were attached to uniaxial load cells (MLP-1K,
Transducer Techniques, CA, USA). The load cells were calibrated immediately prior to
testing by progressively applying known ~200N loads up to a load of ~800N (~600 N forces
are the highest our group has previously recorded in tests of the Nordic hamstring curl. The
distal ends of the straps were placed level with the most prominent point of the lateral
101
malleoli. The ankle braces and load cells were secured to a pivot which allowed the force
generated by the knee flexors to be measured through the long axis of the load cells. From the
initial kneeling position with their ankles secured in yokes, arms on the chest and hips
extended, participants lowered their bodies as slowly as possible to a prone position.
Participants performed only the lowering (eccentric) portion of the exercise and were
instructed to use their arms and flex at the hips and knees in order to reassume the starting
position so as to minimise knee flexor activity. Participants initially performed five
submaximal but progressively more intense repetitions of the bilateral Nordic hamstring
exercise as a way of warming up for subsequent maximal efforts. The NeST involved
performing single repetitions of the NHE as slowly as possible, initially with body mass and
thereafter with extra loads held centred over the xiphoid process. The first extra load was 5
kg and this was increased in 5 kg increments until the sum of the two ankle forces did not
increase. A single repetition was performed at each intensity level and the highest sum of the
left and right leg forces was recorded. Three minutes rest was allowed after warm-up and
three minutes was allowed between single maximal repetitions. Participants were encouraged
to maintain a fully extended hip angle throughout each repetition. To calculate knee flexor
torques from the forces applied at the ankle, a measurement of the distance between the
femoral lateral epicondyle and middle of the ankle strap was made with a flexible steel tape
measure.
Knee flexor passive torque
The resistance of the relaxed knee flexors to passive stretch, measured as gravity corrected
knee flexion torque (Nm), was also assessed via dynamometry (Biodex® System 3, Shirley,
USA) (Knapik, Wright, Mawdsley, & Braun, 1983). Prior to testing, neonatal ECG electrodes
(10 mm in diameter, 15 mm interelectrode distance) (Ambu®, Ballerup, Denmark) were
102
placed over the intersection of the medial and lateral hamstring muscle bellies. Subsequently,
participants were seated on the dynamometer with a hip angle of approximately 85°, a 5 cm
thick foam pad was then placed between the seat and upper back of the participant to increase
hip flexion to approximately 90º. Straps were placed around the tested thigh, waist, and chest
to minimise compensatory movements. All seating variables (e.g., seat height, pad position)
were recorded to ensure the replication of positions. The lever angle range of motion was set
at 0° and 90° (0° representing full knee extension) with gravity correction for limb weight
conducted at 30° from full knee extension. The warm-up prior to the assessment of passive
knee flexor torque and isometric strength involved two sets of four maximal concentric knee
extension and flexion contractions at an angular velocity of 240°.s
-1 and 120°
.s
-1, respectively.
Passive knee flexor torque was determined by measuring the maximum torque produced
during the extension of the knee. Participants were instructed to completely relax their lower
limbs while the dynamometer extended their knee at 10°.s
-1 across three repetitions
(Magnusson et al., 1997).Real-time surface EMG traces displayed to the participant and the
researcher allowed confirmation of hamstring muscle relaxation throughout these tests.
Participants were asked to rate their muscle soreness at the point of full knee extension using
a numerical scale between 0 and 10 (0 = no pain; 10 = severe pain).
Isometric peak torque
Isometric knee flexor strength was assessed via isometric dynamometry (Biodex® System 3,
Shirley, USA) (Knapik et al., 1983). Isometric contractions were performed at five different
lever angles (10°, 30°, 50°, 70° and 90°) with 2 x 3 s maximal voluntary contractions at each
angle. The order of knee angles was randomised for each participant and replicated in post-
training tests. A 30 second rest period was employed between each contraction at the same
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angle and with a one minute rest between angles. The investigators loudly exhorted
participants to exert maximal effort during all contractions.
Sprint times
For both running sessions, all participant’s had their 80 m sprint times measured using timing
gates (SMARTSPEED LITE, Fusion sport, Brisbane, Australia), which have previously been
shown to be reliable (Oliver & Meyers, 2009). The standardised warm-up consisted of a 3
min jog followed by four 80 m run-throughs with increasing speed (60%, 70%, 80% and 90%
of each participant’s perceived maximum) with 3 mins lower limb dynamic stretches. A
recovery period of 3 min was applied between maximum efforts.
Hamstring training program
Participants were required to complete a training program consisting of nine supervised
exercise sessions over the course of five weeks (Table 1) with a minimum recovery period of
48 hours between sessions. The training program design was similar to previous training
studies using the NHE (Mjølsnes, Arnason, Raastad, & Bahr, 2004; Petersen et al., 2011) and
has been shown to produce morphological adaptations (Timmins et al. 2016). To ensure
procedural consistency, all sessions were conducted in the same laboratory, using the same
exercise equipment and supervised by the same investigators.
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Table 1 - Training program variables
Week Training
sessions
Sets Repetitions
1 2 2 6
2 2 3 6
3 2 4 6
4 2 5 6
5 1 5 6
Nordic hamstring exercise (ECC) training
The NHE was performed on a padded board with the ankles individually secured
immediately superior to the lateral malleolus by individual ankle braces attached in-series to
custom made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia). The
instructions given to the participants were to lean forward at the knee at the slowest possible
speed whilst maximally resisting the descent and maintaining the trunk and hips in an upright
position, with only the eccentric phase being performed. The hands were kept pronated and
wrists hyper-extended next to the chest to catch the fall (Bourne, Opar, Williams, Al Najjar,
Shield, 2015). The exercise was performed initially with body mass alone but once
participants displayed adequate strength to completely stop the movement at ~100 from full
knee extension, they were required to hold a weight plate (range = 5 – 10 kg) to their chest
(centred over the xiphoid process). This external load was increased in 5 – 10 kg increments
when participants were again able to stop the movement at ~10º from full knee extension.
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Leg curl (CON) training
The unilateral leg curl was performed on a prone leg curl machine (CALGYM, Australia)
using a load of ~6-7RM. Each repetition started at full knee extension and finished at
approximately 900 of knee flexion. Once this point was reached a research assistant held the
load allowing the participant to return their shank, without external load, to the starting
position of full knee extension. The assistant then lowered the load.
Blood collection and analysis
Blood samples were collected before the first sprint session, <15 min after, 24 h, and 48 h
post sprint. The blood samples were collected from an antecubital vein into an EDTA tube
(BD, Franklin Lakes, NJ). Tubes were then centrifuged at 1000 rpm at 4ºC for 10 min and
then stored at -80ºC until the day of analysis. Creatine kinase activity was measured using a
spectrophotometric assay on an automated analyser (Cobas Mira, Roche diagnostics GmbH,
Germany).
Statistical analysis
All statistical analyses were performed using SPSS version 22.0.0.1 (IBM Corporation,
Chicago, IL). Where appropriate, data were screened for normal distribution using the
Shapiro-Wilk test and homoscedasticity using Levene’s test. T-tests were used to compare
participant age, height and body mass. The forces collected during the NeST were converted
to torque and expressed as torque per kilogram of bodyweight (Nm/kg). Repeated measures
split plot ANOVAs were used to determine changes in measures of BFLH architecture (FL,
PA, MT), NeST peak torque, isometric peak torque, passive knee flexor torque and knee
flexor DOMS, 80 m sprint performance, venous CK and perceived muscle soreness. For the
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analysis of BFLH architecture (FL, PA and MT), Nm/kg and 80 m sprint performance the
within-subject variable was time (pre and post strength training intervention) and the
between-subject variable was group (CON vs. ECC). For isometric peak knee flexor torque
the within-subject variables were time (pre vs. post) and angle (10, 30, 50, 70 and 90º) and
the between-subject variable was group (CON vs. ECC). The between limbs (dominant vs.
non-dominant) BFLH architecture was not significantly different at either time point (p>0.05),
therefore dominant and non-dominant limbs were averaged to provide a single value for each
participant. The 80m sprint performance is reported as minimum, and performance
decrements (maximum time –minimum time) time taken to complete the ten repetitions. To
explore changes in passive knee flexor torque, knee flexor DOMS and CK the within-subject
variable was time (before and after first sprint session then at 24 h and 48 h post) and the
between-subject variable was group (CON vs. ECC). Descriptive statistics were used to
report 24 h post-training session ratings of perceived soreness. For all analyses, post hoc
independent t-tests with Bonferroni corrections were used to determine which comparisons
differed significantly. The mean differences were reported with their 95% confidence
intervals (CIs). Cohen d effect sizes were calculated using the thresholds; trivial < 0.20, small
= 0.20-0.49, medium = 0.50-0.79 and large > 0.80 (Cohen, 1988).
6.5. Results
No significant differences were observed for age, height or body mass between the groups (p
> 0.05; Table 2). Training session compliance rates were 100% and 99.3% for the CON and
ECC groups, respectively.
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Table 2 - Participant characteristics
Group Age (years) Stature (cm) Mass (kg)
CON 22.7 ± 3.9 178.3 ± 5.8 86.2 ± 15.4
ECC 23.7 ± 4.8 181.3 ± 6.9 83.8 ± 14.6
Biceps femoris long head architecture
As a consequence of the strength training intervention, a significant group by time interaction
was found for BFLH fascicle length (p<0.001) (figure 1) and pennation angle (p<0.001)
(figure 2). However, no significant group by time interaction was seen for muscle thickness
(p=0.193). Before training there were no significant between group differences observed for
fascicle lengths (CON = 10.39cm; ECC = 10.22cm; mean difference = 0.17cm; 95% CI -0.32
to 0.66; p=0.484; d=0.25), PA (CON = 14.18º; ECC = 14.16º; mean difference = 0.02º; 95%
CI -0.93 to 0.98; p=0.964; d=0.01) or MT (CON = 2.55cm; ECC = 2.49cm; mean difference
= 0.06cm; 95% CI -0.14 to 0.25; p=0.523; d=0.22). Post hoc analyses showed significant
BFLH fascicle elongation in the ECC group (mean difference= 1.40 cm (13%); 95% CI 1.05
to 1.75; p<0.001; d = 2.0) and shortening in the CON group (mean difference = 0.66 cm
(6%); 95% CI -0.92 to -0.40; p<0.001; d =0.92) (Figure 1). After training the differences
between the group’s fascicle lengths were significant (mean difference = 1.90cm; 95% CI
1.34 to 2.45; p<0.001; d=2.57). There was a significant decrease in PA for the ECC group
(mean difference = 0.77º (5%); 95% CI -0.98 to -0.30; p=0.001; d = 0.52) and an increased
PA in the CON group (mean difference = 1.73º (12%); 95% CI 1.40 to 2.06; p<0.001; d =
1.69) (figure 2). After training, a significant between group difference in PA was found
(mean difference = 2.52º; 95% CI 1.60 to 3.45; p<0.001; d=2.07). A significant increase in
MT was shown for both the ECC (mean difference = 0.19cm (7%); 95% CI 0.12 to 0.29;
p<0.001; d = 0.73) and CON (mean difference = 0.11 cm (4%); 95% CI 0.39 to 0.19;
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p=0.005; d = 0.43) groups (Figure 3). However, there was no significant difference between
groups after training (mean difference = 0.01cm; 95% CI -0.20 to 0.23; p=0.868; d=0.03).
Figure 9 – Pre and post intervention individual and mean changes in bicep femoris long head fascicle lengths.
CON = green squares; ECC = red circles. The dashed red line represents the ECC group average and the green
line shows the CON group average. Shaded areas represent 95% CI.
Figure 10 – Pre and post intervention individual and mean changes in bicep femoris long head pennation
angles. CON = green squares; ECC = red circles. The dashed red line represents the ECC group average and the
green line shows the CON group average. Shaded areas represent 95% CI.
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Figure 11 - Pre and post intervention individual and mean changes in bicep femoris long head thickness. CON
= green squares; ECC = red circles. The dashed red line represents the ECC group average and the green line
shows the CON group average. Shaded areas represent 95% CI.
Eccentric knee flexor strength
There was no significant group by time interaction (p=0.065 d=0.19) detected for the NeST
scores. There were no significant between groups differences found in eccentric knee flexor
strength prior to the intervention (CON = 3.88Nm/kg; ECC = 3.68Nm/kg; mean difference =
0.20Nm/kg; 95% CI -0.18 to 0.58; p=0.299; d=0.39). However, there were strength
improvements for both the ECC (mean difference = 0.83Nm/kg (24%); 95% CI 0.59 to 1.0;
p<0.001; d =1.49) and CON (mean difference = 0.51Nm/kg (13%); 95% CI 0.29 to 0.75;
p<0.001; d = 0.95) groups (figure 1). There was no significant difference between groups at
the completion of training (mean difference = 0.15Nm/kg; 95% CI -2.87 to 0.59; p=0.480;
d=0.25).
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Figure 12 - Pre and post intervention individual and mean changes in Nordic eccentric strength. CON = green
squares; ECC = red circles. Dashed red line represents the ECC group average and the green line shows the
CON group average. Shaded areas represent 95% CI.
Peak isometric knee flexor torque
There was no significant group by time by angle interaction observed for peak isometric knee
flexor torques (p=0.480; d=0.12). There were no significant differences between groups at
any angle before training (10º, CON = 153Nm; ECC = 161Nm; mean difference = 7Nm; 95%
CI -32 to 16; p=0.511; d=0.19; 30º, CON = 151Nm; ECC = 158Nm; mean difference = 7Nm;
95% CI -30 to 16; p=0.542; d=0.22; 50º, CON = 145Nm; ECC = 149Nm; mean difference =
3Nm; 95% CI -23 to 15; p=0.705; d=0.03; 70º, CON = 127Nm; ECC = 130Nm; mean
difference = 3Nm; 95% CI -20 to 13; p=0.705; d=0.08; 90º, CON = 96Nm; ECC = 102Nm;
mean difference = 5Nm; 95% CI -18 to 6; p=0.351; d=0.31) or after training (10º, CON =
154Nm; ECC = 151Nm; mean difference = 3Nm; 95% CI -21to 26; p=0.819; d=0.09; 30º,
CON = 151Nm; ECC = 153Nm; mean difference = 2Nm; 95% CI -25 to 22; p=0.898;
d=0.22; 50º, CON = 140Nm; ECC = 145Nm; mean difference = 5Nm; 95% CI -26 to 17;
p=0.666; d=0.15; 70º, CON = 129Nm; ECC = 128Nm; mean difference = 1Nm; 95% CI -16
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to 17; p=0.931; d=0.04; 90º, CON = 99Nm; ECC = 105Nm; mean difference = 6Nm; 95% CI
-20 to 7; p=0.352; d=0.31).
Peak isometric torques did not change at any angle as a consequence of concentric (10º, mean
difference = 0.60Nm; 95% CI -11.02 to 12.24; p=0.917; d=0.03; 30º, mean difference =
0.53Nm; 95% CI -13.61 to14.68; p=0.939 ; d=0.00; 50º, mean difference = -5.26Nm; 95%
CI -17.75 to 7.22; p=0.395; d=0.16; 70º, mean difference = 2.33Nm; 95% CI -7.79 to 12.46;
p=0.641 ; d=0.08; 90º, mean difference = 2.20Nm; 95% CI -6.26 to 10.66; p=0.599; d=0.15)
or eccentric strength training (10º, mean difference = -10.00Nm; 95% CI -21.62 to 1.62;
p=0.089; d=0.32; 30º, mean difference = -5.00Nm; 95% CI -19.14 to 9.14; p=0.475; d=0.17;
50º, mean difference = -4.33Nm; 95% CI -16.82 to 8.15; p=0.483; d=0.16; 70º, mean
difference = -1.60Nm; 95% CI -11.73 to 8.53; p=0.749; d=0.10; 90º, mean difference =
2.80Nm; 95% CI -5.66 to 11.26; p=0.504; d=0.19).
Passive knee flexor torque and muscle soreness
No significant group by time interactions were observed for the knee flexor’s peak passive
torque (p=0.807 d=0.16) or DOMS (p=0.700). There were no significant pre-intervention
differences in peak passive gravity corrected knee flexor torque (CON = 12Nm; ECC =
13Nm; mean difference =1Nm; 95% CI -4.74 to 6.61; p=0.737; d=0.29) or DOMS (CON =
1.7; ECC = 1.5; mean difference = 0.2; 95% CI -1.25 to 1.72; p=0.750; d=0.10). Similarly,
no between group differences were observed after the training period in passive peak knee
flexor torque (CON = 8Nm; ECC = 10Nm; mean difference = 2Nm, 95% CI -1.17 to 5.37;
p=0.198 ; d= 0.55) or DOMS (CON = 0.9; ECC = 0.7; mean difference = 0.1, 95% CI -0.80
to 1.08; p=0.766; d=0.17), 24 hours post sprinting passive peak torque (CON = 11Nm; ECC
= 11Nm; mean difference = 0Nm, 95% CI -6.99 to 6.72; p=0.96; d=0.01) or DOMS (CON =
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1.0; ECC = 1.3; mean difference = 0.3, 95% CI -1.31 to 0.75; p=0.580; d=0.23), and 48 hours
post sprinting passive peak torque (CON = 10Nm; ECC = 12Nm; mean difference = 2Nm,
95% CI -3.48 to 7.12; p=0.485; d=0.28) or DOMS (CON = 1.8; ECC = 1.4; mean difference
= 0.4, 95% CI -0.83 to 1.81; p=0.643; d=0.30).
Eighty-metre sprint performance
There were no significant group by time interactions for the change in best times between
sprint sessions 1 and 2 or for the performance declines within sessions minimum time
(p=0.556; d=0.24) or performance decrements (p=0.317; d=0.05) in the sprint sessions. No
significant between group differences were observed in the first or second sprint session for
minimum time (first, CON = 11.51sec; ECC = 11.32sec; mean difference = 0.18sec; 95% CI
-0.60 to 0.98; p=0.630; d=0.07; second, CON = 11.70sec; ECC = 11.43sec; mean difference
= 0.27sec; 95% CI -0.52 to 1.07; p=0.483; d=0.27) or performance decrements (first, CON =
3.01sec; ECC = 2.57sec; mean difference = 0.44sec; 95% CI -2.13 to 1.25; p=0.595; d=0.21;
second, CON = 1.97sec; ECC = 1.93sec; mean difference = 0.04sec; 95% CI -0.88 to 0.79;
p=0.910; d=0.04). The performance decline within each training session (the difference in
times for the fastest and the slowest 80m sprint) changed significantly between the first and
second sprint sessions in the CON group (first = 2.92 ± 2.15sec vs. second = 1.98 ± 1.12sec;
mean difference = 1.04sec; 95% CI -1.92 to -0.15, p=0.023; d=0.56) but not the ECC group
(first = 2.46 ± 1.65sec vs. 1.93 ± 0.86sec; mean difference = 0.64sec; 95% CI -1.46 to 1.67;
p=0.114; d=0.48).
Two participants from the CON group were unable to finish the second sprinting due to one
participant sustaining a hamstring strain during the first sprint and the other participant ‘too
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sore’ and fearful of an injury to sprint. All participants from the ECC group were able to
complete both sessions.
Creatine kinase
There was no significant group by time interaction found for creatine kinase (p=0.818). No
significant differences in the levels of CK were observed between groups before the first
sprint session (CON = 177 U·L-1
; ECC = 226 U·L-1
; mean difference = 49 U·L-1
; 95% CI -
158.31 to 59.08; p=0.353; d=0.41), immediately after (CON = 239 U·L-1
; ECC = 327 U·L-1
;
mean difference = 88 U·L-1
, 95% CI -217 to 41; p=0.171; d=0.60), 24 hours after (CON =
951 U·L-1
; ECC = 972 U·L-1
; mean difference = 21 U·L-1
, 95% CI -538 to 495; p=0.933;
d=0.03) or 48 hours after (CON = 607 U·L-1
; ECC = 609 U·L-1
; mean difference = 2 U·L-1
,
95% CI -320 to 316; p=0.990; d=0.00) the first sprinting session. CK levels significantly
increased significantly after running, from baseline measurements for both the concentric
(immediately after sprinting, mean = +62 U·L-1
; 95% CI 11 to 114; p=0.011; d=0.53; 24h
post, mean = +774 U·L-1
; 95% CI 242 to 1305; p=0.002; d=1.80; 48h post, mean = +430
U·L-1
; 95% CI 85 to 775; p=0.009; d=1.53) and eccentric groups (immediately after
sprinting, mean difference = 101 U·L-1
; 95% CI 55 to 146; p<0.001; d=0.67; 24h post, mean
difference = 745 U·L-1
; 95% CI 279 to 1211; p=0.001; d=1.75; 48h post, mean difference =
383 U·L-1
; 95% CI 80 to 685; p=0.008; d=1.44).
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Perceived muscle soreness after strength training sessions
Perceived muscle soreness was reported as group mean and SD as shown in figure 13.
Figure 13 - Group averages for subjective ratings of delayed onset muscle soreness. Error bars represent SD.
6.6. Discussion
This study is the first to investigate the effects of eccentric and concentric hamstring
conditioning on the change in running performance between two sprint sessions held 48 h
apart. Contrary to our hypothesis, there were no between-group differences for sprint
performance decrements or markers of muscle damage. Previous studies have shown that
concentrically trained rat muscles lose significantly more force and have their force-length
relationships shifted significantly further towards longer muscle lengths than eccentrically
trained muscles after a single bout of electrically stimulated and maximal eccentric
contractions (Lynn et al., 1998). Both force loss and the shift in the torque-joint angle curve
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are thought to result from muscle damage. There is evidence in humans that the muscle
soreness and weakness induced by eccentric contractions are elevated after periods of
concentric training (Gleeson, Eston, Marginson, & McHugh, 2003) and there is significant
evidence that eccentric training has the opposite effects (Brockett et al., 2004; Morgan, 1990.
Whitehead et al., 1998). These previous findings led to the hypothesis that eccentrically
trained hamstrings would recover significantly better than concentrically trained muscles and
that this would influence sprint performance, particularly in the second of the two running
sessions conducted in this study.
There is a range of possible explanations for the lack of performance decline and a lack of
differences between the eccentrically and concentrically trained participants in this study.
Firstly, it must be acknowledged that the extent of muscle soreness and the changes to indices
of muscle damage (CK) were modest, although similar to that found after downhill running
(Byrnes et al., 1985). Previous studies employing maximal eccentric contractions of single
muscle groups such as the elbow flexors (Nosaka & Clarkson, 1996) have reported blood CK
levels in the region of 10000 and the enzyme elevations seen in the current study were only
about 7% of this. This suggests moderate muscle damage as a consequence of sprinting and
there is currently no method to measure whether the circulating CK originated in the
hamstring muscles, as many muscle groups of the lower limbs could have experienced
muscle damage as a consequence of the sprinting performed. It may be that any mild damage
to the hamstring muscles does not necessarily lead to a drop in sprint performance because
they provide only a relatively small portion of the impulses involved in sprint running. The
relatively low sprint training status of the sampled participants may have also contributed to
the high variability in performance, thereby masking any effects of hamstring conditioning.
Future studies exploring the effects of concentric and eccentric training may benefit from
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recruiting more highly trained participants or including regular sprint sessions within the
training program. Such studies are difficult, however, because well trained individuals are
typically more reticent to alter their training programs.
There were no between group differences in the measured markers of muscle damage used in
this study. It has previously been reported that muscle damage is greater in concentrically
conditioned muscle than eccentric (Lynn et al., 1998) and that stiff knee flexor muscles
produce high levels of peak passive torque (Magnusson et al., 1997). Therefore, we
hypothesised that the muscle damage sustained during the first sprint session would be
greater in the CON than the ECC group leading to the CON group being stiffer and
consequently display greater peak passive torque, higher levels of soreness and elevated
levels of CK, although these did not differ between groups. The degree of hip flexion during
the passive knee flexor torque assessment was less than that in a previous report (Magnusson
et al., 1997) which may be the reason no between group differences were found for peak
passive torque or DOMS. However, despite no group differences in the measured markers of
muscle damage two participants from the CON group were unable to finish the second sprint
session due to one straining their hamstring in the first repetition of the second session and
the other withdrawing due to hamstring ‘soreness’. These incidents suggest, for at least two
of the participants from the CON group, that there may have been substantial damage caused
by high-speed running in the first session that had not completely recovered 48 h post.
Notably, the participant who strained his hamstring exhibited the shortest muscle fascicles of
the group and perhaps other participants experienced less muscle damage because their
fascicles, although shortened by concentric training, were sufficiently long to minimise
muscle damage.
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The results from the current study provide further evidence that BFLH architecture responds
differently to eccentric and concentric training (Bourne et al., 2016; Timmins et al., 2016).
Distinct changes were observed between groups with BFLH fascicles lengthening or
shortening and pennation angles decreasing or increasing in the ECC and CON groups,
respectively. The elongation of fascicles and reduced PA exhibited in the ECC group is likely
indicative of the addition of in-series sarcomeres (Lynn et al., 1998). This adaptation is
expected to reduce the amount of strain experienced per sarcomere at any given muscle
length. As a consequence, fascicle lengthening with more in-series sarcomeres might be
expected to reduce sarcomere lengths during high-speed running, and thereby lessen the
damage caused by such eccentric contractions (Brockett et al., 2004; Lynn & Morgan, 1994;
Morgan, 1990; Whitehead et al., 1998). In contrast, the CON group displayed shorter
fascicles and greater pennation angles, which are likely representative of reduced sarcomeres
in-series and more in-parallel.
The torque-joint angle relationship and the joint angle at which muscles generate their peak
torques are proposed to change in response to changes in muscle fascicle lengths (Brockett et
al., 2004). Indeed, eccentric training programs have been shown to increase fascicle lengths
(Timmins et al., 2016) and, in separate studies, push the torque-joint angle relationship
towards longer muscle lengths (Kilgallon, Donnelly, & Shafat, 2007). To our knowledge, this
study is the first to have examined biceps femoris fascicle lengths and isometric knee flexor
torque-joint angle relationships simultaneously. It has also been shown that the joint angle of
peak torque can be shifted towards a lesser or greater angle as a consequence of eccentric
(Brockett, Morgan, & Proske, 2001) or concentric (Brockett et al., 2004) training,
respectively. While the aforementioned studies suggest the angle of peak torque is indicative
of fascicle length the current results do not support this proposal. For this phenomenon to
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exist it would be expected the ECC group exhibit larger strength gains at longer lengths and
the CON group exhibit greater strength gains at shorter lengths. In the current study no such
shift occurred, despite a ~2 cm fascicle length difference between groups after training. These
findings suggest that there is not a strong relationship between isometric knee flexor torque-
joint angle relationships and fascicle lengths. The lack of change in isometric strength
occurred despite significant increases in eccentric strength in both groups and significant
increases in loads lifted by the concentric training group. A degree of task specificity is
commonly seen after resistance training (Behm & Sale, 1993), however, we had expected
some improvements in isometric performance.
Low levels of eccentric strength in Australian Rules football (Opar et al., 2015) and soccer
(Timmins, Bourne, et al., 2015) and between limb imbalances in Rugby (Bourne et al., 2015)
using the NeST have been shown to increase the risk of future hamstring strains, while short
biceps femoris long head fascicles are associated with a higher risk of injury in soccer
(Timmins, Bourne et al., 2015). In this study, there were statistically significant
improvements in Nordic eccentric strength for both groups but these were associated with
opposite directions of change in fascicle lengths. This observation may suggest that the use of
NeST alone may not provide the best indicator of future injury risk and should be coupled
with the measurement of BFLH fascicle lengths (Timmins, Bourne, et al., 2015).
We acknowledge that there are some other limitations to the current study. Muscle
architectural change was only assessed in the BFLH and these adaptations may differ between
knee flexors. There is also a degree of estimation required with the measurement of fascicle
length using 2D ultrasound because the entire length of the BFLH fascicles is not visible in
ultrasound images. While the estimation equation used in this study has been validated
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against cadaveric samples (Klimstra et al., 2007), we recognise the room for error and
suggest future studies employ extended field-of-view ultrasonography to reduce this. Despite
our recreationally active participants displaying similar, or higher, levels of Nordic eccentric
strength compared with elite Australian Rules footballers (Opar et al., 2015) and professional
soccer players (Timmins, Bourne, et al., 2015) it remains to be seen if the sprint recovery
results are applicable to other ‘well-trained’ populations.
This is the first study to show that concentric and eccentric knee flexor training do not have
different effects on decrements in sprint performance when two sprint training sessions are
held 48 h apart. It remains possible, however, that concentric and eccentric training for
longer periods or in more highly trained athletes may have different effects to those observed
here.
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Chapter 7: General Discussion
The primary aims of this program of research were to: 1) investigate the effect of
accumulated training and match loads on hamstring strain injury risk within elite Australian
Rules football players; 2) assess the acute effects of repetitive drop punt kicking on hamstring
strength and muscle activation; and 3) compare the effects of concentric and eccentric knee
flexor strengthening exercises on the recovery of performance between consecutive sprint
running sessions.
Study 1 was the first to investigate relationships between running distances and risk of
hamstring strain injuries. This paper showed that players who sustained hamstring injuries
had performed significantly more high-speed running in the four weeks prior to injury. The
results also suggest no relationship between hamstring strain injury risk and the player’s s-
RPE, total running distances, the absolute distance of high-speed running, or accelerating or
decelerating distances. This study highlights the effect unaccustomed distances of high-speed
running has on hamstring strain injury risk whilst also emphasizing the importance of load
monitoring on an individual basis. As high-speed running is the primary mechanism for a
hamstring injury, applying appropriately planned and monitored distances of running may
allow the risk of subsequent hamstring strain injury to be reduced.
Study 2 described the immediate effects of drop punt kicking on isokinetic concentric and
eccentric knee flexor strength and hamstring muscle activation. These results showed that
strength loss after repetitive kicking is highly specific to eccentric muscle contractions and
whilst declines in sEMG were observed in both medial and lateral hamstrings, only
reductions in lateral hamstring sEMG was associated with eccentric strength losses. These
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findings are consistent with previous studies measuring the same variables as a consequence
of repeated sprint running (Greig, 2008; Small et al., 2010; Timmins et al., 2014) and may
provide further insight into why the lateral hamstring muscle is the most commonly injured
(Opar et al., 2015; Timmins, Bourne, et al., 2015).
Study 3 found no differences between the effects of concentric and eccentric knee flexor
strength training on the recovery of sprint performance when two consecutive high intensity
running sessions were performed 48 hours apart. Contrary to the hypothesis, there were also
no differences in the measured markers of muscle damage between eccentrically or
concentrically trained individuals. However, two participants from the concentric-only
training group could not complete the second session due to a hamstring strain and
‘soreness’, which may suggest insufficient recovery between sessions. These data also
suggest no relationship exists between isometric knee flexor torque-joint angle relationships
and hamstring fascicle lengths.
This program of research may provide assistance in determining the direction of future
research. Study 1 produced a model which may, once tested in the practical setting, help
reduce hamstring strain injuries via effective running load monitoring. Study 2 showed the
effects of kicking induced fatigue on hamstring function is similar to how the muscle
responds after running. However, there is currently a dearth of epidemiological data reporting
the most commonly injured hamstring muscle during the kick. Future studies should employ
a prospective design that records the amount and intensity of kicks performed and possible
relationships to injury risk. Study 3 found no differences in the measured markers of muscle
damage that occurred as a consequence of sprinting Future work should focus on the effects
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of eccentric-only, concentric-only and combined eccentric and concentric work (such as that
seen in conventional resistance training) on a wider range of performance variables.
In summary, this program of research has contributed to the hamstring strain injury literature
by providing information based on the two most common mechanisms involved with injury
onset and added further insight into the muscular adaptations induced by eccentrically biased
exercises. Study 1 and 2 highlight the importance of monitoring training session volumes and
intensities of high-speed running and kicking, while Study 3 found that there was no
difference between the effects of concentric nor eccentric knee flexor strength training on the
recovery of sprint performance or indices of muscle damage after a high-intensity sprint
running session. However, further work is needed to explore the effects of strength training
on the performance of and recovery from high-speed running.
124
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Appendices
Appendix A.
Cardiovascular & Injury History Questionnaires
CARDIOVASCULAR RISK FACTOR QUESTIONNAIRE
To be eligible to participate in the experiment you are required to complete the following
questionnaire which is designed to assess the risk of you experiencing any harm during the course of
the study. A full and honest disclosure of your medical history is vital for your own safety.
Name: ________________________________________________Date of Birth: ______________
Age: __________years Weight: __________kg Height: __________cm
Give a brief description of your average weekly activity pattern:
Please tick / answer the appropriate responses for the following questions:
1. Are you overweight? Yes No
Don’t Know
2. Do you smoke? Yes No
Don’t Know
3. Does your family have a history of premature
(<70 years) cardiovascular problems (e.g. heart
attack, stroke)? Yes No Don’t Know
4. Are you asthmatic? Yes No
Don’t Know
5. Are you diabetic? Yes No
Don’t Know
138
6. Do you have high blood cholesterol levels? Yes No
Don’t Know
7. Do you have high blood pressure? Yes No
Don’t Know
8. Do you have low blood pressure? Yes No
Don’t Know
9. Do you have a heart murmur Yes No
Don’t Know
10. Do you have, or have you ever had, any blood-
clots in any of your blood vessels (e.g. deep-
vein thrombosis)? Yes No Don’t Know
11. Do you have, or have you ever had, any
tendency to bleed for long periods after cutting
yourself? Yes No Don’t Know
12. Are you currently using any medication Yes No
If so, what is the medication?
13. Have you ever experienced any of the following during exertion (exercise or physical labour)
or at rest?
Light headedness or dizziness
Pain in the chest, neck, jaw or arm
Numbness or pins-and-needles in any part of your body
Loss of consciousness
14. Do you think you have any medical complaint or any other reason which you know of that
may prevent you from safely participating in intense exercise?
Yes No
If yes, please elaborate.
139
I, ________________________________________________, believe that the answers to these
questions are true and correct.
Signed: ________________________________________________
Date: ________________________
