Zurich Open Repository andArchiveUniversity of ZurichMain LibraryStrickhofstrasse 39CH-8057 Zurichwww.zora.uzh.ch

Year: 2004

Compensatory mechanisms of weightbearing lameness in horses: a novelapproach by measuring vertical ground reaction forces on an instrumented

treadmill

Weishaupt, Michael A

Posted at the Zurich Open Repository and Archive, University of ZurichZORA URL: https://doi.org/10.5167/uzh-23374Habilitation

Originally published at:Weishaupt, Michael A. Compensatory mechanisms of weightbearing lameness in horses: a novel approachby measuring vertical ground reaction forces on an instrumented treadmill. 2004, University of Zurich,Vetsuisse Faculty.

Compensatory mechanisms of weightbearing lameness

in horses – A novel approach by measuring vertical ground

reaction forces on an instrumented treadmill

Michael Andreas Weishaupt, Dr. med. vet.

Zurich, 2004

Equine Hospital, Equine Performance Centre

Vetsuisse-Faculty University of Zurich

Weishaupt, Michael Andreas

Compensatory mechanisms of weightbearing lameness

in horses – A novel approach by measuring vertical ground

reaction forces on an instrumented treadmill

Michael A. Weishaupt

Equine Hospital, Equine Performance Centre

Vetsuisse Faculty University of Zurich

Winterthurerstrasse 260

CH-8057 Zurich

Switzerland

Phone: ++41 44 635 84 34; Fax: ++41 44 313 03 84

Email: mweishaupt@vetclinics.unizh.ch

PhD-Thesis Vetsuisse Faculty, University of Zurich

ISBN: 3-9521627-3-6

Subject: Horse, lamness, gait analysis, kinetics

Copyright by M. A. Weishaupt 2004

all rights reserved

Publication l “Instrumented treadmill for measuring vertical

ground reaction forces in horses” has been reproduced with kind

permission of the American Journal of Veterinary Research.

Publication ll “Assessment of gait irregularities in the horse:

eye vs. gait analysis” has been reproduced with kind permission

of the Equine Veterinary Journal.

Publication lll “Vertical ground reaction force-time histories of

sound Warmblood horses trotting on a treadmill” and publication

lV “Compensatory load redistribution of horses with induced

weightbearing forelimb lameness trotting on a treadmill” have

been reproduced with kind permission of the Veterinary Journal.

To my parents and Moni

«Sei der Gang deines Pferdes unter Dir wie die Bahn eines Sterns.

In deiner fühlenden Hand, in deinem schwingenden Leib,

in deinem schwebenden Herzen liegt die Kurve und pfeilgerader Weg,

liegt Anfang und Ende, liegt die unermessliche Poesie der Bewegung,

liegt die lebendige Kraft»

Rudolf G. Binding, Reitvorschrift für eine Geliebte, 1926

Supervisor

Prof. Dr. Jörg A. Auer

Equine Hospital, Vetsuisse Faculty University of Zurich

Switzerland

Co-Supervisors

Prof. Dr. Edgar Stüssi

Laboratory for Biomechanics, ETH Zurich, Switzerland

Dr. Jachen Denoth

Laboratory for Biomechanics, ETH Zurich, Switzerland

Prof. Dr. Hans Hoppeler

Institute of Anatomy, University of Berne, Switzerland

Content

General Introduction 9

Introduction 11

Locomotion analysis of equine lameness 12

Purpose and outline of the thesis 20

PhD-Publications 23

I. Instrumented treadmill for measuring vertical 25

ground reaction forces in horses

Summary 26

Introduction 27

Materials and Methods 28

Results 34

Discussion 37

ll. Assessment of gait irregularities in the horse: 43

eye vs. gait analysis

Abstract 44

Introduction 45

Materials and Methods 46

Results 51

Discussion 52

Glossary of abbreviations 55

lll. Vertical ground reaction force-time histories 57

of sound Warmblood horses trotting on a treadmill

Summary 58

Introduction 59

Material and Methods 60

Results 62

Discussion 66

lV. Compensatory load redistribution of horses 71

with induced weightbearing forelimb lameness

trotting on a treadmill

Abstract 72

Introduction 73

Material and methods 74

Results 76

Discussion 87

General Discussion and Conclusions 95

Methodological aspects 96

Compensatory mechanisms of weightbearing lameness 98

References 105

Summary 123

Scientific Presentations on the PhD-Topic 128

PhD Courses and Exams 129

Acknowledgments 130

Curriculum vitae 131

l 9

General Introduction

Introduction

The functional integrity of the locomotor apparatus is the most

important prerequisite for successful athletic performance in

sports horses. Nevertheless, musculoskeletal injuries are the main

reason for wastage in the horse industry worldwide121. Epidemi-

ological studies revealed that more than 50% of race horses expe-

rienced some period of lameness and in 20% of those cases the

lameness was sufficient to prevent the individuals from racing

after the injury67. Furthermore, it is estimated that three quarters

of poorly performing horses have subclinical disorders of the loco-

motor system101. Even with the best supervision of rider or trainer,

training programs, conformation and accidents can all contribute

to injury that may influence the ability to exercise successfully54.

Therefore, prevention and early identification of locomotor dis-

eases have a high priority in equine sports medicine and animal

welfare and justify profound research81,83.

Lameness is a symptom and can be defined as an alteration of

the normal gait pattern caused by a functional or structural dis-

order in the locomotor system. The locomotor behaviour of the

horse is far from being fully understood. Further refinement of

lameness diagnosis and information describing the effects of

lameness on mechanics and energetics in relation to athletic per-

formance are still needed. Clinicians assess lameness by associat-

ing the actual gait with experienced reference movements of nor-

mal subjects, by matching the partial movements of the left with

the right body side or by comparing the gait before and after an

intervention such as a diagnostic nerve block. Visual assessment

carries all risks that are inherent in subjectivity and therefore,

interpretation of clinical signs depends directly on the expertise of

the observer. Because gait compensations made by lame horses

may occur fast or in a subtle manner, the temporal resolution of

the human eye is easily overtaxed. Thus, subtle lameness and the

complex nature of compensatory movements can not always be

assessed adequately and lack reliability, especially when re-evalu-

ated repeatedly. Gait analysis in a quantitative manner offers a

better reproducibility, higher spatio-temporal resolution and is

less dependent on the experience of the observer. Kinematic

and kinetic techniques proved to be reliable, but to date, clinical

application of these techniques has still been limited by the

time and expertise required to extract appropriate and clinically

relevant data.

l 11G E N E R A L I N T R O D U C T I O N

12 l

Locomotion analysis of equine lameness

Human beings have always been attracted by the outstanding

athletic capabilities and locomotor skills of the horse. Basic ques-

tions such as the footing sequence of the different gaits or the

relationship between conformation and performance, puzzled

early researchers and stimulated the development of technical

gadgets, able to assess the horse in motion. Eadweard Muybridge

(1830–1904) invented the synchronised series photography. He is

regarded as the founder of modern kinematic research and of

locomotor research in general82. It was also Muybridge, who in his

fascinating book “Animals in Motion”104, documented horses suf-

fering from fore- and hindlimb lameness. On these serial photos

compensatory movements, such as the characteristic asymmetric

head nodding can be already recognised (Figure 1).

With the booming economy in the early nineteen-seventies a

fresh impetus was given to equestrian sports and related sciences.

Two special fields of biomechanics evolved and were established

in equine gait analysis: the kinematics and the kinetics.

G E N E R A L I N T R O D U C T I O N

Figure 1: Serial photographs made by Eadweard Muybridge.104

l 13

Kinematics

Kinematic analysis measures the geometry of movement without

considering the forces that cause the movement.

The trajectories of body segments are quantified by temporal,

linear and angular variables. These variables contain essentially

the same information which is presented to the eye of the

observer but with a higher spatio-temporal resolution. In the last

fifteen years, electrogoniometry1 and cinematography51,52 which

required manual digitising of marked body landmarks, were

replaced by auto-tracking video-based or optoelectronic systems.

Three-dimensional analyses and digital high-speed videography

with temporal resolutions up to 1000 frames per second were

established parallel with the availability of more and more pow-

erful personal computers.

Buchner and co-workers at the Utrecht University conducted

possibly the most comprehensive kinematic investigation on com-

pensatory movements of lame horses. Synonymous for an uncom-

plicated weightbearing lameness, the sole pressure model,

designed by Merkens and Schamhardt95 was used for a detailed

study of the changes caused by an unilateral forelimb, hindlimb

or a bilateral forelimb lameness in walking and trotting

horses17,18,20,21. Other researchers also made use of this lamenes

model to study the compensatory movements of weightbearing

lameness43,53,71.

The ongoing ambition to diagnose the precise localisation of the

ailment with the help of gait analysis parameters, led to the inves-

tigation of specific lamenesses, such as navicular disease57–59,69,114,

tendinitis of the superficial digital flexor tendon37, carpal lame-

ness6,108,109 or spavin13,79. Hereby, either clinical cases or lameness

models (synovitis /arthritis, tendinitis models) were used.

Clayton27–31 documented other specific orthopaedic diseases

more on the basis of case reports. It is obvious, that these single

cases made it difficult to relate the kinematic changes with the

clinical symptomatology and therefore did not allow the drawing

of a general conclusion. Repeated recordings before and after

diagnostic nerve blocks and the assessment of a statistically

relevant number of identical cases are necessary to identify the

characteristic features of a specific disease.

A number of studies addressed the question of correspondence

between clinical judgment of lameness and the degree of asym-

metry of head, trunk or limb movement. To achieve this, groups

of horses with various causes and severity of forelimb or hindlimb

G E N E R A L I N T R O D U C T I O N

14 l

lamenesses were analysed 3,14,70,106,107,133. Correspondence

between subjective and objective assessment increases with the

expertise of the observer and is generally better in forelimb than

hindlimb lameness. Furthermore, subjective judgement corre-

sponds better with measures of vertical movement of the head

and trunk than with measures of the limbs. The amplitude of ver-

tical displacement and acceleration of the head, sternum or

sacrum can be assessed without much technical expenditure by

the use of locally mounted accelerometers10,11,72,137.

With exception of the aforementioned studies of Buchner, all

other studies were confined to observations at the trot, the pre-

ferred gait in the clinical lameness evaluation.

Combing kinematic data with a segmental body model of a

standard horse, the 3-dimensional movement of the body centre

of mass (BCM) can be reconstructed23. The BCM is a key factor in

the analysis of equine gait, as its position and trajectory deter-

mines the distribution of loads within the limbs. The movement

of the BCM of horses with forelimb lameness followed closely the

movement of the trunk and experienced the same apparent

changes in the vertical axis; sagittal and transversal shifts were less

pronounced25,26.

Kinematic lameness studies in horses demonstrated, that a par-

ticular problem, e.g. in a joint, changes not only the movement

of the respective limb, but also influences the entire movement

pattern of the horse. It is not surprising that the compensatory

changes in the movement pattern of the limbs, the head, and the

trunk, which are the key observation points in visual gait assess-

ment, also turned out to be to most reliable ones for kinematic

analyses.

In weightbearing lamenesses, the most consistent changes

were observed in the maximal fetlock hyperextension and the

maximal coffin joint flexion angles, as well as in the vertical dis-

placement and maximal vertical acceleration of the head and

tuber sacrale. Asymmetric vertical head nodding is probably the

most obvious sign of weightbearing asymmetry between fore-

limbs21,69,109,137. Correspondingly, the vertical movement of the

tuber sacrale shows less lowering and lifting during the stance

phase of the lame hindlimb21. The trajectory of the tuber coxae is

composed of a vertical translational movement of the trunk and

the rotational movements of the pelvis around the vertical and

longitudinal axes. During a stride cycle at the trot, the tuber coxae

shows a biphasic movement with a slightly smaller amplitude of

G E N E R A L I N T R O D U C T I O N

motion during the stance, than during the swing phase of the

concerning limb14. Lameness amplifies the difference between

these two amplitudes in the affected hindlimb: the vertical ampli-

tude is diminished or even absent during the stance phase and

enlarged during the swing phase 21,85. Maximal fetlock hyperex-

tension was shown to be an indirect measure for the vertical

ground reaction force117,118 and is reduced in the lame limb at mid-

stance proportionally to the degree of lameness. In a group of

horses with moderate to severe lameness, fetlock hyperextension

decreased in average by 10°, which corresponded to a reduction

in peak vertical force of 27%37. In the contralateral, sound limb,

a compensatory increase of fetlock hyperextension could be

observed20. In proximal joints, such as the shoulder and tarsal

joints, flexion increases in the lame limb, indicating smooth limb

loading controlled by extensor muscles7,63.

The mixed lameness is defined as a lameness where pain or pain

reactions are obvious during the stance and swing phases. During

the stance phase the above mentioned typical changes of weight-

bearing lameness can be observed. During the swing phase, flex-

ion of the affected and linked joints (carpal, tarsal) is decreased.

Both the maximal flexion, as well as the total range of motion is

restricted, which gives the impression of a stiff leg, which is pro-

tracted with a lower flight arc of the hoof.

Temporal stride variables are claimed to be of questionable

value in detecting lameness because of controversial results

reported in former investigations. Firstly, mild lamenesses do not

show significant temporal deviations from the sound stride pat-

tern and secondly, key parameters, such as stance duration or the

time of diagonal advanced placement maintain their left-to-right

symmetry with increasing lameness18,59,132. Temporal asymmetry

is better interpreted as a sign of individual locomotor pattern, also

known as sidedness, handedness or laterality41,49,91. However,

Buchner et al.18 could show that in trotting horses with moderate

forelimb lameness, the contralateral step and suspension dura-

tions become slightly asymmetric.

Selected kinematic parameters allow identification of the

affected limb, quantification of the degree of lameness, and clas-

sification into supporting or swinging limb lameness. The ambi-

tious goal to find characteristic compensatory patterns that relate

to the localisation of the aliment has still not been reached. It is

speculated that horses adapt their movement to pain in a limb in

l 15G E N E R A L I N T R O D U C T I O N

a rather uniform way, possibly because of the limited degree of

freedom in their locomotion patterns24.

Kinetics

Kinetic analysis studies the forces that are responsible for a move-

ment.

The first evidence of kinetic studies in equine locomotion dates

back to 1874. Etienne Jules Marey’s (1830–1904) “Chaussure

exploratrice”, which consisted essentially of an India rubber ball,

filled with horsehairs and fitted between the branches of the

horseshoe over the frog was able to record pressure changes at

hoof placement (Figure 2A). The pressure fluctuations were trans-

mitted by airtight rubber tubes and registered on a charcoal-

blackened rotating cylinder, which the rider held in his hand

(Figure 2B). Marey was the first to refer to vertical forces while

describing equine stride patterns.

Kinetics distinguish between internal and external forces and

torques. Internal forces, such as tendon forces or bone strains may

be determined directly; however, these measurements are invasive

and therefore limited to research applications. A more elegant

way to estimate internal forces and torques is to make use of

inverse dynamic models. Inverse dynamics calculate the forces

that were the cause of an observed motion, applying the laws of

dynamics (Isaac Newton, England, 1642–1727) to a linked multi-

segment model (Joseph Louis Lagrange, France/Italy, 1736–1813)

16 l G E N E R A L I N T R O D U C T I O N

Figure 2A, 2B: Etienne Jules Marey’s “Chaussure exploratrice”

of the respective object of study. The bases for these calculations

are time-locked kinetic and kinematic data of a particular move-

ment. After having established normative data for the

horse35,36,39,40,75,76, this approach has been applied to clinically

relevant questions 38,89,90.

The work presented here, focuses on the external forces, i.e. the

forces exerted between the hoof and the ground. The ground

reaction force (GRF) can be measured directly by the use of a force

plate77,112,131, force shoes9,45,65,68,116,119 or more sophisticated force

measuring systems such as the Equine Gait Analysis (EGA)5,66,132.

The most established technique nowadays, is the force plate.

Other concepts are reserved for research investigations. Standard

force plates are able to split the input force into its three orthog-

onal components, the transverse-horizontal (Fx), the longitudinal-

horizontal (Fy) and the vertical force (Fz) component.

Force plate reference data exist from sound walking92,94, trot-

ting8, 99, cantering horses98, of ridden horses at the walk and

trot124 and of horses at take-off and landing, clearing a 0.8–1.3m

high fence125. At the walk, peak vertical forces (Fzpeak) reach in

average 66% (second peak) of bodyweight (bwt) in the forelimbs

and 51% (first peak) in the hindlimbs92, at the trot 118% in the

fore- and 104% in the hindlimbs99 and at the canter 101% in the

trailing hindlimb, 115% in the leading hindlimb, 147% in the

trailing forelimb and 122% in the leading forelimb98. When clear-

ing a 0.8 m high fence, vertical GRF amplitudes were of similar

magnitude to those measured at the canter. Data of a single horse

indicated that peak forces increased sharply with greater fence

heights. Attacking a 1.3 m high fence at the right hand canter, the

highest Fzpeak at take-off were observed in the trailing fore- and

hindlimb (173% and 143% of bwt, respectively); during landing

Fzpeak decreased gradually along the step sequence from 204% of

bwt in the trailing forelimb, to 153% in the leading fore- and

trailing hindlimb, to 122% in the leading hindlimb125. Particularly

remarkable was the observation that the magnitude of forces

would vary according to the jumping technique of the specific

horse. Ratzlaff et al.115 measured vertical forces in horses at rac-

ing speed (13.7–15.8 m/s) using instrumented horseshoes. On the

straight the greatest vertical forces were exerted on the leading

forelimb, followed by the leading hindlimb; on the banked turns

the greatest forces were taken by the leading and the trailing fore-

limb confirming the observation that the majority of racing

injuries occur in the forelimbs. Unfortunately, the absolute figures

l 17G E N E R A L I N T R O D U C T I O N

of the reported Fzpeak are hardly plausible, probably because of the

central localisation of a single piezoelectric transducer over the

frog of the hoof.

Kinetic methods and their derived parameters have proven to

detect and quantify locomotor unsoundness in horses19,37,46,65,66,

95–97,100,144,145 and to monitor clinical progress reliably 4, 60,73,129.

However, when using force plates, problems in obtaining repeat-

able, constant speed trials 88 or targeting the platform may pres-

ent substantial restrictions, especially when dealing with

quadrupeds. Depending on the gait, two to 20 attempts are

needed to register a single, valid foot strike92, 93, 98, 99. Conse-

quently, data acquisition and processing are extremely time-

consuming, which often limited the former kinetic studies to the

comparison between contralateral limbs 37,46. Furthermore, with a

single force plate, GRF data of only one limb can be recorded at

the time while at least one other limb supports the body concur-

rently. To get a complete indication on the load redistribution in

case of lameness, force-time histories of all 4 limbs are indispen-

sable. A valuable approach was undertaken by Merkens and

Schamhardt95,96, who compiled mean representative GRF data of

each of the 4 limbs with time information derived from high-

speed film analyses of lame horses at the walk. For the trot,

Morris and Seeherman100 reported on load distribution in horses

with unilaterally induced carpal lameness, however, without con-

sidering the underlying temporal mechanisms.

From these studies it is known that weightbearing lameness

affects mainly the vertical and longitudinal-horizontal forces,

whereas changes in the transverse-horizontal forces are negli-

gible. At the walk, in unilateral forelimb as well hindlimb lame-

nesses, reduced loading of the lame limb is primarily compensated

by the contralateral limb and in a lesser extent by concurrently

loaded limbs93. At the trot, in unilateral forelimb lameness, load

is redistributed to the contralateral limb and, within the lame

diagonal stance, to the diagonal hindlimb100,132. Hereby the com-

pensatory movements of the head and neck play a central role.

The head and neck represents about 10 % of the total body

mass22,80 and has, with the long lever arm relative to the body

centre of mass, an important effect on loading and unloading the

forelimbs. Model calculations estimate that differences of only

10 cm in the vertical amplitude of the head during the stance

phases of the lame and sound forelimb cause differences in verti-

18 l G E N E R A L I N T R O D U C T I O N

cal force of nearly 500 N and differences in the sagittal torque act-

ing on the trunk of about 230 Nm136.

The current quantitative analysis of sound and lame horses

revealed subtle deviations in gait pattern caused by lameness. In

contrast, intra-individual and even wider inter-individual variabil-

ity was observed. To differentiate reliably between a gait irregu-

larity and an individual gait asymmetry or between different

degrees of lameness, the intra- and inter-individual gait variabil-

ity has to be smaller than the changes caused by the lameness.

Inter-individual variability may be controlled by normalisation

algorithms, which account for body mass and size 64,74. Intra-indi-

vidual variability depends strongly on the experimental set-up.

Particularly, locomotion velocity has a direct influence on gait

parameters, both, in dogs 86,87,122,123 and horses33,34,42,74–76,84,88,135.

Regularity of gait and standardisation of speed are a prerequisite

for comparison of multiple assessments of the same animal or

between different patients. An approach to minimise the factors

which contribute to the extrinsic variability is to study the subject

on a treadmill.

Treadmill

Possibly the first equine treadmill used scientifically was operated

in the late 19 th century by N. Zuntz and C. Lehmann147 (Figure 3).

The belt of the treadmill had to be either moved by the horse itself

or was driven by a steam engine. The German scientists con-

ducted calorimetric experiments of horses during walking and

trotting work, at various inclines and while pulling different

l 19G E N E R A L I N T R O D U C T I O N

Figure 3: First research treadmill for horses used by N. Zuntz

and C. Lehmann (late 19th century).147

20 l

draught loads148. They even managed to measure the energy

requirements of a horse backing up!

Since the re-introduction of the modern high-speed treadmill in

the mid-sixties by Persson110, this instrument gained a wide spread

acceptance as a research tool in equine exercise physiology126,127.

Increasingly, the treadmill is used for clinical investigations such

as exercise testing, dynamic evaluation of the upper airways or to

support qualitative gait analyses, i.e. the visual subjective assess-

ment, if necessary, combined with video recordings128. Once

adapted to the treadmill, horses have regular gait patterns with

minimal intra-individual variations16. The variation of extrinsic fac-

tors such as ground condition, environmental diversions, and

especially subject velocity 74–76, 88 are known as a source of inter-

ference when performing gait analyses in the overground situa-

tion. On the treadmill those variables can be easily controlled and

consequently standardised.

Theoretically, treadmill locomotion does not differ from over-

ground locomotion as long as the treadmill belt velocity is con-

stant134. Nevertheless, differences in energetic130 and biomechan-

ical15 variables were documented for horses. Comparative kine-

matic analyses of treadmill and overground locomotion revealed

reduced stride frequency, prolonged relative stance duration of

the forelimbs, preceding forelimbs at diagonal touch-down,

greater retraction of both fore- and hindlimbs and reduced verti-

cal excursion of the hoofs during the swing phase and of the with-

ers at midstance on the treadmill15. The noticed alteration of gait

has to be interpreted like those observed between different sur-

face conditions, such as rubber mat and concrete15. These small

differences in temporal and spatial parameters do not disqualify

the treadmill for gait analyses as long as cross-comparisons are

avoided.

Purpose and outline of the thesis

The review of the current state of knowledge and methodologi-

cal possibilities in equine locomotor research led to the following

purpose of the presented thesis. It can be divided into 3 parts:

In a first part, a novel kinetic gait analysis system, capable of

simultaneously determining the vertical GRF of all 4 limbs had to

be developed and validated. The technology should be imple-

mented on a treadmill to benefit from the standardisation of

G E N E R A L I N T R O D U C T I O N

extrinsic factors affecting locomotion. The instrumentation of the

horse had to be held to a minimum. The system should meet the

required standards of precision for locomotor research and should

be operational in a clinical set-up, i.e. data acquisition should be

rapid and safe for the patient as well as yielding the relevant data

instantaneously.

The validation procedure should comprise (1) the comparison of

the treadmill forces with synchronised force traces measured with

force shoes and (2) the comparison of the measured weightbear-

ing symmetries/asymmetries of sound and unsound riding horses

with the consensus statement of 3 experienced clinicians. Hereby,

not only the affected limb, but also to degree of lameness should

be considered.

The second objective was to establish representative treadmill

GRF and inter-limb co-ordination time data of clinically sound

horses at the trot. In a research setting, data may be collected

from the same horses before and after induction of lameness. In

a clinical environment, the procedure is to compare data before

and after diagnostic nerve blocks or to reference data of clinically

normal horses moving at the same gait and velocity.

The third part of this thesis aimed to quantify the load and time

shifting mechanisms of weightbearing lamenesses and to identify

the most sensitive parameters determinable with the instru-

mented treadmill. In a first stage, the investigation focused on

analysis of changes at the trot caused by forelimb lameness.

To study these compensatory mechanisms, the use of horses

with naturally occurring lamenesses may pose some problems.

Former studies showed that changes in the locomotion pattern

caused by lameness are rather small compared to the large inter-

individual variations observed within a group of “reference”

horses. This complicates the task of distinguishing between the

individual motion pattern and the standard compensatory pattern

resulting from lameness. A way to overcome this problem is to use

the individual horse as its own control and study the changes in

motion pattern after an intervention which normalises gait, such

as a selective positive nerve block. However, even horses suffer-

ing from very similar diseases or syndromes show heterogeneity

of clinical presentations. Additionally, it is difficult to further

exclude additional problems possibly located in the same limb or

in other limbs.

A technique whereby a more or less transient lameness is

induced in groups of sound horses, offers an important and reli-

l 21G E N E R A L I N T R O D U C T I O N

22 l G E N E R A L I N T R O D U C T I O N

able method to study the locomotion pattern of specific well-

defined lamenesses in a controlled manner that minimizes indi-

vidual variation. The locomotion pattern of each horse before

inducing lameness serves as an individual control and small but

expectantly typical deviations can be attributed to a specific loco-

motor disorder.

A number of methods have been described in the literature to

induce experimental lamenesses in horses:

Intra-articular injection of Lipopolysaccharides50 or Ampho-

tericin109 was used to induce synovitis / arthritis in joints, such as

the carpus6,108,109 or the tarsus79. A chip fracture of the radial

carpal bone was created surgically in an osteoarthritis lameness

model 4,55,100. These models were used to induce a mixed, sup-

porting-swing limb lameness. Intra-tendinous injection of Colla-

genase was used to induce superficial digital flexor tendini-

tis 37,60,129. All methods, except the Lipopolysaccharide-arthritis,

resulted in a relatively long-standing lameness, sometimes with-

out a return to normal function. Furthermore, these methods do

not allow the induction of more than one degree of lameness in

the same horse within a short time.

A more elegant method, which imitates the well-known effect

of a stone trapped under the horseshoe, was described by

Merkens & Schamhardt 95. Bolts are screwed into nuts welded to

the inner rim of each branch of the horseshoe. While loading the

limb, pressure is applied to the corium of the sole. By tightening

or loosening the bolts, various degrees of lameness can be

elicited. This mechanical lameness model has been used in many

studies17,18,20,21,25,26,43,53,71. It has a distinct advantage over chem-

ical models, since the degree and duration of lameness can be

controlled easily. From the ethical point of view, this transient

lameness model has to be favoured and was, therefore, chosen

for this study.

l 23

PhD-Publications

I. Instrumented treadmill for measuring vertical ground reaction forces

in horses

Michael A. Weishaupt, Hermann P. Hogg, Thomas Wiestner, Jachen Denoth,

Edgar Stüssi, Jörg A. Auer.

American Journal of Veterinary Research (2002) 63(4); 520–527.

II. Assessment of gait irregularities in the horse: eye vs. gait analysis

Michael A. Weishaupt, Thomas Wiestner, Hermann P. Hogg, Patrick Jordan,

Jörg A. Auer, Eric Barrey.

Equine Veterinary Journal (2001) Supplement 33; 135–140.

III. Vertical ground reaction force-time histories of sound Warmblood horses

trotting on a treadmill

Michael A. Weishaupt, Thomas Wiestner, Hermann P. Hogg, Patrick Jordan,

Jörg A. Auer.

Veterinary Journal (2004) 168; 304–311.

IV. Compensatory load redistribution of horses with induced weightbearing

forelimb lameness trotting on a treadmill

Michael A. Weishaupt, Thomas Wiestner, Hermann P. Hogg, Patrick Jordan,

Jörg A. Auer.

In press at the Veterinary Journal (2004).

l 25

I. Instrumented treadmill for measuring vertical ground reaction forces in horses

Michael A. Weishaupt, Hermann P. Hogg, Thomas Wiestner, Jachen Denoth,

Edgar Stüssi, Jörg A. Auer

American Journal of Veterinary Research (2002) 63(4); 520–527.

Summary

Objective – To develop and validate a novel instrumented tread-

mill capable of determining vertical ground reaction forces of all

4 limbs simultaneously in horses.

Procedure – 18 piezo-electric force transducers were mounted

between the treadmill frame and supporting steel platform to

measure the actual forces at the corresponding bearing points.

Each of the 18 sensor forces is equal to the sum of the unknown

hoof forces weighted with the transfer coefficients of the corre-

sponding force application points. The 4 force traces were calcu-

lated, solving at each time point the resulting equation system,

using the Gaussian least-squares method. System validation com-

prised the following tests: determination of the survey accuracy

of the positioning system, determination of the natural frequen-

cies of the system, linearity test of the force transfer to the

individual sensors, determination of superimposed forces with the

treadmill-integrated force measuring system (TiF) in a static

configuration, and comparison of vertical ground reaction forces

determined simultaneously by use of TiF and force shoes mounted

on the forelimbs of a horse.

Results – Comparison between static test loads and TiF-calculated

forces showed deviations of < 1.4%. Force traces calculated by

TiF and those recorded by use of the force shoes were highly

correlated (r>_ 0.998).

Conclusions and Clinical Relevance – This instrumented treadmill

allows a reliable assessment of the load distribution and interlimb

coordination in a short period of time and is therefore suitable for

experimental as well as clinical investigations.

26 l P h D - P U B L I C AT I O N I

l 27P h D - P U B L I C AT I O N I

Introduction

Disorders of the equine locomotor apparatus are the most wide-

spread reason for wastage in sports horses67,121. It is estimated

that three-fourths of poorly performing horses have subclinical

musculoskeletal problems101. Therefore, the early detection and

resolution of these conditions have high priority within the con-

text of sport medical care and animal welfare. Traditionally, the

evaluation of gait abnormalities is based on subjective assess-

ments and, therefore, relies strongly on the expertise of the ortho-

pedic clinician70. Various kinetic techniques for gait analysis

such as force plates37,46,95-97,100,144 force-measuring horseshoes65,

the Kaegi Equine-Gait-Analysis System66, and accelerometric

devices11,137 have been used to quantify locomotor unsoundness

and provide additional assistance in the interpretation of delicate

subclinical conditions139. Advanced diagnostic procedures or

therapeutic interventions could be monitored more objec-

tively 4,60,73. However to date, clinical application of all these tech-

niques including kinematic gait analysis has been limited.

Basically, all of the aforementioned kinetic measuring concepts

are confined by the number of ground contacts of simultaneously

measured limbs. When using force plates, problems in obtaining

repeatable, constant speed trials88 or targeting the platform may

present substantial restrictions especially, when dealing with

quadrupeds. Two to 6 attempts are needed to assess a single, valid

foot strike, depending on whether the horse is at a, walk, trot, or

canter 92,98,99. Consequently, data acquisition and processing are

extremely time consuming. Therefore, time-related ground reac-

tion force distributions of concurrently loaded limbs are still poorly

documented94, and alterations attributable to locomotor disorders

have not been systematically studied96. With the use of force-

measuring horseshoes, the number of consecutive strides is unre-

stricted, but the nature of instrumentation is delicate, and weight

and height of the horseshoe may alter the physiologic motion pat-

tern of a horse.

High-speed treadmills for horses are an established instrument

in equine exercise physiology and have proved to be useful for

visual gait and kinematic assessments128. Once adapted to the

treadmill, horses have regular gait patterns with minimal interindi-

vidual variations16. External factors such as ground condition,

environment, and subject velocity74,88 that influence gait charac-

teristics can be highly standardized.

28 l

The objective of the study reported here was to develop a system

that would combine the advantages of a treadmill with a force-

measuring system, able to record the vertical ground reaction

forces of all 4 limbs simultaneously over multiple strides138. Instru-

mentation of a horse was to be held to a minimum, and the data

should be instantly available. The design and validation of the sys-

tem are documented and possible applications proposed.

Materials and Methods

Compared to the classic force plate system, the treadmill-

integrated force measuring system (TiF) is based on a completely

different measuring principle. Single detached sensing compo-

nents for each individual hoof do not exist. On the contrary, the

horse walks entirely on a single, load-sensitive platform. Because

up to 3 hoof forces are acting simultaneously on this platform dur-

ing locomotion, the direct determination of the different forces is

not feasible. However, if each force application point (FAP) of the

acting forces is known, the individual forces can be calculated

from the entire ground reaction by solving a linear equation sys-

tem for the unknown hoof forces. Thus, the required components

of the TiF are: a load-sensitive treadmill platform, a positioning

system to localize the FAP on the treadmill platform, a force-

transfer coefficient matrix that represents the transfer character-

istics from every possible FAP to every sensor of the load-sensing

treadmill platform, a measuring system for time-locked data

acquisition, and a fast computer system for calculation of the

4 individual force traces of each limb.

Treadmill and force measuring system

An equine high-speed treadmilla was modified. The standard trac-

tion motor was exchanged for an engine with larger mass and

50 % more power (30 kW) to ensure constant velocity of the

treadmill belt15. The original treadmill platform was replaced by a

custom-designed, lightweight steel plate of only 5 mm in thick-

ness, transversally reinforced with underlying T-shaped steel sup-

porting structures. This plate has a high transversal (370 cm4/m)

and low longitudinal (1.04 cm4/m) moment of inertia. On each

long side, the platform was affixed on 12 equally distributed bear-

ing points to the treadmill frame. At each bearing, a damping ele-

ment (70 shore, spring constant 3 kN/mm) was inserted. Eighteen

P h D - P U B L I C AT I O N I

bearing points (9 on each side) were supplied with piezo-electric

force sensorsb, which measure the vertical force component at the

respective location (Figure 4). Force sensor outputs are amplified

with charge amplifiersc. The determined temperature behavior of

this sensor-amplifier arrangement was <±0.3 N/C for temperatures

between 20° and 50 °C and baseline drift was < ± 6.0 N/30 minutes.

Positioning system

The x- and y-coordinates of the FAPs (i.e., the position of each

hoof on the treadmill platform) are calculated by trigonometry,

based on angle values determined with incremental angular

encodersd. The combined quadrature outputs of each encoder has

an angular resolution of 0.09°. The goniometers are mounted on

aluminum rails, which are placed in parallel orientation and

defined positions relative to the treadmill’s system of coordinates.

For each limb, a thin rubber string connects 2 goniometers via the

corresponding hoof (Figure 4). Shafts of the goniometers have

lightweight needles (6 cm long) to which the rubber strings are

l 29P h D - P U B L I C AT I O N I

L8

R1 R2 R3 R4 R5 R6 R7 R8 R9

angu la r encoders

rubber s t r ing

force sensor t readmi l l p la t fo rm

Z

X

Y

L1 L2 L3 L4 L5 L6 L7 L9

no sensor

0 .5 m

treadmi l l f rame

t readmi l l f ramtreadmi l l p la t fo rmtrac t ion be l t shock absorber

R1 R2 R3 R4 R5 R6 R7 R8 R9

Figure 4: Schematic illustrations representing the top and the cross-

sectional lateral view of the instrumented treadmill for use in

determination of vertical ground reaction forces in horses. Notice

the 18 force sensors and the 4 triangulation units to determine the

hoof coordinates.

affixed. At the hoof, the rubber string is attached to a hook on a

L-shaped stainless-steel plate, which is inserted between the bear-

ing surface of the lateral hoof wall and the horseshoe. The out-

ward tension of this rubber string is on average 5 N.

Computer hardware and signal processing

Force-sensor signals are filtered with a 200-Hz anti-aliasing low-

pass filter (Bessel, second-order) and digitized with a 12-bit ana-

log-to-digital converter. The incremental angular values from the

8 encoders are processed by a hardware decoder to yield contin-

uous absolute values. All channels are sampled within 30µs at a

rate of 433 Hz. Input and timing operations are executed by a sin-

gle-board high-performance microprocessore. The resulting data

are transferred over a link (20 megabits /s) to a host computer f.

The measuring softwareg is programmed in C++ and performs the

calculations of the 4 vertical ground reaction force traces and

their analysis.

Force-transfer coefficient matrix

To calculate the forces acting on the treadmill platform, force-

transfer coefficients from each spot on the running area of the

treadmill to each of the 18 force transducers has to be known.

This array of coefficients was generated by rolling a single-wheel

calibration trolley with a weight of 2.85 kN longitudinally over the

entire treadmill platform. The procedure was repeated for approx-

imately every 3 cm in a transverse direction, resulting in a dense

array of calibrated points. After gauging, the coefficient matrix

was scanned for missing items, completed by linear interpolation,

and smoothed. The matrix covers a total walking sector of

3.5 x 1.3m and has a resolution of 0.5 cm. It also implements the

constructive characteristic of the platform suspension, having

non-measuring bearing points and, therefore, force shunts at

both edges of the platform.

Data processing and principles of the TiF force-calculation

procedure

Data processing starts immediately after initiation of a measure-

ment and involves several steps. The 18 force and 4 x and 4 y posi-

tional data strings are filtered, using a Finite Impulse Response

(FIR) low-pass filter with a cut-off frequency of 20 Hz (Kaiser-

Bessel window; 129 taps; transition range 5 to 95 % attenuation,

from 14.5 to 25.5 Hz; sampling frequency 433 Hz). The x and y

30 l P h D - P U B L I C AT I O N I

l 31

positions are corrected for the distance between the point of

attachment of the rubber string at the lateral hoof wall to the cen-

ter of the hoof. Preliminary tests showed that for multiple forces

at various FAPs, the response of a specific sensor is the linear

superposition of the respective weighted input forces. Therefore,

for each sampling moment, 18 linear equations can be formu-

lated, each containing 1 of the 18 sensor forces and the 4

unknown hoof forces:

Sn + rn = Ffl Cn {xfl; yfl} + Ffr Cn {xfr; yfr} + Fhl Cn {xhl; yhl} + Fhr Cn {xhr; yhr}

where Sn is the sensor response of the force transducer n; n is the

number of the force sensor (1 to 18); rn is the error term of the

corresponding equation; Ffl, Ffr, Fhl, and Fhr are the 4 unknown

forces for the left forelimb, right forelimb, left hind limb, and right

hind limb, respectively, at each of their respective x and y posi-

tions; and Cn {xfl; yfl}, Cn {xfr; yfr}, Cn {xhl; yhl}, and Cn {xhr; yhr} are the trans-

fer coefficients from the 4 x and y positions for the left forelimb,

right forelimb, left hind limb, and right hind limb, respectively, to

the force transducer n. This equation system is highly overdeter-

mined. On the basis of positional data, only the hooves at stance

are considered in the equation system; this controls the number

of unknowns in the equations. Furthermore, the calculation algo-

rithm includes only those force sensors in the equation system

which are in close vicinity to the hoof positions; this reduces the

number of equations. The linear equation system is solved, using

the Gaussian least-squares method.

As reported elsewhere146, the center of pressure moves during

stance phase within the hoof, from the heel region to the toe

region. Because the goniometer positioning system supplies the

coordinates of the center of the hoof, the FAP has to be corrected.

For each time sample, the entire calculation procedure is itera-

tively repeated while adjusting the hoof coordinates in the hori-

zontal plane by increments of 0.5 cm until the sum of the absolute

values of the residuals (rn in the equation) attains a minimum

value. For each of the 4 resulting force curves (Figure 5), stance

phases are detected and temporal (stride frequency, stance time,

time to peak vertical force), spatial (stance length), and force

(peak vertical force, vertical impulse) variables as well as the left-

right symmetry indices are extracted automatically. The numeric

results and a selection of graphic displays are immediately avail-

able after data collection.

P h D - P U B L I C AT I O N I

32 l P h D - P U B L I C AT I O N I

Figure 5: Representative curves of calculated vertical ground reac-

tion forces of all 4 limbs in a horse at the walk (top) and at the trot

(bottom). Notice the differing force patterns between the left and

the right forelimb at the walk. The left forelimb has a normal force

dip at approximately midstance, whereas it is missing in the right

forelimb. This was the result of a mechanically limited hyperexten-

sion of the right metacarpophalangeal joint of this specific horse.

FL = left forelimb. FR = right forelimb. HL = left hind limb. HR = right

hind limb.

Time ( s )

0 1 2 3 4 5 6

Forc

e (1

kN/D

ivis

ion

)

F L

FR

HL

HR

Time ( s )

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Forc

e (2

kN/D

ivis

ion

)

FL

FR

HL

HR

Accuracy of the goniometer positioning system

Because the distance between the paired goniometers and their

lateral distance to the treadmill are identical for all 4 pairs of

goniometers, the accuracy test was conducted only for a single

pair.

Nine predetermined positions within the stance area of the left

hind limb were surveyed repeatedly (n = 10), using the goniome-

ter system. Target points were approached in the direction that

the treadmill belt moves, and alternatively from the opposite

direction. Averaged goniometer coordinates were compared with

known x and y positional coordinates.

Determination of the natural frequency of the system

The natural frequency spectrum of the treadmill platform was

determined by Fast Fourier Transformation (FFT) amplitude-

spectrum analysis of the raw sensor outputs (20 Hz FIR software

filter disabled). Sensor responses were recorded during a mechan-

ical stimulus (i.e. the treadmill was hit with a rubber mallet) and

while a horse was trotting on the treadmill.

Linearity test of the force transfer to the individual sensors

The individual sensor responses were recorded while test loads

ranging from 0.1 to 2.6 kN were applied to a defined location on

the treadmill. The procedure was repeated along the longitudinal

axis of the treadmill; loads were placed at the level of and be-

tween each sensor row (y-direction). Two series of measurements

were made with respect to the x-direction: exactly in the middle

between sensors of a row and at positions one-fourth of the dis-

tance between sensors of a row. Linearity was assessed by linear

regression and qualified by the coefficient of determination (R2).

Determination of superimposed forces with TiF in a static

configuration

Test loads were positioned within the running area of the tread-

mill platform to simulate realistic footfall combinations for horses

at a walk, trot, and gallop. For each record, the location within

the corresponding sector of a specific limb and the magnitudes of

the loads (range, 0.4 to 1.2 kN) were varied. Various 1-, 2-, and

3-test load configurations were investigated. Positions of the test

loads and force-sensor data were sampled and processed with TiF.

Correspondence was tested, using a 2-sided t-test with signi-

ficance defined as P≤0.05.

l 33P h D - P U B L I C AT I O N I

Comparison of vertical ground reaction forces determined by

use of a force shoe and TiF

Vertical ground reaction forces were measured simultaneously

with TiF and 2 strain-gauge force shoes65 that were tightly

screwed to the horseshoes of the forelimbs of a horse. The force

shoes were calibrated with a calibration pressh up to 7kN. Within

this range, the force shoes had fully linear characteristics

(R2>0.999) and a calibration error of ≤ ±0.25%. To guarantee

time synchronicity, signals of the force shoes were sampled and

processed under equal conditions and with the same software as

that used for sensor signals of the treadmill. However, unlike the

sensor forces of the treadmill, bandwidth of the force shoe sig-

nals was 200Hz. The horse was measured at the walk (1.5 m/s)

and the trot (3.4 m/s), twice over 20 and 30 strides, respectively.

Correlation between the force curves of the 2 measuring meth-

ods was assessed with Pearson correlation matrix. Correspon-

dence between the results of the main temporal, spatial and force

variables was tested either with a 2-sided t-test or a Wilcoxon

signed-rank test, depending on results of preceding tests for

normality of distribution. Values were considered significant at

P≤0.05.

Results

Accuracy of the goniometer positioning system

Mean (± SD) absolute difference between known coordinates of

the target points and those measured with the goniometer posi-

tioning system (n = 90) amounted to –0.8±3.0 mm in x-direction

and –2.3 ±1.6 mm in y-direction. Extremes were 4.8 mm for the

x- and 4.4 mm for the y-direction.

Determination of the natural frequency of the system

For both testing conditions, main natural frequencies were

observed between 40 and 60 Hz. In contrast, the relevant har-

monic components of the information signal of the trotting horse

were <15 Hz (Figure 6).

Linearity test of force transfer to individual sensors

For any test location, output voltage of each sensor was highly

linear with increasing test loads (R2 > 0.995; Figure 7). The longi-

34 l P h D - P U B L I C AT I O N I

l 35P h D - P U B L I C AT I O N I

Figure 6: Response of force sensor L3 (see Figure 1) after a mallet

thump (top) and while a horse was trotting on the treadmill (mid-

dle). The superimposed FFT amplitude spectra of these two signals

are shown in the bottom graph (mallet, thick line; horse, fine line).

Notice the frequency components of the information signal during

trot are in the band between 0 and 12.5 Hz whereas the mechani-

cal noise is located between 40 and 60 Hz.

tudinal spread of the input load from a single FAP to the different

sensors reveals the nature of the force-transfer coefficient matrix

(Figure 8).

Determination of superimposed forces with TiF in a static

configuration

In the 1-test load configuration, loads were determined with a

mean (± SEM) precision of 100.3 ± 0.76% (n = 10). In the 2-test

load configuration, loads were determined with a mean precision

of 99.7 ± 0.47 and 99.5 ± 0.33% (n = 15), and in the 3-test load

configuration, loads were determined with a mean precision of

98.9 ± 0.36, 98.6 ± 0.50, and 99.0 ± 0.37%, respectively (n = 10).

Only in the 3-test load configuration, 2 of the 3 calculated forces

differed significantly from the test loads.

Comparison of vertical ground reaction forces determined by

use of a force shoe and TiF

Examples of directly measured ground reaction forces recorded

with the force shoes were superimposed on calculated force

curves obtained with TiF (Figure 9). Pearson product-moment cor-

36 l P h D - P U B L I C AT I O N I

-0 .2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Refe rence we ight (kN)

y = 0.3533x

R 2 = 0.9997

Sen

sor

resp

on

se (

kN)

y = 0.0777x

R 2 = 0.9976

Figure 7: Simultaneously measured sensor responses of the fifth

(black circles) and sixth (gray circles) force transducer on the right

side of the treadmill to increasing loads. Test loads were applied in

the center of the treadmill on the fifth sensor row. Thus, all

together the loaded sensor row measured 70.7%, and the adjacent

row 15.5% of the input weight.

l 37

relation coefficient (r) of force curves at the walk was ≥0.999 and

of force curves at the trot ≥0.998. Absolute values and percent-

age differences of the main temporal, spatial, and force variables

were calculated (Table 1). Mean differences did not exceed 4.5%.

Nevertheless, values for all variables differed significantly.

Discussion

To achieve a high standard in safety and locomotor convenience

for the horse, the running platform is mounted on elastic, shock-

absorbing bearings, which compress slightly under load. This

allows training, exercise physiology, and sports medical investiga-

tions without harming the horses. Because of the high forces

involved, the platform bends very locally at the places where

hooves are in contact with the treadmill. This type of construction

P h D - P U B L I C AT I O N I

0

No of sensor

-10

10

20

30

40

60

50

Sen

sor

resp

on

se (

%)

0 1 2 3 4 5 6 7 8 9 10

Figure 8: Longitudinal spread of the force of a test load as applied

at 4 different locations. A test load was centered on the third

(diamond), fifth (circle), and seventh (square) sensor row, and cor-

responding responses were measured in the sensors on the left

side of the treadmill. In additon, the test load was placed halfway

between the center and the left sensor in row 5, and responses of

the left (gray circle) and right (open circle) sensors were measured.

Sensor responses are normalized relative to the test load. Notice

the symmetric spreading independent of the position of the test

load and the negative forces at the subsequent alternate rows from

the point of load application.

has an inherent tendency to oscillate. From the technical point of

view, the flexibility of the bearing-platform construction appears

to be suboptimal. Normally, a force measuring system for undis-

torted signals should have maximal rigidity and be lightweight to

attain a high resonant frequency. Also, the 15-mm textile-armored

rubber belt of the treadmill contributes to the damping charac-

teristics of the treadmill. This limited the frequency components

38 l P h D - P U B L I C AT I O N I

Figure 9: Comparison between vertical force curves measured by

use of a force shoe (gray line) and the treadmill-integrated force

measuring system (fine black line) for the left forelimb of a horse

at a walk (top) and at a trot (bottom). Notice the small force spikes

on the force shoe curve at impact when the horse was walking and

the more prominent impact peaks when the horse was trotting.

l 39

of the hoof forces and, therefore, decreased the requirements for

signal bandwidth of the measuring system. Whereas a classic

force plate immediately supplies the contact force of a hoof strike,

TiF must calculate the input forces from numerous sensor

responses. Each of these signals is contaminated by mechanical

vibration noise, which is different at each bearing as well as phase

shifted. We redesigned the platform construction for 25% less

weight and increased the longitudinal flexibility and number of

bearing points. This resulted in more-localized, less-energetic

oscillations. Analysis of the results revealed that the natural fre-

quencies were located between 40 and 60 Hz (Figure 6), which

was 3 times higher than the frequency components of the infor-

mation signal (<15 Hz). To eliminate noise from the raw sensor

signals, a steep 20-Hz FIR filter was used. In comparison to the

200-Hz bandwidth signal of the force shoe, this did not alter the

principal characteristics of the force curve (e.g., slope rate, peak

values; Figure 9) and, therefore, had an insubstantial influence on

the main variables (Table 1). The only exception was the impact

peak, which was smoothed compared to the signal obtained by

use of the force shoe.

P h D - P U B L I C AT I O N I

Left forelimb Right hindlimbs

Variable Force shoe TiF Mean difference* Force shoe TiF Mean difference*

Walk

Tstance (ms) 801 ± 12 791 ± 11 –1.2 % 811 ± 11 796 ± 9 –1.8 %

TFzpeak (ms) 513 ± 11 508 ± 10 –1.1 % 507 ± 11 498 ± 9 –1.9 %

SL (m) 1.177 ± 0.020 1.165 ± 0.017 –1.0 % 1.225 ± 0.016 1.217 ± 0.016 – 0.7 %

Fzpeak (N) 3906 ±82 3852 ± 83 –1.4 % 3756 ± 58 3612 ± 54 – 4.1 %

Iz (Ns) 2041 ± 28 2008 ± 29 –1.6 % 1913 ± 26 1824 ± 24 – 4.5 %

Trot

Tstance (ms) 347 ± 11 337 ± 8 –2.8 % 334 ± 7 333 ± 7 – 0.3 %

TFzpeak (ms) 172 ± 6 166 ± 5 –3.6 % 160 ± 4 161 ± 4 +1.1 %

SL (m) 0.983 ± 0.025 0.966 ± 0.022 –1.7 % 0.982 ± 0.025 0.976 ± 0.024 – 0.7 %

Fzpeak (N) 5566 ± 189 5679 ± 197 +2.0 % 5687 ± 168 5698 ± 157 – 0.2 %

Iz (Ns) 1136 ± 43 1143 ± 43 +0.6 % 1110 ± 44 1099 ± 40 – 0.9 %

Table 1: Comparison between mean ± SD values of temporal, spatial,

and force variables measured by use of the treadmill-integrated

force measuring system (TiF) or force shoes in a horse walking

(n = 40 strides) or trotting (60 strides) on a treadmill.

* Mean differences were calculated relative to values of force shoes.

Tstance = Stance time. TFzpeak = Time of vertical force peak. SL = Stance length.

Fzpeak = Peak vertical force. Iz = Vertical impulse.

We observed only a narrow spread of forces to neighboring sen-

sor rows. Almost the total force was detectable within 3 sensor

rows (i.e., the force spreads out from the FAP over 1 sensor space

of 36 cm in either direction; Figure 8). The loaded sensor row

measured approximately two-thirds of the input weight, and the

adjacent rows each one-sixth of the input weight. This resulted in

high force amplitudes of selected sensors and, therefore, in a

favorable signal-to-noise ratio. On the other hand, this sharp force

distribution enhances the demands on determination of the FAP.

Because of the resolution of the coefficient matrix, the accuracy

and repeated precision of the positioning system must be

< 0.5 cm. This prerequisite was met. As during the stance phase,

the center of pressure moves through the hoof in the longitudi-

nal axis by approximately 5 cm146, the x- and y-coordinates meas-

ured by the goniometer positioning system must be corrected.

The validation experiments revealed a good correspondence

between static test loads and TiF-calculated forces (deviations

≤1.4%) as well as high correlations between continuous force

data of TiF and force shoes (r≥0.998). Maximal mean differences

of temporal variables between the two measuring methods did

not exceed 1.9% when the horse walked and 3.6% when it trot-

ted. Maximal mean differences of spatial variables were ≤1% at

the walk and < 2% at the trot. Mean differences of force variables

did not exceed 4.5% at the walk and 2.0% at the trot. However,

all mean differences reported in Table 1 were significant. Because

of the high reproducibility of the 2 measuring systems, their dif-

ferences had narrow standard errors. Therefore, even marginal

differences (<1%) between the input and output values were sig-

nificant. For biomedical measurements, an error of 2 to 5% is nor-

mally acceptable. Also, some of the differences between reference

and calculated loads of the 3-load configuration in the static con-

figuration were significant, although the mean precision was

≥98.6%.

The force shoe was calibrated for full ground contact and the

vertical component only. Especially during late stance, when the

hoof tilts over the toe region of the force shoe, the force vector

moves out of the calibrated central area (42 mm in diameter) of

the force shoe; additionally a fraction of the fore-aft push-off

force is transmitted erroneously as vertical values. This may

explain a certain amount of the slight divergence between the

2 curves (Figure 9).

40 l P h D - P U B L I C AT I O N I

Extrapolation of treadmill data to equivalent overground condi-

tions is inadequate. Although the patterns of the TiF force curves

resemble closely the force traces obtained from overground loco-

motion92,99, small differences between these 2 approaches in peak

force magnitudes, similar to those observed in humans78,143,

should be expected also in horses. Lower push-off forces should

be considered when interpreting treadmill data, particularly for

higher speeds and heavier horses, since forces associated with belt

friction increase with body mass. However, this does not disqual-

ify this measuring system, especially when looking for left-right

asymmetries. Because the treadmill belt imposes the path of the

limbs, horizontal forces (fore-aft and lateral) are not measurable

with this concept. This limits certain applications in gait analyses

as e.g., advanced inverse dynamic calculations.

One of the main operational areas of TiF is in the field of ortho-

pedic diagnostics and research, where quantification of load dis-

tribution within the 4 limbs is essential. The ability to discriminate

between physiologic left-right asymmetry and natural predisposi-

tion for a limb49 from mild pathologic deviations in the locomo-

tion pattern may be addressed more objectively. In general, horses

with a marked lameness should not be exercised on a treadmill.

This implies that TiF measurements or gait analyses in general

make more sense when performed on patients with subtle or mild

lameness of complex nature. During an orthopedic work-up, the

degree of lameness can be quantified and documented. Within

sports medical care and preventive medicine, a general locomo-

tion status can be assessed periodically. Finally, curative proce-

dures as well as rehabilitative training can be monitored more

closely.

Horses are carefully adapted to treadmill-walking during short

sessions of 10 to 20 minutes. The majority feels comfortable after

2 to 3 repeated sessions of walking and trotting and shows their

characteristic gait pattern, similar to that manifested on the con-

crete runway. Frequently, the lameness is more pronounced on a

treadmill. The gait rhythm is more regular, especially at lower

velocities, which decreases inter-subject variability. The patients

have to be minimally equipped with the hook for attachment of

the rubber string. The outward tension of the rubber strings is

minimal and was tolerated well by all horses. Data acquisition can

be performed in a short time frame. Normally, for walking and

trotting horses, up to 50 strides each can be recorded within

<5 minutes. This and the rapid availability of temporal, spatial,

l 41P h D - P U B L I C AT I O N I

42 l

and kinetic data are important advantages in a clinical situation

and when dealing with orthopedic or other rehabilitation patients

that cannot physically tolerate long trotting examinations. The

large number of successive strides can be averaged to determine

more representative values, thereby increasing statistical power.

The distribution of load between the 4 limbs can be assessed

under highly standardized conditions. The noticeable drift stabil-

ity of the charge amplifiers we used allows us to conduct repeated

measurements for up to 30 minutes without resetting the force

sensors.

Information on load distribution and interlimb coordination cre-

ates novel perspectives for assessing gait quality and efficiency.

The influence of a rider on the horse’s center of mass and the

effect of fatigue on the impulse pattern are interesting aspects

that can be studied.

The measurement system described here has the capability to

measure the vertical ground reaction force of all 4 limbs simulta-

neously and assess interlimb coordination during successive

strides. Factors influencing the pattern of ground reaction forces,

such as soil condition, velocity of locomotion, and duration of

exercise, can be strictly controlled in this experimental configura-

tion, guaranteeing high reproducibility and standardization. This

allows reliable follow-up studies of a subject as well as compari-

son among individuals.

Acknowledgments

The authors thank Vreni Hänni and Rainer Vogt for technical assis-

tance. This study was supported by Kagra AG and Kistler Instru-

ments AG.

Footnotes

a Mustang 2200, Kagra AG, Fahrwangen, Switzerland.b Type Z17135, Kistler Instruments, Winterthur, Switzerland.c Type 5037A3, Kistler Instruments, Winterthur, Switzerland.d Type TK 162/1000, Tekel Instruments, Roletto, Italy.e Inmos T805-30MHz, STMicroelectronics, Genève, Switzerland.f IBM compatible PC (Intel PIII, 512 kB cache, 1.2 GHz, 128 kB RAM),

Micropose AG, Zurich, Switzerland.g HP2, Department of Veterinary Surgery, University of Zurich,

Zurich, Switzerland.h Zwick 1484, Zwick GmbH & Co, Ulm, Germany.

P h D - P U B L I C AT I O N I

l 43

ll. Assessment of gait irregularities in the horse: eye vs. gait analysis

Michael A. Weishaupt, Thomas Wiestner, Hermann P. Hogg, Patrick Jordan,

Jörg A. Auer, Eric Barrey

Equine Veterinary Journal (2001) Supplement 33; 135–140.

Abstract

The purpose of this study was to verify the sensitivity of two gait

analysis methods in detecting subtle lameness and to compare the

results to the traditional orthopaedic evaluation. Twenty two

horses were evaluated (1) subjectively by three different expe-

rienced clinicians and (2) objectively with synchronised ground

reaction force and accelerometric gait measurements on a tread-

mill. The horses were assigned for each of the 3 methods inde-

pendently to one of the following three groups (GR): sound, lame-

ness front limb, lameness hind limb. Additionally for each horse

the affected limb (AL) and the degree of lameness (DL) were

defined. The accordance between the 3 assessment methods for

the categorical variables was tested with Spearman correlation

analysis. The relationship between vertical ground reaction forces

and dorsoventral as well as mediolateral accelerations were

studied using Pearson correlation matrix.

Significant correlation was found between the clinical GR and

GR based on force (r = 0.51, P< 0.05) and acceleration data

(r= 0.47, P<0.05) respectively, as well between AL based on clini-

cal and ground reaction force (r=0.65, P<0.05) assessment. No

significant correlation was found neither for GR between the two

measuring methods nor for DL between the three assessment

methods. Pearson correlation matrix revealed significant correla-

tions between peak vertical forces and dorsoventral acceleration

in the hind limbs.

Conclusion: The measurements of kinetic and kinematic param-

eters represent a helpful complementary tool in the assessment

of subtle gait alterations. However this information needs to be

carefully interpreted and always related to the clinical observa-

tions.

44 l P h D - P U B L I C AT I O N I I

Introduction

Musculoskeletal injuries are world-wide the main reason why

horses are forced to interrupt or even quit their athletic career. Epi-

demiological studies revealed that more than 50% of race horses

experienced some period of lameness, and in 20% of those cases

the lameness was sufficient to prevent the individuals from racing

after the injury 67. Furthermore, subclinical disorders of the loco-

motor system are the most frequent causes (74%) responsible for

poor performance101. Therefore, prevention and early identifica-

tion of locomotor inadequacies prior to the appearance of overt

clinical signs have a high priority in equine sports medicine and

animal welfare. Clinical assessment of subtle gait irregularities in

the horse and their interpretation are often delicate or complex.

They depend strongly on the expertise of the orthopaedic clini-

cian. The quantitative assessment of those gait asymmetries and

of subtle changes of the locomotion pattern as assessed during

an orthopaedic work-up or after a therapeutic intervention would

provide additional assistance.

Kinetic methods and their derived parameters have proven to

detect and quantify locomotor unsoundness in horses 37,65,66,95-

97,100,144 and to monitor clinical progress reliably 4,60,73. Likewise,

the documentation of gait asymmetries with the help of accelero-

metric measurements was investigated11,137.

With the exception of the work by Dow et al.46, which addres-

ses the problem of early diagnosis of subclinical biomechanical

abnormalities, other investigations were mainly evaluating horses

with obvious gait alterations. Moreover, conditioned by the

chosen lameness model or orthopaedic problem, all studies were

focused on either the fore respectively the hind limb pair, assess-

ing the ground reaction force side asymmetry. To our knowledge,

only comparisons between clinical judgement and kinematic gait

analysis methods were studied70,106.

The purpose of this study was to test if it would be possible to

detect subtle lamenesses relying only on vertical force distribution

within all four limbs or body trunk acceleration. A further aim was

to detect the affected limb, suspected by the clinicians and to

determine the degree of gait irregularity. The results of the two

gait analysis methods would be compared to the results of a

traditional orthopaedic evaluation.

l 45P h D - P U B L I C AT I O N I I

46 l

Materials and Methods

Horses

Twenty two riding horses of different breeds, aged between 4 and

22 years with a mean (± SD) bodyweight of 561±51.7 kg and a

mean height at the withers of 165±5.5cm were evaluated. The

horses were ridden daily for pleasure and regularly in show jump-

ing or dressage competitions at various levels. According to the

owner’s or rider’s opinion, none of the them showed actually

evident gait abnormalities. All horses were accustomed to the

experimental equipment and the treadmill exercise following the

guidelines described by Buchner et al.16.

Orthopaedic examination

Horses were examined at two different days at the walk and trot,

on a concrete runway and on a highspeed treadmilla. The ortho-

paedic assessment included additional information as gait pattern

on the circle, trotting regularity after flexion tests as well as

obvious visual and palpatory findings. Horses were evaluated

independently by three different experienced clinicians and sub-

sequently assigned to one of the following three groups (GRC):

group 1 = sound or indefinable subtle gait irregularity

group 2 = lameness front limb

group 3 = lameness hind limb.

Additionally, for each horse the affected limb (ALC ) and the

degree of weightbearing lameness (DLC ) were defined. The clini-

cal scoring system applied in this study contained the following

5 lameness grades:

Grade 1/5 No irregularity at the walk; slight irregularity not

visible on every stride at the trot

Grade 2/5 No irregularity at the walk; slight lameness on every

stride at the trot

Grade 3/5 Slight irregularity at the walk; moderate lameness

at the trot

Grade 4/5 Moderate lameness at the walk; severe lameness at

the trot

Grade 5/5 No weightbearing on the affected limb

If the three specialist examiners assessed the affected limb differ-

ently, the limb with the most clinical findings was selected.

P h D - P U B L I C AT I O N I I

l 47

Gait analysis

Gait analysis was performed on the treadmill. Kinetic data were

collected during 20 seconds which amounted to 25 or more con-

secutive motion cycles. Data sampling was started after 2 minutes

of warm-up and as soon as the horses were trotting regularly in a

straight head-body position at a pre-set treadmill speed of

3.5 m/s. This speed proved to be an appropriate trotting velocity

to obtain a natural gait in all tested horses. The horses were

aligned and centred on the treadmill with the help of two lead

ropes attached on either side of the halter.

Vertical ground reaction forces (GRFz) were measured for all

four limbs simultaneously with the treadmill integrated force

measuring system (TiF) described by Weishaupt et al.138. Briefly,

18 piezo-electric force transducersb, mounted between the tread-

mill frame and the supporting steel platform over which the rub-

ber belt is pulled, measure the actual vertical reaction forces at

their corresponding bearing points. The XY-co-ordinates of each

hoof on the treadmill surface are calculated by triangulation based

on angle values determined with 2 electro-goniometersc con-

nected to the corresponding hoof with a thin rubber string. The

4 individual force traces of each limb are derived mathematically

by weighting the 18 sensor forces with the transfer coefficients of

the corresponding force application points on the treadmill sur-

face. For each sampling time an optimal sub-set of equations is

solved for each limb and averaged to increase precision. Sampling

frequency was set to 433 Hz to avoid aliasing with multiples of the

50/100 Hz power line frequencies.

Dorsoventral (Ad–v) and mediolateral (A m–l) accelerations were

measured at the sternum and sacrum using the Equimétrix sys-

temd, where two bi-axial accelerometers were connected to a

data-logger fixed on to an elastic girth. Data were filtered at a cut

off frequency of 25 Hz and sampled with 50 Hz per channel. Four

synchronised channels recorded continuously the Ad–v and Am–l

signals of the horse during the test. After completion of the test,

the data-logger was removed from the horse and the data trans-

ferred to a laptop. A trigger signal, sent at the onset of the force

recordings to the data-logger, ensured the identification and syn-

chronisation for post-processing of corresponding data sets.

Data processing

Prior to data processing, raw force and positional data were digi-

tally filtered with a 12 Hz FIR filter (– 6 dB, 0.3 s Kaiser-Bessel

P h D - P U B L I C AT I O N I I

48 l

window). Peak vertical forces (Fzpeak) and vertical impulse (Iz; ver-

tical force integrated over time during an entire stance phase)

were extracted automatically and averaged over 25 consecutive

strides. Symmetry considerations were based on differences

P h D - P U B L I C AT I O N I I

Figure 10: Force symmetry plot.

GRFz distribution of horse N° 6 with a clinically scored grade 2 left

forelimb lameness. The longitudinal “symmetry“ axis is shifted to

the right side. The deviation is more pronounced in the front (SI1)

than in the back (SI2). The absolute differences of Fzpeak and Iz were

383.4 N and 84.0 Ns respectively between the forelimbs and 4.8 N

and 29.7 Ns respectively between the hind limbs. This accounts for

a 3.7% Fzpeak and 4.3% Iz asymmetry in the front limbs (note: in the

graph only the central part (±15%) of the maximal symmetry field

(±100%) is displayed, therefore the markers for SI1 and SI2 (SI, sym-

metry index) are plotted as linearly interpolated values at the grid

lines of ±15%). Comparing both diagonal stances, load is shifted

more to the front during the front right – hind left diagonal stance

(SI4) than during the front left – hind right diagonal stance (SI3).

This results in a shift of the mean centre of force to the right front.

SI1 = (fl - fr) / (fl + fr); forelimb symmetry index

SI2 = (hl - hr) / (hl + hr); hind limb symmetry index

SI3 = (fl - hr) / (fl + hr); diagonal left fore – right hind limb

symmetry index

SI4 = (fr - hl) / (fr + hl); diagonal right fore – left hind limb

symmetry index

SI5 = ((fl + fr) – (hl + hr)) / (fl + fr + hl + hr); mean centre of force

For abbreviations please refer to glossary.

15%

10%

5%

0%

-5%

-10%

-15%-15% -10% -5% 0% 5% 10% 15%

S I 3

S I 5S I 4

S I 1

S I 2

f l fr

hl hr

between the fore and the hind limbs respectively, and between

diagonal limb pairs. The symmetry indices of forces and impulses

were expressed as a quotient, where the difference between the

limbs was divided by the sum of the limb pair. For each horse these

various symmetry indices were displayed graphically visualising

the longitudinal and diagonal force symmetry axes as well as the

resulting mean centre of force. Figure 10 displays an example of a

horse with a clinically scored left forelimb grade 2 lameness.

Based on the force symmetry indices and plots, GRF, ALF and DLF

were defined analogous to the clinical evaluation. Threshold

values for the gradation into lameness grades were 0–2% mean

difference in force and impulse for degree 1, 2–4% difference for

degree 2 and > 4% difference for degree 3.

The Ad–v of the sternum and sacrum provided quantitative infor-

mation about the symmetry and regularity of the trot. The pat-

terns of the Ad–v curve are systematically affected when the horse

is lame. The gait symmetry (Sym) and regularity (Reg) of the trot

was calculated using correlation coefficients C1 and C2 which are

derived from the autocorrelation function calculated on 20.48

seconds of the Ad-v signal. C1 expresses the similarity of dorso-ven-

tral accelerations between the movement of the right and left

diagonals. C2 expresses the similarity of dorsoventral accelera-

tions between successive movements of diagonals. Sym is calcu-

lated as the ratio (C1 /C2) x 100 and Reg as the sum (C1+C2) x 100.

At normal trot, a complete stride consists of two similar acceler-

ation patterns corresponding to each half-stride. When the suc-

cessive strides of the trot are poorly repeated, the gait is distin-

guished by a low regularity value. When the strides are continu-

ously asymmetric, the coefficient of correlation between the right

and left motions (C1) becomes lower than the coefficient of cor-

relation between the motions of two successive strides (C2); hence

the stride symmetry parameter is lower than 100%. Symmetry

and regularity variables were calculated for the forelimbs (FSym,

FReg) and the hind limbs (HSym, HReg). According to our refer-

ence data base obtained in young sound horses, a sound horse

should have at the trot:

Symmetry > 98%; 98–96% the symmetry is suspicious and

< 96% symmetry is abnormal

Regularity >185/200;185–175 /200 the regularity is suspicious

and <175 regularity is abnormal

Based on these symmetry and regularity indices it was possible to

sort the horses in each group (GRA) and to determine a degree of

l 49P h D - P U B L I C AT I O N I I

50

lP

hD

-P

UB

LIC

AT

ION

II

Horse N° Clinical assessment Kinetic symmetry variables Kinetic based rating Accelerometric symmetry indices Accelerometric

Peak vertical force Vertical impulse Symmetry Regularity based rating

GRC ALC DLC SI1 [%] SI2 [%] SI1 [%] SI2 [%] GRF ALF DLF FSym HSym FReg HReg GRA DLA

1 2 fl 1 – 2.0 –1.3 – 3.8 – 2.7 2 fl 2 96.7 99.3 182.7 194.2 2 3

2 3 hr 1 1.3 2.2 1.4 2.2 3 hr 2 99.2 98.3 187.1 188.5 2 1

3 2 fl 2 1.7 1.8 3.9 3.8 2 fr 2 98.0 99.5 189.4 192.5 1 1

4 2 fr 1 3.9 1.0 3.1 1.9 2 fr 2 99.7 97.5 192.8 187.9 3 2

5 2 fr 2 2.4 2.4 2.0 0.9 2 fr 2 98.5 99.0 191.3 193.8 1 1

6 2 fl 2 – 3.7 –0.1 – 4.3 – 2.0 2 fl 3 95.1 96.8 187.3 190.3 2 3

7 2 fl 2 0.3 –0.3 – 0.5 –1.4 1 hl 1 97.7 98.6 187.9 191.2 2 2

8 1 fr 1 – 2.1 –0.3 – 2.3 –1.6 2 fl 2 98.7 99.5 190.5 192.6 1 1

9 1 hr 1 0.8 0.1 0.9 1.7 1 hr 1 95.4 96.1 190.1 191.2 2 3

10 3 hr 2 – 0.3 0.4 0.0 1.6 1 hr 1 97.4 97.8 190.9 189.7 3 2

11 2 fr 1 – 0.9 –0.4 – 0.4 –0.1 1 fl 1 99.9 100.0 177.3 186.3 2 2

12 2 fl 2 – 3.5 –0.9 – 3.4 – 2.1 2 fl 2 98.3 98.6 190.8 191.2 1 1

13 1 fl 1 1.7 1.1 0.6 –1.3 1 fr 1 99.8 98.1 191.2 192.2 1 1

14 3 hr 2 0.6 1.2 1.0 2.1 3 hr 1 98.5 98.1 187.7 189.9 2 1

15 3 hl 2 – 2.7 –1.6 – 2.2 –1.9 2 fl 2 98.3 97.0 184.7 186.4 2 3

16 1 fl 1 – 1.1 0.0 – 2.0 0.0 1 fl 1 99.0 98.7 191.2 189.8 1 1

17 2 fr 2 1.8 0.3 2.4 1.4 2 fr 2 93.9 97.5 187.8 194.4 2 3

18 2 fr 2 – 0.1 –1.3 – 0.1 –1.1 3 hl 1 98.9 98.8 190.7 191.1 1 1

19 2 fl 2 – 1.2 –0.5 – 0.3 –1.0 1 fl 1 95.5 96.8 188.4 190.7 2 3

20 3 hl 2 1.3 –5.4 1.9 – 6.9 3 hl 3 97.7 97.3 190.4 190.7 2 2

21 2 fl 1 0.3 1.8 –1.8 – 0.2 2 fl 1 97.7 98.2 188.7 183.0 3 2

22 2 fr 3 6.1 0.8 3.7 0.6 2 fr 3 96.0 97.2 187.9 190.6 2 2

Table 2: Scoring results based on clinical judgement, force and accelerometric variables

For abbreviations please refer to glossary.

l 51

lameness (DLA). No affected limb was defined. In order to com-

pare the acceleration and force data, the dorsoventral and medio-

lateral components of the acceleration vectors were calculated for

the forelimbs and hind limbs over 20.48 seconds for each of the

following phases of the locomotor cycle: right and left stance

phase, right and left suspension phase.

Statistics

The correspondence between the 3 assessment methods for the

variables GR, AL and DL was tested with Spearman correlation

analysis. The relationship between GRFz and Ad–v as well as Am–l

were studied using Pearson correlation matrix. In all statistical

tests the significance level was set at P<0.05.

Results

All horses showed a subtle or more pronounced gait asymmetry

and therefore always a suspected or affected limb could be deter-

mined clinically. No lameness was rated above grade 3. The results

of the clinical assessment as well as the rating based on kinetic

and accelerometric variables are listed together with the kinetic

and accelerometric symmetry indices in Table 2.

The correspondence between the clinical assessment and the

ratings based on the two gait analysis methods are summarised

in Table 3. Significant correlations were found between GRC and

GRF (r=0.51) and GRA (r= 0.47) respectively. However, no signifi-

cant correlation was found for GR between the two measuring

methods. In 15 out of the 22 cases the AL was assigned corre-

spondingly based on the clinical assessment and force measure-

ments; this was confirmed with a significant correlation (r=0.65).

No correlations were found for DL between the three assessment

methods.

The relationship between raw force and accelerometric data

calculated with Pearson correlation matrix showed significant cor-

relations between Fzpeak of the left hind limb and Ad–v of the left

(r = 0.51; P < 0.05) and the right (r = 0.52; P < 0.05) hind limbs

respectively, and a trend was observed between Fzpeak of the right

hind limb and Ad–v of the left (r = 0.41; P < 0.1) and the right

(r = 0.40; P < 0.1) hind limbs respectively. No correlations were

found between force and acceleration variables neither in the

forelimb nor between GRFz and Am–l variables.

P h D - P U B L I C AT I O N I I

52 l

Discussion

Twenty two presumably sound horses were assessed for gait sym-

metry, subjectively with an orthopaedic work-up and objectively

by means of GRFz and body-trunk acceleration measurements. For

each horse and method the involved limb and the lameness

degree were defined, whereupon horses were classified as sound,

lame in a fore or lame in a hind limb. Moderate concordance was

found comparing the two gait analysis methods to the clinical

evaluation. A rather disappointing concordance was found

between the kinetic and accelerometric based ratings, as well cor-

relating force and acceleration variables.

The comparison between clinical evaluation and gait analysis

remains methodologically difficult because the clinical rating of

the patient represents a purely subjective judgement, relying

mainly on the individual expertise of the observer. The fact that in

this study horses were not obviously lame, rendered the task even

more delicate: based on the GRFz data set, 10 out of 22 horses

were rated as grade 1, i.e. had a load asymmetry of less than 2%;

out of these 10 horses, 7 were assigned to the sound group.

Although the clinical assessment was a synthesis of 3 clinicians’

observations, in certain cases a definite rating was hardly possi-

ble either because of an unclear manifestation of the gait irregu-

larity or a suspected involvement of a second limb. Furthermore,

the orthopaedist integrates mainly the 3-dimensional displace-

ment of specific body parts as head or croup in his evaluation. This

of course, reflects not directly the dynamics of ground reaction

forces and body accelerations. On the other side, the clinical

relevance of load redistribution between contralateral limbs of

less than 2% still has to be studied within a larger population in

the context of individual laterality or handedness.

P h D - P U B L I C AT I O N I I

Clinic vs. TiF Clinic vs. Equimétrix TiF vs. Equimétrix

Group (GR) 15 /22* 11/22* 7/22

Affected limb (AL) 15 /22* – –

Degree of lameness (DL) 11 /22 7/22 6/22

*significant Spearman correlation coefficient (P < 0.05).

Bernoulli type I error probability PB < 0.05.

Table 3: Concordance between clinical, kinetic and accelerometric

based ratings.

Slightly asymmetric weightbearing patterns have to be interpreted

with caution as they may be induced artificially by a lateral head

position or dissimilar hoof conformation between contralateral

limbs. It was emphasised to maintain the horse during data

recording in a straight head-body position, although the leading

person was standing always on the left side of the horse. Further-

more, an unproportionally higher number of horses with a load

transfer to the left front limb was not observed. Instead, we could

observe that in horses with unequal fore hoof angles the load was

shifted to the limb with the flatter hoof.

The synchronised time histories of GRFz and Ad-v coincide as pre-

sumed from mechanical law (Figure 11). However, the low corre-

lations between GRFz and Ad-v indicates that the compensatory

mechanisms, reflected in the ground reaction force distribution

within the 4 limbs and body segment accelerations, must be much

more complex and especially variable for different lameness ori-

l 53P h D - P U B L I C AT I O N I I

Figure 11: Synchronised time histories of vertical ground reaction

forces and dorsoventral acceleration.

GRFz of the left and right hind limbs and Ad-v measured at the

sacrum of horse No 20 with a clinically scored grade 2 left hind limb

lameness. Positive peak acceleration occurs a few milliseconds

delayed after Fzpeak of the left and right hind limb, respectively.

Negative peak acceleration occurs during mid suspension phase.

GRFz hl, vertical ground reaction force of the left hind limb; GRFz

hr, vertical ground reaction force of the right hind limb; Ad-v hind,

dorsoventral acceleration at the sacrum

GRFz hl

GRFz hr

Ad-v hind

0

1000

2000

3000

4000

5000

vert

ical

gro

und

reac

tion

forc

e [N

]

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5

dors

oven

tral

acc

eler

atio

n [g

]

5500

500

1500

2500

3500

4500

-500

6000

time [s]

gins and causes. Figure 11 demonstrates that in the case of grade

2 weightbearing lameness the differences in the acceleration peak

values are not as apparent as reflected by the obvious Fzpeak asym-

metry. Furthermore, the accelerometric values represent the mean

accelerometric vectors calculated over 20 seconds and are there-

fore not directly time-correlated to the Fzpeak data. This may reduce

the power of the selected acceleration parameters. Nevertheless,

the significant correlation between the GRFz and Ad-v of the hind

limbs demonstrates the anatomical conformation of the hindquar-

ters where the sacrum and pelvis are more “directly“ connected

to the ground and hardly allow for dissipation of compensatory

movements to closely located body segments. Whereas in the

front, the thorax is suspended between the forelimbs. Addition-

ally the neck and head takes over a significant part of the com-

pensatory movements 21.These anatomic differences may be the

reason why no correlations were found between GRFz and Ad–v in

the forehand.

The determination of kinetic and accelerometric parameters

represent a helpful complementary tool in the assessment of gait

irregularities. Especially the GRFz information of all four limbs

together, proved to be effective. However, this information needs

to be carefully interpreted and always related to the clinical obser-

vations. The compensatory mechanisms of weightbearing lame-

ness have to be further investigated for each specific orthopaedic

problem separately, taking into account multiple validated bio-

mechanical parameters.

Acknowledgment

The authors would like to thank Vreni Hänni and Reto Stump for

their technical assistance and all the owners, which kindly left

their horses at the study’s disposal.

54 l P h D - P U B L I C AT I O N I I

Glossary of abbreviations

Categorical variables:

AL, affected limb

DL, degree of lameness

GR, group assignment

Force variables:

Fzpeak, peak vertical force [N]

GRFz, vertical component of the ground reaction force [N]

Iz, vertical impulse [Ns]

SI1, forelimb symmetry index [%]

SI2, hind limb symmetry index [%]

SI3, diagonal left fore – right hind limb symmetry index [%]

SI4, diagonal right fore – left hind limb symmetry index [%]

SI5, mean centre of force [%]

Accelerometric variables:

Ad–v, dorsoventral acceleration [g]

Am–l, mediolateral acceleration [g]

FReg, forelimb regularity index [score on a scale of 0–200]

FSym, forelimb symmetry index [%]

HReg, hind limb regularity index [score on a scale of 0–200]

HSym, hind limb symmetry index [%]

Reg, regularity index [score on a scale of 0–200]

Sym, symmetry index [%]

Manufacturer’s addresses

a Mustang 2200; Kagra AG, 5615 Fahrwangen, Switzerland.

b Typ Z17135, Kistler Instruments, 8408 Winterthur, Switzerland.

c Typ TK 162/1000, Omni Ray AG, 8600 Dübendorf, Switzerland.

d Equimétrix, INRA, Station de Génétique Quantitative et Appliquée,

78352 Jouy-en-Josas, France.

l 55P h D - P U B L I C AT I O N I I

l 57

lll. Vertical ground reaction force-time histories of sound Warmblood horses

trotting on a treadmill

Michael A. Weishaupt, Thomas Wiestner, Hermann P. Hogg, Patrick Jordan, Jörg A. Auer

Veterinary Journal (2004), 168; 304–311.

Summary

The objective of this study was to establish representative tread-

mill ground reaction force (GRF) and interlimb co-ordination time

data of clinically sound horses at the trot. These normative stan-

dards would provide a reference data base against which lame

horses could be compared. GRF-time histories were collected from

30 Warmblood riding horses with easy, wide natural gaits. Data

were recorded of all four limbs simultaneously by the use of an

instrumented treadmill. A total of 912 stride cycles per limb were

analysed for force, time and spatial parameters and averaged.

Shape and amplitude of the treadmill force curves were very

similar to force traces recorded with a stationary force plate. The

horses showed a high degree of symmetry in all investigated para-

meters (95% reference interval of left-right asymmetry

±1.8–6.8%); no significant differences were found between left

and right mean values. Intra-individual coefficients of variance of

the various parameters did not exceed 2.7%. Inter-individual

coefficients of variance were 2.5 – 3.5 times larger than the

respective intra-individual coefficients. An instrumented treadmill

provides a number of decisive advantages, such as time-efficient

data acquisition of all four feet simultaneously over successive

strides, or the high regularity of the horse’s gait pattern at con-

trolled velocities, which allow the clinical assessment of loco-

motor performance of horses.

58 l P h D - P U B L I C AT I O N I I I

Introduction

The trot is a symmetric, running gait and is characterised by a

synchronous movement of the diagonal limb pairs which results

in a two-beat rhythm103. The footfalls of the contralateral limbs

are evenly spaced in time, thus, a half cycle out of phase. At

impact as well at lift-off, the diagonal limb pairs are slightly time

dissociated and the diagonal stance phases are separated by a

short period of suspension42,48. Because of its symmetry and the

number of concurrently loaded limbs, the trot is the preferred gait

for lameness evaluation.

The assessment of lameness is usually a subjective procedure

and relies strongly on the expertise of the orthopaedic clinician.

For early treatment and prevention of irreversible damage there is

a need for techniques to identify abnormalities of locomotor

performance prior to the appearance of overt clinical signs81–82.

The analysis of ground reaction forces (GRF) forms a part of the

descriptive data characterising the mechanics of a specific gait. It

proved to be very sensitive in quantifying weightbearing asym-

metries at the walk4,95,96 and the trot37,46,100,144,145. A serious short-

coming of using a single force plate is that the GRF data of only

one limb can be recorded while at least one other limb supports

the body at the same time. In addition, no time relations between

the GRF of the different limbs can be measured in this way. One

approach to assess the force distribution and time relation

between concurrently loaded limbs is to combine averaged

representative GRF patterns with ground contact time data

derived from kinematic analysis. This was done for walking horses

by Merkens and Schamhardt94,96. The method is very laborious, as

repeated recording sessions are necessary to obtain a sufficient

number of representative stride data. At the trot, in average 44

attempts were needed to obtain five valid ground contacts for

each of the four limbs99. Trial-to-trial speed variability, fatigue or

overuse of the patient and therefore test-retest reliability may be

of concern in measures of overground motion analysis.

The highspeed treadmill is widely accepted as a useful tool in

equine exercise physiology and locomotion research, as well as

increasingly for clinical investigations. It allows examinations in

combination with sophisticated measurement equipment and

under highly standardised conditions. A treadmill, instrumented

with a GRF measuring system140, enables the acquisition of loco-

motor data of all limbs simultaneously and over multiple motion

l 59P h D - P U B L I C AT I O N I I I

cycles, which greatly assists the evaluation of locomotor per-

formance of sound and lame horses.

This investigation presents for the first time simultaneously

recorded vertical GRF-time histories of all four limbs of sound

Warmblood horses trotting on a treadmill.

Material and Methods

Animals

Thirty Warmblood riding horses were selected on the basis of

their representative natural gait pattern and physical soundness

(age range 4–14, mean 7.3 years; bwt (mean±sd) 573±54.9 kg;

wither height 1.69 ± 0.05 m). The orthopaedic examination

included the inspection and palpation of the limbs and a careful

visual assessment of their gait at the walk and trot on hard sur-

face, before and after flexion tests, and on the treadmill. Contra-

lateral hoofs had to be symmetrical in shape and size and prop-

erly shod. The horses were accustomed to the treadmill during

two days through at least three 30 min sessions16.

Treadmill integrated force measuring system (TiF)

Gait analysis was performed on a high-speed treadmill (Mustang

2200, Kagra AG) instrumented with piezoelectric force trans-

ducers (Type Z17135, Kistler Instruments). The details of this

measuring system and the raw data processing have been

described elsewhere140. The system measures the four vertical

force curves simultaneously within their lapse of time (resolutions:

force 12 bit (6 N), time 2.3 ms, spatial 0.01 m). The velocity of the

treadmill belt was measured on the front coil of the treadmill by

use of an inductive revolution counter. The accuracy of this

speedometer was ±0.8% at 3.5 m/s belt speed.

Data acquisition and analysis

The horses were warmed-up during a period of 15 min at walk and

trot. Measurements were taken at 3.5 m/s; under overground

condition, this speed is considered as the working trot33 and

proved to be an appropriate trotting velocity to obtain an easy and

natural gait in all tested horses. The horses were aligned and cen-

tred on the treadmill using lead ropes attached on either side of

the halter. Data sampling was started as soon as the horse was

trotting with a regular pace and in a straight head-body position.

60 l P h D - P U B L I C AT I O N I I I

Recording time lasted 20 or 30 s which corresponded to 24 and

more consecutive motion cycles. Measurements during which the

horse stumbled or visibly changed its relative position on the

treadmill were rejected. Force-time histories were analysed with

in-house developed software (HP2, Equine Performance Centre,

University of Zurich) which allowed the automatic extraction of

force, time and spatial parameters for each limb separately. The

terminology refers to the one proposed by Deuel 44 and further

definitions are explained Figure 12.

Force parameters were normalised to the horse’s bodyweight.

The beginning and the end of stance phase was determined by the

intersection of the linear approximation to the initial and terminal

l 61P h D - P U B L I C AT I O N I I I

Figure 12: Force and time parameters extracted from the force

traces at the trot. FL, left forelimb; FR, right forelimb; HL, left

hindlimb; FR, right hindlimb. Fzpeak, vertical force peak; TFzpeak, time

of vertical force peak; Iz, vertical impulse; SD, stride duration; StD,

stance duration; SwD, swing duration; StpDcl, contralateral step

duration; StpDil, ipsilateral step duration; SpD, suspension dura-

tion; TAP, time of advanced placement; TAC, time of advanced com-

pletion.

0 0.5 1.0 1.5 2.0

HR

HL

FR

FL

Time [s]

Impact

Kink

TAP right TAC right

SpD left SpD right

TFzpeak

Fzpeak

SD

Iz

SwDStD

StpDcl FR StpDcl FL

StpDi l HL

TAP left TAC left

62 l

slope of the force curve with the zero-baseline. Two threshold

levels were set to define the respective approximation line.

Because of the initial impact peak and the kink in the terminal part

of the force curve (Figure 12), levels were applied differently with

regard to the beginning (7.5% and 15% of Fzpeak) and end (10%

and 3% of Fzpeak) of the stance phase.

An average of 30±6.0 motion cycles per horse were analysed

and averaged. Contralateral limb asymmetry was quantified with

the asymmetry index (ASI) according to Herzog et al.62

ASI = (Pleft – Pright) / (0.5 x (Pleft+Pright)) x 100%

where P denotes any optional parameter. In a fully balanced situ-

ation between contralateral limbs, ASI is zero.

The individual results were then averaged to “grand” means

(n=30 horses), for each parameter and limb separately. By anal-

ogy, for each limb a mean force curve was calculated represent-

ing the “standard“ treadmill force curves for sound Warmblood

horses at the working trot.

Statistics

Statistical analysis was performed with SigmaStat 2.0 (SPSS

Science). Descriptive statistics (mean, sd) were computed for each

parameter. Normality of data was tested with the Kolmogorov-

Smirnov test. Left-right (L–R) asymmetry and differences between

fore- and hindlimbs were tested using paired t-tests. The signifi-

cance level was set at P=0.05. Intra-individual variation (variabil-

ity between strides of the same horse) and inter-individual varia-

tion (variability between horses) of selected parameters were eval-

uated using the coefficient of variance (CV).

Results

The mean velocity of the treadmill belt was 3.5 ± 0.05 m/s. The

4 mean “standard” treadmill force curves within their mean time

history are shown in Figure 13. The typical vertical force curve of

the trot showed a dampened impact force peak immediately after

touch-down, a single force maximum around midstance and a dis-

creet kink during breakover of the hoof. Impact force peaks were

more distinct in the forelimbs than in the hindlimbs. In the fore-

P h D - P U B L I C AT I O N I I I

l 63

limbs the impact peak occurred 37±1.7 ms and in the hindlimbs

35 ± 2.6 ms after beginning of stance.

Vertical GRF parameters

Mean values of Fzpeak, time of peak vertical force (TFzpeak) and

vertical impulse (Iz) are summarised in Table 4. The differences

between fore- and hindlimbs were significant. Fzpeak as proportion

of bwt was 112 ±7.2% in the forelimbs and 100±5.8% in the

hindlimbs. Thus, Fzpeak was in average 12.3% larger in the fore-

limbs than in the hindlimbs corresponding to 52.9 ±1.2% of total

vertical force carried by the forelimbs at midstance. In the fore-

P h D - P U B L I C AT I O N I I I

Figure 13: Mean “standard“ treadmill force curves with the 95% ref-

erence interval (± 2.04 x sd) of 30 sound Warmblood horses at the

trot (3.5 m/s). FL, left forelimb; FR, right forelimb; HL, left hindlimb;

FR, right hindlimb.

0

HR

HL

FR

FL

Time [s]

0.80.60.40.2

64 l

limbs the vertical force peak occurred after 47% of stance and in

the hindlimbs after 50%. The total stride impulse (sum of the

4 vertical impulses over an entire motion cycle; IS) amounted to

7.56 ± 0.35 Ns /kg of which, 28% were attributed to each fore-

limb and 22% to each hindlimb.

Temporal parameters

Stride duration (SD) lasted 759 ± 34 ms which corresponds to a

stride frequency (SF) of 79.2 ± 3.5 min–1. Mean values of stance

duration (StD), swing duration (SwD), contralateral (StpDcl) and

ipsilateral step duration (StpDil) are listed in Table 5. Except for

StpDcl, the differences between fore- and hindlimbs were signifi-

cant. Stance phases in the forelimbs occupied 41.8 ± 2.4% of SD

compared to 36.5 ±1.9% in the hindlimbs. Suspension duration

(SpD) lasted 59 ±18 ms or 7.8 ± 2.18% of SD and 60±18 ms or

7.8 ± 2.22% of SD for the left and right suspension, respectively;

L– R differences were not significant. Of the 30 horses, 14

impacted within both diagonals first with the forelimbs (time

of advanced placement; TAPleft –12 ± 7ms and TAPright –15 ± 8 ms

for the left and right diagonal stance, respectively) and five

impacted first with the hindlimbs (TAPleft 8 ± 4 ms and TAPright

12 ± 5 ms). In 11 horses the forelimb preceded the hindlimb in the

one diagonal and vice versa in the other. At diagonal lift-off, the

hindlimbs preceded the forelimbs in all horses (time of advanced

completion; TAC left 36±10 ms and TAC right 35±11 ms for the left

and right diagonal stance, respectively); L–R differences were not

significant.

Spatial parameters

As spatial measures are a function of time and speed, only the

directly determined spatial parameters as stance length (StL) and

gauge width (G) are reported in Table 6. For the sake of com-

P h D - P U B L I C AT I O N I I I

forelimbs hindlimbs

Parameter left right ASI [%] left right ASI [%]

Fzpeak [N/kg] 10.95 ± 0.73 10.97 ± 0.70 – 2.0 ± 2.0 9.75 ± 0.58a 9.78 ± 0.57a – 0.4 ± 1.8

TFzpeak [ms] 149 ± 11 151 ± 11 – 0.9 ± 2.3 138 ± 7a 139 ± 7a – 0.7± 2.6

Iz [Ns/kg] 2.13 ± 0.09 2.13 ± 0.10 – 0.4 ± 2.8 1.64 ± 0.10a 1.66 ± 0.10a – 1.0 ± 2.9

a significant difference between ipsilateral limbs.

Table 4: Force parameters (mean ± sd) of 30 Warmblood horses

trotting (3.5 m/s) on a treadmill

l 65

pleteness, stride length (SL) calculated from SD and treadmill belt

velocity amounted to 2.659 ± 0.128 m. Gauge width between

forelimbs was significantly larger than between hindlimbs at the

beginning of stance; at the end of stance no significant difference

was found between front and back. In the forelimbs as well as in

the hindlimbs the width increased significantly towards the end

of stance shifting the treadmill belt slightly to the side. Of the 30

horses, 17 showed a positive ipsilateral overreach distance (OR)

where the hind hoofs were placed in average 0.07± 0.04 m in

front of the imprint of the ipsilateral fore hoofs. In 11 horses OR

was negative (–0.04 ± 0.03 m) and in two horses the imprints of

the forelimbs were covered by those of the respective ipsilateral

hindlimb. Differences between left and right OR were not signif-

icant.

Asymmetry index

The horses showed a high degree of symmetry in GRF, time and

spatial parameters (Tables 4–6). No significant differences were

P h D - P U B L I C AT I O N I I I

forelimbs hindlimbs

Parameter left right ASI [%] left right ASI [%]

StD [ms] 317 ± 16 316 ± 16 0.0 ± 1.2 277± 13a 277 ± 14a – 0.1 ± 1.8

SwD [ms] 442 ± 34 442 ± 32 0.0 ± 0.9 482 ± 32a 482 ± 32a 0.1 ± 1.0

StpDcl [ms] 380 ± 19 379 ± 17 0.2 ± 2.9 380 ± 18 379 ± 19 0.5 ± 3.4

StpDil [ms] 375 ± 24 375 ± 23 – 0.2 ± 2.0 384 ± 16a 384a ± 17a 0.2 ± 1.9a significant difference between ipsilateral limbs.

Table 5: Temporal parameters (mean ± sd) of 30 Warmblood horses

trotting (3.5 m/s) on a treadmill

forelimbs hindlimbs

Parameter left right ASI [%] left right ASI [%]

StL [m] 1.09 ± 0.06 1.09 ± 0.06 0.0 ± 1.3 0.93 ± 0.04a 0.93 ± 0.04a 0.2 ± 2.5

forelimbs hindlimbs

Gstart [m] 0.16 ± 0.06 0.12 ± 0.06b

Gend [m] 0.17 ± 0.06c 0.15 ± 0.07c

a significant difference between ipsilateral limbsb significant difference between fore and hindc significant difference between start and end of stance

Table 6: Spatial parameters (mean ± sd) of 30 Warmblood horses

trotting (3.5 m/s) on a treadmill

found between left and right mean values. Also the diagonal sums

of Fzpeak and Iz were highly symmetric (ASI – 0.1±1.3% and

–0.2±1.5%, respectively).

Intra- and inter-individual variability

Mean intra- and inter-individual variations of Fzpeak, Iz and StD are

listed in Table 7.

The intra-individual CVs were 2.5 to 3.5 times smaller than the

respective inter- individual ones. Intra- individual variations of

force parameters were larger in the hindlimbs compared to the

forelimbs.

Discussion

Theoretically, treadmill locomotion does not differ from over-

ground locomotion as long as treadmill belt velocity is constant134.

Nevertheless, differences in biomechanical15 and energetic130

variables were documented also for horses. Comparative kine-

matic analysis of treadmill and overground locomotion revealed

reduced SF, prolonged relative StD of the forelimbs, preceding

forelimbs at diagonal touch-down, greater retraction of both,

fore- and hindlimbs and reduced vertical excursion of the hoofs

during the swing phase and of the withers at midstance on the

treadmill15. A comparable study on kinetic parameters exists only

for walking humans143; although isolated sections of the vertical

force curves at mid- and late stance were slightly different, the

overall pattern of GRF were highly correlated between force plate

and treadmill (r=0.998– 0.983 for different walking speeds), indi-

cating that segment motion acceleration is similar during the

stance phase. The differences in the second half of stance were

attributed to treadmill belt speed fluctuations15,143. To consider

this problem, the standard traction motor of our treadmill was

66 l P h D - P U B L I C AT I O N I I I

intra-individual CV inter-individual CV

StD forelimbs 1.6% 5.0%

hindlimbs 1.7% 4.7%

Fzpeak forelimbs 2.0% 6.5%

hindlimbs 2.3% 5.8%

Iz forelimbs 1.9% 4.4%

hindlimbs 2.7% 6.1%

Table 7: Mean intra- and inter-individual coefficients of variance

exchanged for an engine with larger mass and 50% more power

to ensure constant velocity of the treadmill belt.

Conformity of surface condition, speed of locomotion, breed

and the familiarity of the horse to treadmill exercise are the most

relevant prerequisites for direct comparison of locomotor data of

different studies and particularly when comparing overground

with treadmill locomotion. On first exposures to the treadmill,

horses tend to move with higher cadence and relative longer

stance phases16. Speed variations are known as a source of inter-

ference when performing gait analyses in the overground situa-

tion74,88. The comparison of the treadmill force data with pub-

lished force plate data was therefore limited to investigations with

a similar methodological set-up. A suitable match was found

with the reference data of Dutch Warmblood horses trotting

(3.9–4.3 m/s) on a rubber matted runway with integrated force

plate99. Furthermore, comparisons were focused on relative data.

GRF parameters

The general shape of the treadmill force curves was very similar

to force plate curves of comparable type of horses37,46,99. Curve

landmarks such as impact peak, the single force peak and the kink

during breakover were recognisable also on the treadmill force

curve.

The impact peak is a high frequency, passive force observed to

occur in human athletes around 30–50 ms after the beginning

of stance (6–17% of StD). Neuromuscular reflexes are not fast

enough to adjust limb stiffness by altering muscle tone and there-

fore impact forces are implicated in running injuries102,105. Time

and magnitude of the impact are influenced by the nature of the

subsoil, the shock-absorbing features of the interface (damping

sole), the mass of the limb and velocity and direction of the hoof

hitting the ground. In the present study, impact forces were

observed to occur around 11–13% of stance which corresponds

with the force plate findings of Dow et al.46.

Compared to the data of Merkens et al.99, absolute Fzpeak

values on the treadmill were slightly lower in correspondence to

the slower mean velocity (3.5m/s vs. 3.9–4.3m/s). On the tread-

mill, Fzpeak of the forelimbs was 12.3% larger than of the hind-

limbs. This difference was barely different from the overground

situation (13.5%), indicating that also during treadmill locomo-

tion the centre of mass at midstance is located in the same pro-

portion closer to the forehand. Forces peaked slightly later on the

l 67P h D - P U B L I C AT I O N I I I

treadmill (forelimbs 47%, hindlimbs 50% of stance) than during

overground locomotion (forelimbs 44 %, hindlimbs 46 % of

stance). On the treadmill, Iz of the forelimbs was 29.1% larger

than of the hindlimbs. This difference was smaller in the over-

ground situation (22.6 %) and can be explained by a relative

longer StD of the forelimbs on the treadmill (14.4%) compared to

the overground situation (11.1%). This observation agrees with

the findings of Buchner et al.15. Drevemo et al.47 reported for trot-

ters at higher speeds (12 m/s) a relative longer StD of the

hindlimbs compared to the forelimbs. Thus, the proportion of StD

between fore- and hindlimbs seems to be velocity-dependent.

Temporal parameters

The SD were similar to mean values published by Buchner et al.15

of horses trotting between 3.0 and 3.9 m/s on a rubber matted

runway and on the treadmill, but clearly longer than the values of

the same horses trotting at their respective velocities on an asphalt

runway. Our findings support the conclusion, that it is not the type

of locomotion but rather the type of surface with its grip and

damping features which affects the cadence of the gait. In agree-

ment, Clayton32 reported clearly shorter SD and SpD for horses

trotting at comparable speeds on a concrete runway.

Diagonal limb dissociation varies, both, between and within

individuals and tends to be inverse and more pronounced with

increasing speed 48. At impact, the majority of our horses com-

pleted the suspension phase by landing first with the forelimb and

subsequently with the diagonal hindlimb. At lift-off, all horses

protracted the hindlimb prior to the diagonal forelimb. Also

Buchner et al.15 observed the same diagonal limb dissociation pat-

tern (negative TAP, positive TAC) on the treadmill, whereas in the

overground situation, the hindlimb preceded more often the

diagonal forelimb at impact as well as at lift-off.

Contralateral asymmetry

Taking into account the different methods of computing the

symmetry indices, contralateral asymmetries of Fzpeak, Iz and StD

were slightly lower on the treadmill compared to overground data

of Merkens et al.99. This may be attributed to speed and body

position consistency. The mean ASI values of the investigated

parameters did not deviate significantly from ASI=0, confirming

that the selected group of horses represented orthopaedically

sound subjects. Nevertheless, asymmetries between 1.8 % and

68 l P h D - P U B L I C AT I O N I I I

6.8% (95% reference interval of ASI) for the key force and tem-

poral parameters corroborated that sound horses may show a

natural locomotion asymmetry at the trot 47,48,99,111.

Intra- and inter-individual variation

Primary factors contributing to variance in kinetic analysis include

inherent subject variation such as morphology (breed-specific)

and size (limb length), and variation in trial repetitions for each

subject (e.g. speed). Speed control and regularity of locomotion

are two of the outstanding advantages of the treadmill. Accord-

ingly, intra-individual CVs did not exceed 2.7% and were appar-

ently lower than those reported for overground locomotion99.

Intra-individual variability of force parameters was slightly

higher in the hindlimbs compared to the forelimbs (Table 7). This

may be attributed to the mainly propulsive function of the

hindlimbs responsible to meet a stationary position of the subject

on the treadmill.

Inter-individual variations were 2.5–3.5 times higher than the

intra-individual variations which corroborates previously reported

findings15,99. Inter-individual CV of time, velocity and impulse

parameter may be further reduced by normalising them to dimen-

sionless quantities by taking into account the individual leg

lengths as proposed by Hof64 and Khumsap et al.74. However,

with the narrow range of wither heights in our group of horses

(1.69 ± 0.05 m), such a normalisation would reduce the CV only

by about 0.15%.

GRF-time histories recorded simultaneously of all four limbs

provide a complete indication of limb loading reflecting the accel-

eration pattern of the body centre of mass. To date, only an instru-

mented treadmill enables the time-efficient data acquisition and

analysis of these parameters. This presents an important advan-

tage in a clinical situation when dealing with orthopaedic or other

rehabilitation patients that cannot physically tolerate long trotting

examinations. The treadmill force curves were very similar in shape

and amplitude to force plate curves. The small differences in tem-

poral and spatial parameters do not disqualify the treadmill for

gait analysis as long as cross-comparisons are avoided. The large

number of successive strides and the standardisation of factors

influencing the GRF’s, such as velocity of locomotion or duration

of exercise, contribute decisively to the reliability of the data. This

allows dependable follow-up studies of a subject as well as com-

parisons among individuals. The high regularity of the horse’s gait

l 69P h D - P U B L I C AT I O N I I I

pattern on the treadmill was reflected in low intra-individual CVs

of all investigated parameters. This facilitates a more objective dis-

crimination between different degrees of lameness. The drawback

of this gait analysis system remains the restriction to the vertical

component of force and impulse parameters.

This data set may be used as reference values to assess the loco-

motor performance of unsound horses or in charting the progress

of patients during a recovery process following injury. However,

comparisons are restricted to data recorded at the same velocity

(3.5 ± 0.05m/s) and to horses with wither heights around 1.7 m.

Reference data for a wider range of velocities and other exterieur

conformations will be established.

Acknowledgments

The authors wish to thank the Swiss National Centre, Berne,

particularly Dr. Beat Wampfler, and all owners for kindly leaving

their horses at the study’s disposal, as well as Vreni Hänni and

Anne Stuart for technical assistance.

70 l P h D - P U B L I C AT I O N I I I

l 71

lV. Compensatory load redistribution of horses with induced weightbearing

forelimb lameness trotting on a treadmill

Michael A. Weishaupt, Thomas Wiestner, Hermann P. Hogg, Patrick Jordan, Jörg A. Auer

In press at Veterinary Journal (2004).

Abstract

The study was performed to obtain a detailed insight into the load

and time shifting mechanisms of horses with unilateral weight-

bearing forelimb lameness. Reversible lameness was induced in

11 clinically sound horses by applying a solar pressure model.

Three degrees of lameness (subtle, mild and moderate) were

induced and compared with the sound control measurement. Ver-

tical ground reaction force-time histories of all four limbs were

recorded simultaneously on an instrumented treadmill.

Four compensatory mechanisms could be identified that served

to reduce structural stress, i.e. peak vertical force on the affected

limb: (1) with increasing lameness, horses reduced the total verti-

cal impulse per stride; (2) the diagonal impulse decreased selec-

tively in the lame diagonal; (3) the impulse was shifted within the

lame diagonal to the hindlimb and in the sound diagonal to the

forelimb; (4) the rate of loading and the peak forces were reduced

by prolonging the stance duration. Except in the diagonal

hindlimb, where peak vertical forces increased slightly in the mod-

erate lameness condition, no equivalent compensatory overload

situation was observed in the other limbs. Specific force and time

information of all four limbs allow the unequivocal identification

of the affected limb.

72 l P h D - P U B L I C AT I O N I V

l 73

Introduction

Injuries of the locomotor system are commonly sustained by sports

horses and cause considerable interference with training and

competition67,121. While lameness-induced alterations in inter-

limb timing and/or movement patterns of defined body seg-

ments have been identified in numerous kinematic stud-

ies 6,17,18,20, 21,27–31,43,53,56–59,71,79,109,133, kinetic investigations are still

limited in number. Understanding weight-shifting mechanisms is

essential in clinical lameness evaluation, especially when dealing

with quadruped animals. The compensatory movements caused

by lameness of a hindlimb and transmitted to the forehand or vice

versa, are visually recognisable; however, the underlying redistri-

bution of limb loading has not yet been investigated in detail.

Furthermore, there is speculation about the potential of second-

ary injuries resulting from the animal’s attempts to compensate

the lameness by altering its gait pattern.

Time series of ground reaction force (GRF) traces of all four

limbs provide a complete picture of the load redistribution in case

of lameness. Because of the time and effort involved in recording

a sufficient number of strides of all limbs with a single force plate,

kinetic studies were often limited to the comparison between

contralateral limbs 37,46. Furthermore, concurrent ground contacts

of two or three limbs can not be recorded in this way and there-

fore the temporal relationship between limbs has to be deter-

mined with a supplementary measuring system. A valuable

approach was undertaken by Merkens and Schamhardt94–96 who

compiled mean representative GRF data of the four limbs with

corresponding time information derived from kinematic analyses

of sound and lame horses at the walk. For the trot, Morris and

Seeherman100 reported on GRF redistribution in horses with uni-

laterally induced carpal lameness, however, without considering

the underlying temporal relationship.

This study is part of a larger investigation which evaluates the

vertical GRF, timing and selected spatial changes of horses walk-

ing and trotting on a force-measuring treadmill140, before and

after inducing different degrees of weightbearing lameness. This

report focuses on the changes of forelimb lameness at the trot. It

analyses the interrelationship between load redistribution and

changes of intra- and inter-limb timing and pinpoints the most

sensitive indicators for characterisation of supporting forelimb

lameness.

P h D - P U B L I C AT I O N I V

Material and methods

Horses

Eleven clinically sound horses, nine Warmblood riding horses and

two Franche Montagne light draught horses, were used in this

study (age mean ± sd 8.5 ± 4.6 years; body weight 579 ± 55.3 kg;

wither height 1.67 ± 0.07 m). The orthopaedic examination

included inspection and palpation of the limbs and careful visual

assessment of their gait at the walk and trot on a hard surface,

before and after flexion tests, and on the treadmill. Both front and

hind hooves had to be symmetric in shape and size and properly

shod. The time since the last hoof trimming was <4 weeks. At

arrival, 3 of the 4 limbs were chosen at random; the correspon-

ding horseshoes were modified by welding a nut (thread M10) to

the inner rim of each branch between the quarters and the bars,

at the point of greatest width95. The soles were cleaned and

trimmed to a consistent thickness by evaluating the elasticity of

the sole and the sensitivity of the corium with a hoof tester. Horses

were accustomed to the treadmill during two days in at least three

30 minutes sessions16.

Data collection

Gait analysis was performed on a high-speed treadmill (Mustang

2200, Kagra AG) with an integrated force measuring system (TiF,

Equine Hospital, University of Zurich). This system decomposes the

reaction force response at the multiple bearing points of the

treadmill platform into the 4 vertical hoof forces. A detailed

description of this measuring system and the raw data processing

procedure has been published elsewhere140. The system resolu-

tions are: force 12 bit (6 N), time 2.3 ms, spatial 0.01 m.

Data acquisition and analysis

Data acquisition was started after a warm-up period of 15 minutes

at a walk and trot. Measurements were taken at 3.5 m/s. Record-

ing time lasted 20 s which resulted in 22–29 consecutive motion

cycles. Data sampling was started as soon as the horse was trot-

ting at a regular pace with the head and body aligned. Horses

were centred on the treadmill using two lead ropes attached on

either side of the halter. Measurements during which the horse

stumbled or visibly changed its position on the treadmill were

rejected.

74 l P h D - P U B L I C AT I O N I V

After recording the sound control measurement (‘sound’),

3 degrees of lameness were induced in the selected forelimb. Bolts

with flat tips were screwed into the nuts of the modified horse-

shoe thereby inducing pain on the corium of the sole during the

stance phase. The procedure was controlled using a torque meter

with 0.1 Nm increments (Type 757, Rahsol Dremotec). The same

pressure was induced to the medial and lateral side of the hoof.

After tightening the bolts, the horse was trotted on the treadmill

to evaluate the degree of lameness. The degree of lameness was

scored by an experienced veterinarian according to the following

definitions:

‘1’ – subtle lameness: irregularity not visible on every stride at

the trot;

‘2’ – mild lameness: visible on every stride at the trot;

‘3’ – moderate lameness: distinctly visible at the trot where,

however, the cadence of the movement is not obviously

disturbed.

If the degree of lameness corresponded to the protocol, gait

analysis was carried out, otherwise the bolts were readjusted.

After removing the bolts and walking the horses 15 min on the

treadmill, a final control measurement (‘post’) was recorded.

Experiments of the hindlimbs are not reported in this article. The

experimental protocol was approved by the Animal Health and

Welfare Commission of the canton of Zurich.

In total, 17 trials were conducted, in which eight left and nine

right forelimbs were set experimentally lame. In six horses unilat-

eral lameness was induced both in the left and in the right fore-

limb. Trials performed on the same horse were separated either

by ≥24 h or were conducted on the same day if baseline values

were re-attained. In order to pool all trials, the data of the induced

right forelimb lameness were mirrored mathematically, i.e. right

forelimb and hindlimb data were categorised as left forelimb and

hindlimb data, respectively, and vice versa. Data are reported as

results of the “lame” (left fore) limb, its contralateral limb (right

fore), its ipsilateral limb (left hind) and its diagonal limb (right

hind), and of the lame (front left – hind right) and sound diago-

nal (front right – hind left), respectively.

Analysed parameters and their abbreviations are listed in Table 8.

All force parameters were normalised to the horse’s bodyweight.

Contralateral limb asymmetry was expressed with the asymmetry

index (ASI) according to Herzog et al.62:

l 75P h D - P U B L I C AT I O N I V

ASI = ( Pleft – Pright )

100%0.5 (Pleft + Pright )

where ‘P’ denotes the parameter being assessed. In a fully bal-

anced situation between contralateral limbs, ASI is zero. Left-side

deficit results in a negative ASI.

On average 27±3.7 motion cycles per horse were analysed and

averaged. The individual results were then averaged (n=17 trials),

for each parameter, limb and lameness condition separately.

Statistics

Statistical analysis was performed with SigmaStat 2.0 (SPSS

Science). Descriptive statistics (mean, sd, sem) were computed for

the presentation of data. Normality of data was tested with the

Kolmogorov-Smirnov test. The influence of lameness was tested

for each limb separately using a parametric or a non-parametric

(Friedman) one way ANOVA for repeated measures, depending on

the result of the normality test; pairwise multiple comparison tests

were performed with the Student-Newman-Keuls method. ASI of

the sound control values were tested against ASI = 0 using one

group Student t-test. Paired t-test was used to test concordance

between left and right limbs. Significance level was set at P = 0.05.

Results

Mean tread-belt velocity during all trials was 3.49 ± 0.05 m/s and

statistically equal between conditions. An average pressure of

0.8 ± 0.3 Nm, 1.0 ± 0.4 Nm and 1.3 ± 0.5 Nm was applied with

each of the 2 bolts to induce the subtle, mild and moderate lame-

ness, respectively. In the moderate lameness group, three trials

were scored as moderate to severe; however, the horses could still

keep up the pre-set treadmill speed. In all investigated parame-

ters, no significant difference was found between the ‘sound’

and ‘post’ measurements.

With the exception of StD and Fzpeak of the lame limb, Izdiag%,

ASI SpD, ASI Iz, ASI Izdiag, ASI Fzpeak, parameters did not change

significantly in the subtle lameness condition. Thus, lameness-

induced changes reported in the text refer to statistically signifi-

cant differences between the sound and the mild and/or the

moderate lameness condition.

76 l P h D - P U B L I C AT I O N I V

l 77P h D - P U B L I C AT I O N I V

Temporal parameters

SF stride frequency [1/min]

SD stride duration [ms]

StD stance duration [ms]

StDrel relative stance duration [%] duration of stance phase as proportion of stride duration

SwD swing duration [ms]

StpDcl contralateral step duration [ms]

SpD suspension duration [ms]

TAP time of advanced placement [ms] time dissociation between diagonal limbs at initial

ground contact

TAC time of advanced completion [ms] time dissociation between diagonal limbs at toe-off

TFzpeak time of vertical force peak [ms]

Force parameters

Fzpeak peak vertical force normalised to the ‘bwt’ bodyweight

horse’s bwt [N/kg]

Fz fore% vertical force distribution between diagonal percentage of diagonal peak force carried

limbs at midstance [%] by the forelimb

Iz vertical impulse normalised to the horse’s integral of force over stance duration

bwt [Ns/kg]

Izdiag diagonal vertical impulse normalised sum of vertical impulses of a diagonal limb pair

to the horse’s bwt [Ns/kg]

Iz fore% distribution of Izdiag within the diagonal percentage of Izdiag carried by the forelimb

limb pair [%]

IzSD vertical stride impulse normalised sum of the 4 vertical impulses over an entire

to the horse’s bwt [Ns/kg] motion cycle

Spatial parameters

Gx fore gauge at impact between forelimbs [m] lateral distance between contralateral limbs

Gx hind gauge at impact between hindlimbs [m] lateral distance between contralateral limbs

dPtrac difference in limb protraction between if the value is positive, the left limb is further

contralateral limbs [m] protracted

dRtrac difference in limb retraction between if value is positive, the right limb is further

contralateral limbs [m] retracted

Table 8: Index of abbreviations of force, temporal and spatial

parameters

Temporal parameters

With increasing lameness, SD decreased and thus, SF increased

proportionally (Table 9).

StD increased equally in the affected as well in the contralateral

forelimb, but remained constant in the hindlimbs (Figure 14). Tak-

ing into account the continuous decrease of SD with increasing

lameness, StDrel increased significantly in all limbs (Table 9). SwD

decreased in all four limbs; the decrease was more pronounced in

the forelimbs because of the prolonged forelimb StD (Figure 15).

StpDcl decreased significantly in all four limbs; however, the

decrease was more distinct during/following the stance of the

affected diagonal limb pair (Figure 16). Relative to SD, StpDcl was

reduced during the transition from the lame to the sound diago-

nal stance and proportionally prolonged from the sound to the

lame diagonal stance.

With increasing lameness, SpD decreased after push-off of both

diagonal limb pairs; however, the reduction was more distinct

after the push-off of the lame diagonal (Table 9).

In the sound condition, the average TAP was only just negative

in both diagonals; i.e. in the majority of the horses the forelimbs

preceded the hindlimbs or impacted almost synchronously at

78 l P h D - P U B L I C AT I O N I V

Parameter Condition

sound subtle mild moderate

SF [1/min] 82.0 ± 0.9 82.5 ± 0.9 83.2 ± 0.8 a, b 85.7 ± 1.1 a, b (+ 4.5%)

SD [ms] 733 ± 7.9 728 ± 7.9 722 ± 7.4 a, b 702 ± 9.4 a, b (– 4.3%)

StDrel [%] forelimbs 41.2 ± 0.56 41.6 ± 0.68 42.3 ± 0.56 a, b 44.2 ± 0.56 a, b

hindlimbs 36.8 ± 0.45 37.2 ± 0.52 37.5 ± 0.50 a 38.6 ± 0.48 a, b

SpD [ms] lame diagonal 62 ± 4.6 57 ± 5.6 49 ± 4.6 a, b 29 ± 5.1 a, b (– 54%)

sound diagonal 62 ± 4.6 60 ± 5.6 58 ± 4.6 a 52 ± 4.2 a, b (– 17%)

TAP [ms] lame diagonal – 3 ± 2.4 – 3 ± 2.4 – 7 ± 2.7 – 16 ± 3.3 a, b

sound diagonal – 4 ± 2.6 – 4 ± 2.5 – 8 ± 3.1 a – 18 ± 3.0 a, b

TAC [ms] lame diagonal 29 ± 2.1 29 ± 2.5 28 ± 2.2 23 ± 2.4 a, b

sound diagonal 28 ± 2.4 28 ± 2.9 27 ± 2.3 22 ± 2.0 a, b

Refer to Table 8 for abbreviations. Data are presented as mean ± sem; percentage difference to reference value (sound) is given in

brackets.a significant difference (P < 0.05) compared to the sound condition.b significant difference (P < 0.05) compared to the preceding condition.

Table 9: Changes in temporal parameters with increasing

weightbearing lameness.

l 79

diagonal touch-down. With increasing lameness, the forelimbs of

both diagonals impacted even earlier (Table 9, Figure 19B). At

diagonal lift-off, the hindlimbs always preceded the forelimbs in

the sound as well as in the lame condition. TAC decreased in both

diagonals equally in the moderate lameness condition indicating

a slight prolongation of the double support (Table 9, Figure 19B).

P h D - P U B L I C AT I O N I V

Figure 14: Changes in stance duration (StD) with increasing left

forelimb lameness in horses at the trot (n=17 trials).

Data are presented as mean±sem. Percentage information as pro-

portion of the “sound” value.

‘sound’ = condition before inducing unilateral forelimb lameness,

‘1’= subtle lameness, ‘2’= mild lameness, ‘3’= moderate lameness;

‘FL’= left forelimb, ‘FR’= right forelimb, ‘HL’= left hindlimb,

‘HR’= right hindlimb.

a=significant difference (P<0.05) compared to the sound condition.

b=significant difference (P<0.05 ) compared to the preceding con-

dition.

240

260

280

300

[ms]

320

a

a,ba

FL

a,ba

FR

HL HR

240

260

280

300

[ms]

320

sound 1 2 3

302

100% 100% 101% 103%

302

100% 100% 101% 102%

270

100% 100% 100% 100%

269

100% 100% 100% 101%

GRF parameters

With increasing lameness, IzSD decreased (Table 10) proportionally

to the decrease of SD. Iz decreased in the affected forelimb and

in its ipsilateral hindlimb, while it increased in the contra-

lateral and diagonal limb (Figure 17). Fzpeak decreased in the lame

forelimb, while remaining unchanged in the contralateral limb; in

the hindquarters, Fzpeak decreased in the ipsilateral limb but

increased slightly in the diagonal limb (Figure 18).

With increasing lameness, Izdiag decreased during the lame diag-

onal stance while it remained unchanged during the opposite

diagonal stance (Table 10). Expressing Izdiag relative to the corre-

sponding IzSD of each condition, the deficit in Izdiag of the lame

diagonal was fully compensated by the sound diagonal. Accord-

ingly, in the subtle, mild and moderate lameness condition, the

80 l P h D - P U B L I C AT I O N I V

350

380

410

440

470

350

380

410

440

470

[ms]

FL FR

HL HR

[ms]

500

500

a,b

aa,b

a,b

a

a,b

a,b

a,b

sound 1 2 3

431

100% 99% 97% 91%

431

100% 99% 97% 91%

464

100% 99% 98% 93%

464

100% 99% 97% 93%

Figure 15: Changes in swing duration (SwD) with increasing left

forelimb lameness in horses at the trot (n=17 trials).

Refer to Figure 14 for shared caption and abbreviations.

diagonal impulse shifts were 0.4, 0.9 and 2.8%, respectively. With

increasing lameness, Iz fore% decreased in the lame diagonal and

increased in the sound diagonal (Table 10); i.e. in the lame diago-

nal weight was shifted towards the hindlimb, whereas, in the

sound diagonal, it shifted towards the forelimb. This oscillating

load distribution along the longitudinal axis of the horse was also

reflected in the distribution of peak vertical forces within the diag-

onal limb pair (Fz fore%; Table 10).

Representative vertical force curves within their time history of the

sound and the moderate lameness (‘3’) condition are presented

in Figure 19A. In the affected limb (left fore) the reduced impact

force and rate of loading in the first half of stance, the lower Fzpeak

and the prolongation of StD are obvious. In the contralateral limb

l 81P h D - P U B L I C AT I O N I V

Figure 16: Changes in contralateral step duration (StpDcl) with

increasing left forelimb lameness in horses at the trot (n=17 trials).

Refer to Figure 14 for shared caption and abbreviations.

FL>FR indicates transition from FL to FR; FR >FL, HL>HR and HR>HL

analogous.

320

330

340

350

360

370

320

330

340

350

360

370

[ms]

FL > FR FR > FL

HL > HR HR > HL

[ms]

380

380

a,b

a,b

a,b

a,b

a

a

sound 1 2 3

367

100% 99% 99% 98%

367

100% 100% 100% 99%

366

100% 99% 98% 93%

367

100% 99% 98% 93%

82 l

(right fore) only the prolonged StD and therefore an enlarged

impulse could be observed. In the diagonal limb (right hind) Fzpeak

increased slightly whereas it decreased in the ipsilateral limb (left

hind).

In the sound as well as the lame condition, TFzpeak occurred in

the forelimbs slightly before midstance whereas it coincided with

midstance in the hindlimbs (Figure 19B). Comparing the midstance

times of the diagonal limbs, a closer coincidence could be

observed in the lame condition. Thus, with increasing lameness,

the stance phase of the hindlimb was timewise centred to the

stance phase of the forelimb.

Spatial parameters

Left-right difference in limb protraction (dPtrac) and retraction

(dRtrac) are illustrated in Figure 20. Compared to the lame fore-

limb, the contralateral limb showed a prolonged anterior phase

and a shortened posterior part of stance. In the hind, the pro-

longed anterior and shortened posterior part of stance was

observed in the ipsilateral limb.

Contralateral asymmetry indices

Asymmetry indices of selected force and temporal parameters are

listed in Table 11.

In the sound condition, none of the ASIs differed significantly

from zero, indicating a high degree of symmetry in weightbear-

P h D - P U B L I C AT I O N I V

Parameter Condition

sound subtle mild moderate

IzSD [Ns/kg] 7.23 ± 0.10 7.15 ± 0.10 7.10 ± 0.09 a, b 6.90 ± 0.11a, b (– 4.5%)

Iz diag [Ns/kg] lame diagonal 3.62 ± 0.05 3.55 ± 0.05 3.48 ± 0.04 a, b 3.25 ± 0.06 a, b (– 10%)

sound diagonal 3.62 ± 0.05 3.60 ± 0.05 3.62 ± 0.05 3.64 ± 0.05

Iz fore% [%] lame diagonal 55.9 ± 0.2 55.1 ± 0.3 53.7 ± 0.4 a, b 49.6 ± 0.7 a, b

sound diagonal 55.9 ± 0.3 55.9 ± 0.4 56.9 ± 0.6 a, b 59.3 ± 0.4 a, b

Fz fore% [%] lame diagonal 53.1 ± 0.4 52.3 ± 0.4 50.6 ± 0.6 a, b 45.8 ± 0.8 a, b

sound diagonal 53.1 ± 0.4 53.1 ± 0.5 53.6 ± 0.5 55.0 ± 0.4 a, b

IzSD (9.81 SD) –1 1.005 ± 0.004 1.000 ± 0.005 1.002 ± 0.005 1.001 ± 0.005

Refer to Table 8 for abbreviations. Data are presented as mean ± sem; percentage difference to reference value (sound) is given in

brackets.a significant difference (P <0.05) compared to the sound condition.b significant difference (P <0.05) compared to the preceding condition.

Table 10: Changes in force parameters with increasing

weightbearing lameness

l 83

ing and inter-limb timing. Values corresponded with data pub-

lished for sound Warmblood horses141.

StD and SwD showed no side-specific changes with increasing

lameness. However, time parameters defining the transition

between diagonals turned out to change asymmetrically

(ASI StpDcl, ASI SpD) which allowed the assignment of the diago-

nal that comprised the affected limb. ASI SpD showed excep-

tionally large standard deviations in all conditions. With increas-

ing lameness, the contralateral asymmetry of force parameters

increased significantly and unequally in fore- and hindlimbs,

allowing the unequivocal detection of the affected limb.

P h D - P U B L I C AT I O N I V

Figure 17: Changes in vertical impulse (Iz) with increasing left

forelimb lameness in horses at the trot (n = 17 trials).

Refer to Figure 14 for shared caption and abbreviations.

1.3

1.5

1.7

1.9

2.1

98% 93%1.3

1.5

1.7

1.9

2.1

[Ns/

kg]

FL FR

HL HR

[Ns/

kg]

2.2

2.2

a,b

a

a,b

a,b

a,ba,b

a,b

sound 1 2 3

1.59

100% 99%

2.02

100% 97% 93% 80%

2.02

100% 100% 102% 107%

1.60

100% 100% 101% 103%

84 l P h D - P U B L I C AT I O N I V

Figure 18: Changes in peak vertical force (Fzpeak) with increasing left

forelimb lameness in horses at the trot (n =17 trials).

Refer to Figure 14 for shared caption and abbreviations.

11.07

100% 99% 99% 101%

11.07

100% 96% 91% 76%7

8

9

10

11

9.75

100% 99% 98% 94%7

8

9

10

11

9.76

100% 99% 101% 102%

[N/k

g]

FL FR

HL HR

[N/k

g]

12

12

a,b

a

a,b

a,ba

a,b

sound 1 2 3

Parameter Condition Trend

sound subtle mild moderate

ASI StD [%] forelimbs – 0.1 ± 1.4 0.2 ± 1.9 0.1± 1.6 0.3 ± 2.3 =

hindlimbs 0.1 ± 1.8 – 0.1 ± 2.0 – 0.1 ± 1.7 – 0.9 ± 2.3a,b =

ASI SwD [%] forelimbs 0.0 ± 1.1 – 0.1 ± 1.4 – 0.1 ± 1.2 – 0.3 ± 1.9 =

hindlimbs – 0.1 ± 1.1 0.1 ± 1.2 0.0 ± 1.0 0.5 ± 1.5a,b =

ASI StpDcl [%] diagonals 0.2 ± 3.3 0.8 ± 3.2 2.2 ± 3.5 a,b 6.5 ± 5.9 a,b �

ASI SpD [%] diagonals – 1.2 ± 12.5 – 6.2 ± 15.7a – 19.0 ± 21.7a,b – 82.1± 71.2 a,b ��

ASI lz [%] forelimbs 0.0 ± 2.2 – 3.0 ± 2.9 – 9.5 ± 6.0 a,b – 29.2 ± 11.0 a,b ��

hindlimbs – 0.1 ± 4.3 – 0.6 ± 4.9a – 3.5 ± 5.6 a – 10.0 ± 8.4 a,b �

ASI lz diag [%] diagonals 0.0 ± 1.8 – 1.4 ± 2.2 a – 3.7 ± 2.7a,b – 11.4 ± 5.1a,b �

ASI Fzpeak [%] forelimbs 0.0± 1.8 – 2.8 ± 2.7 a – 8.7 ± 4.7a,b – 28.0 ± 11.0 a,b ��

hindlimbs – 0.1 ± 2.9 – 0.3 ± 3.3 – 3.1 ± 4.4 a,b – 8.9 ± 6.3 a,b �

Refer to Table 8 for abbreviations. Data are presented as mean ± sd;a significant difference (P<0.05) compared to the sound condition. b significant difference (P<0.05) compared to the preceding condition.

Table 11: Effect of weightbearing lameness on the contralateral

index (ASI) of selected force and temporal parameters.

l 85P h D - P U B L I C AT I O N I V

Figure 19A: Representative vertical force curves of a horse trotting

on an instrumented treadmill. Time line normalised to units of

stride duration.

Bold line: initial condition before inducing lameness;

thin line: moderate left forelimb lameness.

Black: lame diagonal (FL–HR); grey: sound diagonal (FR–HL).

Figure 19B: Inter-limb co-ordination of horses (n=17) trotting on an

instrumented treadmill. Black line: condition before inducing lame-

ness; grey line: moderate left forelimb lameness. Stance and swing

durations are expressed relative to stride duration.

Refer to Figure 14 for shared abbreviations. ▼ time of peak vertical

force, sound condition; ∇ condition of moderate lameness; ● mid-

stance, sound condition; ❍ condition of moderate lameness.

FL

HR

HL

FR

B

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

units of stride duration (SD)

02468

101214

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

units of stride duration (SD)

fore

limbs

[N/k

g]

A

02468

101214

hin

dlim

bs

[N/k

g]

-0.2

lame diagonal sound diagonal

86 l P h D - P U B L I C AT I O N I V

Figure 20: Difference in limb protraction (dPtrac) and retraction

(dRtrac) with increasing left forelimb lameness in horses at the

trot (n=17 trials).

Refer to Figure 14 for shared caption and abbreviations.

[m]

0.06

a

- 0.04

- 0.02

0.00

0.02

0.04

0.06

- 0.04

- 0.02

0.00

0.02

0.04

[m]

protraction forelimbs retraction forelimbs

protraction hindlimbs retraction hindlimbs

a,b

a,b

a,b

a

sound 1 2 3

l 87

Discussion

It was hypothesised, that the compensatory mechanisms of

weightbearing lameness, aim primarily to diminish the structural

stress at the affected site, i.e. to reduce peak forces. In the pres-

ent study, subtle, mild and moderate lameness led to a continu-

ous decrease of Fzpeak in the affected limb (–4%, –9.0% and

–24 %, respectively). However, no compensatory increase was

observed in the contralateral limb and in the diagonal hindlimb

only a 2% increase of Fzpeak in the moderate lameness condition

could be recorded. In the ipsilateral hindlimb, Fzpeak even

decreased by 2% and 6% in the mild and moderate lameness

condition, respectively. Morris and Seeherman100 also reported

reduced Fzpeak not only in the affected limb, but also in the ipsi-

lateral hindlimb. Furthermore, a similar redistribution pattern for

the longitudinal forces was observed: reduced horizontal braking

forces in the lame forelimb, increased horizontal braking forces in

the contralateral limb, increased horizontal propulsive forces in

the diagonal hindlimb and reduced horizontal propulsive forces in

the ipsilateral hindlimb.

In this study, 4 mechanisms were perceived as contributing to

the reduction of Fzpeak at the affected site:

Altering the cadence – reducing stride impulse

In the overground situation, humans with gait abnormalities as

well as lame horses at the trot tend to prefer slower velocities of

the gait2,37,43. On the treadmill, the horses were constrained to

adopt the preset belt velocity. With increasing lameness, the

horses increased SF and thus, decreased SD proportionally. In the

moderate lameness condition, this decrease was around 4.3%

accompanied by a 4.6% decrease of IzSD (Table 9 und 10). Accor-

ding to the momentum theorem, the total vertical impulse is:

∆ J z = ∫ FGRF (t) dt – m g T = m∆ v

P h D - P U B L I C AT I O N I V

Time Force

1. altering the cadence reducing stride impulse

2. asymmetric transitions between redistribution of load between

the lame and sound diagonal the two diagonals

3. redistribution of load within

the diagonal

4. prolongation of stance duration changing the rate of loading

T

O

where ‘∫FGRF (t) dt’ is the time integral of the vertical forces, ‘m’

the body mass of the subject, ‘∆v’ its change of velocity, ‘g’ the

gravitational acceleration and ‘T’ the time during which the forces

act.

During a complete stride cycle (SD), ∆Jz has to equal zero, oth-

erwise the horse would have a net downward or upward velocity

at the end of each stride which would be equivalent with a net

vertical displacement of the body centre of mass. Thus, during a

complete stride cycle, the time-integrated vertical ground reaction

forces equal the gravitational force throughout stride duration.

Expressing the impulse (∆ Jz) as body-weight (bwt) normalised

impulses (∆IzSD= ∆Jz /bwt ) the following equation can be devel-

oped:

∆ Iz = ∆ Iz FL + ∆ Iz FR + ∆ Iz HL + ∆ Iz HR = gSD

Our data showed an excellent conformity with this theorem where

the quotient IzSD /(g SD) equalled 1 for the sound and all lameness

conditions (Table 10).

The shorter SD resulted in a shorter SwD of all four limbs.

Because absolute StD and, therefore, stance length remained

unchanged in the hindlimbs and even increased in the forelimbs,

a faster protraction or a more direct flight arc of the limb has to

be assumed. There is evidence that the cost of locomotion is pro-

portional to SF at any given speed 61. The higher cadence and the

faster protraction observed in this study substantiate the pre-

sumption that lame horses have, for a given speed, a greater

expenditure of energy.

As temporal and GRF parameters depend strongly on the move-

ment velocity74–76,88, it is essential to account for differences in

speed, particularly when comparing observations between normal

and unsound subjects, or before and after diagnostic and thera-

peutic interventions. In this study, treadmill speed was held con-

stant (3.49 ± 0.05 m/s) for all lameness degrees which allowed

direct statistical comparison between the sound and lame condi-

tions.

Asymmetric transitions between the lame and sound diagonal

– redistribution of load between the two diagonals

The reduction of IzSD involved only the lame diagonal stance. Izdiag

of the lame diagonal stance decreased by 10%, whereas the Izdiag

of the sound diagonal stance remained constant (Table 10). In

88 l P h D - P U B L I C AT I O N I V

correspondence, Buchner et al.21 observed reduced vertical trunk

movement during the lame diagonal stance and barely unaffected

vertical trunk movements during the sound diagonal stance. In the

time domain, this asymmetry of Izdiag was reflected in an asym-

metric transition between the two diagonals: both, StpDcl and

SpD were shorter at the transition from the lame to the sound

diagonal which resulted in an earlier support of the body by the

sound diagonal. All changes in temporal parameters reported in

this study were in full agreement with the findings from kinematic

analyses18.

Redistribution of load within the diagonal

In the sound condition, 56% of the Izdiag were carried by the fore-

limb. Lameness changed this distribution in a characteristic fash-

ion. During the lame diagonal stance, load was shifted to the rear.

Compared to the sound situation, the diagonal hindlimb carried

6.3 % more of Izdiag during moderate lameness (Table 10).

Although statistically significant, the compensatory increase of

Fzpeak and Iz in the diagonal hindlimb was in relative and absolute

values small. During the sound diagonal stance, Izdiag was shifted

to the front limb by over 3.4%. The weight-shifting mechanism

along the longitudinal axis of the horse corresponds with the

changes in body centre of mass movement25 and the conclusions

drawn from the inverse dynamic model calculation of compensa-

tory head movements in lame horses136. The reduction or even

suppression of the downward head acceleration during the lame

diagonal stance phase decreases the momentum in the trunk,

thus unloading the lame forelimb, but increasing the loading of

the diagonal hindlimb. During the sound diagonal stance, the dis-

tinct vertical head nod together with the higher horizontal brak-

ing forces in the contralateral forelimb37,100 produce a momentum

in the trunk, which increases the loading of the contralateral fore-

limb and decreases that of the ipsilateral hindlimb.

In moderate and more accentuated forelimb lamenesses, this

weight-shifting mechanism can be observed as a compensatory

movement at the sacrum and the croup. This may mislead the

visual gait assessment and delude a lameness in a sound hindlimb.

In contrast to a kinematic study, which, for forelimb lamenesses

postulated a false lameness in the diagonal hindlimb133, the pat-

tern of load redistribution seen in this study is more indicative of

an ipsilateral compensatory strategy. Furthermore, the moderately

lame horses in this study were observed showing a distinct low-

l 89P h D - P U B L I C AT I O N I V

ering of the sacrum at midstance of the diagonal hindlimb which

was interpreted as a projected weightbearing lameness in the

ipsilateral hindlimb. Comparing the respective forelimb and

hindlimb asymmetry indices of Fzpeak and Iz, the “cross-talk” of a

forelimb lameness to the hindquarters amounted to approxi-

mately one third (Table 11).

Prolongation of stance duration – changing the rate of

loading

A way to reduce the loading rate and peak forces while main-

taining a given impulse, is to extend StD. Prolonged StD was not

only observed in the affected limb (+3%) but also in the con-

tralateral limb (+2%). These observations agree with treadmill18

as well overground data100 of lame horses, however, contradict

the statement that shortened stance duration is indicative of a

weightbearing lameness27,113. The first three mechanisms suc-

ceeded in reducing the Iz of the affected limb by 20% in the mod-

erate lameness situation. As a result of the slight prolongation of

StD, Fzpeak decreased by as much as 24%. In the contralateral

limb, the prolongation of StD parried the compensatory shift of

the vertical impulse (+7%) almost entirely without exerting higher

peak vertical forces. The hindlimbs, in particular the diagonal

hindlimb did not profit from this compensatory mechanism, as StD

remained unchanged. In mild to moderate lamenesses, this strat-

egy plays a decisive role in preventing the contralateral limb from

potential secondary injury. Despite this fact, larger maximal fet-

lock hyperextension angles at midstance were reported for the

contralateral sound limb20 and interpreted as increased compen-

satory vertical load bearing. A more probable explanation is the

increased longitudinal braking force, which extend beyond mid-

stance 37,100.

As long as the lameness is mild to moderate, the increase of StD

is a possible compensatory mechanism. In more severe lameness

conditions the horse will be forced to change entirely to another

compensatory mechanism, reducing StD on the affected limb

which will obviously disturb the continuous cadence of the gait

and may induce compensatory overload in the diagonal and con-

tralateral limbs.

Shortening of StD is often empirically claimed to be an acoustic

indicator for weightbearing lameness. As the conclusion of stance

is silent, StD is not assessable acoustically. Furthermore, the bilat-

eral equal prolongation of StD also refutes this theory. However,

90 l P h D - P U B L I C AT I O N I V

the asymmetric shortening of StpDcl and, therefore, the shorter

transition interval from lame to sound limb impact (Figure 16),

reinforced by the louder impact of the sound limb, are the more

plausible leads for acoustic detection of lameness.

The momentum theorem explains the interdependence of

impulse and the time during which the forces act. Under the

assumption that the basic shape of the force curve stays unaf-

fected, changes in impulse must result in proportional changes in

the time domain. There is as yet only speculation as to whether

the lame horse initially changes its loading pattern, which then

results in the adjustments of cadence and inter-limb timing or vice

versa. If the dictating principle is economy, i.e. preserving func-

tion without consuming much more energy, the tuned application

of force may be the triggering factor.

Spatial arrangement of the limbs

With increasing lameness, the limbs of the sound diagonal were

more protracted at impact and less retracted at lift-off. A pro-

tracted orientation of the compensating limbs at initial ground

contact facilitated the support of the trunk to compensate for the

loss on height of the body centre of mass attained during the lame

diagonal stance37. However, if the site of the sole pain is located

at the toe, horses increase protraction in the affected limb to

avoid initial ground contact with the cranial part of the hoof 71.

For analytical purpose, the vertical force parameters Fzpeak and

Iz proved to have the highest sensitivity and limb-specificity. From

simultaneously determined force profiles of the four limbs, the

affected limb could be identified unambiguously even when the

lameness was subtle. Time parameters such as SpD and StpDcl

could only discriminate between the lame and the sound diago-

nal stance phases, whereas the basic temporal parameters SF, StD

and SwD changed bilaterally or in all limbs proportionally. With

increasing lameness, the majority of parameters changed in a

rather nonlinear way. Although in the subtle lameness condition,

the initial changes followed the respective trends, they did not

reach statistical significance when compared to the sound condi-

tion. Exceptions were Fzpeak and StD of the lame limb. This reflects

the linear constraints of clinical lameness scoring and the defini-

tion of subtle lameness whereby as little as a few lame strides out

of 25 are enough to classify the horse as subtly lame.

The presented data set may allow a first attempt at “calibrat-

ing” the degree of the clinical lameness scoring with vertical GRF

l 91P h D - P U B L I C AT I O N I V

92 l

data. When compared to the sound situation, subtle gait irregu-

larities showed a mean decrease of 4% in Fzpeak of the affected

limb, in mild lamenesses it decreased by 9% and in moderate

lamenesses by up to 24%; ASI Fzpeak, amounted to –2.8± 2.7%,

–8.7± 4.7% and –28.0±11.0% (mean±sd), respectively, for the

different grades. Peak force difference between contralateral fore-

limbs correlated best with the clinical lameness score (r2 = 0.87).

Peak force deficits in a similar range were published previously:

Morris and Seeherman100 reported a decrease in Fzpeak of 11.5%

(ASI Fzpeak, –10.4 %) for an “easily observable lameness” and

Clayton et al.37 a reduction of 28.5% (ASI Fzpeak, –36.3%) for a

moderate lameness. Huskamp et al.66 proposed, based on data

measured with the EGA-system of horses with navicular disease,

a rating system where the differences between lame and con-

tralateral sound limb amounted to 5–10% in subtle, 10–20% in

mild and over 30% in moderate lamenesses.

The regularity of the gait pattern and the reproducibility of mov-

ing velocity on the treadmill are a decisive factor in reducing intra-

individual variability of gait parameters, which again contribute to

the discrimination between the different degrees of lameness.

ASI of force parameters differed significantly between the 3 lame-

ness degrees. Overlaps of the standard deviations between the

slight and mild and the mild and moderate lameness group were

small. However, the cut-off between “normal asymmetry” and

subtle weightbearing asymmetry remains a difficult task as fore-

limb ASI Fzpeak of sound Warmblood horses trotting at the same

velocity was –0.2 ±2.0%141 and 0.0±1.8% in this study.

The horses’ attempt to cope with weightbearing forelimb lame-

ness comprised the reduction of IzSD and its redistribution to the

contralateral fore- and diagonal hindlimb without producing an

equivalent compensatory increase of Fzpeak in these limbs. Future

studies have to examine how far these compensatory mechanisms

may be generalised for other defined orthopaedic problems.

P h D - P U B L I C AT I O N I V

Acknowledgements

The authors wish to thank the Swiss National Equestrian Centre

Berne, particularly Dr. Beat Wampfler, for kindly leaving their

horses at the study’s disposal, the farriers Simon Alt and

Hans-Ueli Schneeberger for preparing the hooves and horseshoes

and Vreni Hänni, Tanja Rietmann and Anne Stuart for technical

assistance.

l 93P h D - P U B L I C AT I O N I V

l 95

General Discussion and Conclusions

Methodological aspects

The instrumentation of an equine highspeed treadmill for GRF

measurements is technically – because of the involved high forces

exerted by 500–700 kg heavy horses – a particular challenge. A

partitioned bearing plate, where 4 separate force measuring sec-

tors are assigned to each limb had, therefore, to be abandoned.

The concept of determining the vertical GRFs of the 4 limbs indi-

rectly from a single load-sensing platform by weighting the force

information from the multiple bearing points of the platform with

the transfer coefficients of the respective force application points

of the hoofs and calculating the unknown hoof forces by the use

of a linear equation system, was realised successfully. The com-

parison between static test loads and the forces calculated with

the treadmill system revealed deviations of <1.4%. The conform-

ity with the force traces measured simultaneously with force shoes

was excellent and highly correlated (r≥0.998). The force resolution

of the system is 6 N, the time resolution 2.3 ms and the spatial

resolution 0.01 m. The natural frequency of the system is located

between 40 and 60 Hz: three times higher than the frequency

components of the information signal (<15 Hz). In comparison to

force plate data published in the literature, the treadmill force

traces correspond in shape and amplitude to a high degree. Even

discreet details of the force curve, such as the impact force peak

immediately after touch-down or the kink during breakover of the

hoof can be recognised on the treadmill force curves.

In contrast to a single force plate, this treadmill integrated force

measuring system allows the simultaneous determination of the

vertical forces of the 4 limbs within their time history. Typically, up

to 50 strides per limb, each at the walk and trot, are recorded

within less than 5 minutes. The calculation of the force traces

starts during data acquisition and is presented in real time on the

computer screen. Fourteen force, 11 temporal and 6 spatial

parameters are derived from the raw data within 30 seconds and

are the basis for further parameter calculations (e.g. relative

parameters, left-to-right symmetry indices). The time-efficient

data acquisition and analysis present an important advantage in

a clinical set-up when dealing with orthopaedic and other reha-

bilitation patients that cannot physically tolerate long trotting

examination. Even the evaluation of the effect of certain diag-

nostic procedures, such as blocking of the coffin joint or the nav-

icular bursa, where the instantaneous effect may provide specific

96 l G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

diagnostic information, can be monitored by the use of this sys-

tem. The large number of consecutive strides can be averaged to

determine more representative values, thereby increasing statisti-

cal power. Horses, once adapted to the treadmill, show a very reg-

ular gait pattern with minimal intra-individual variations. The

intra-individual variability of sound trotting horses was, for the

majority of the key parameters, smaller than 2.0% and did not

exceed 2.7%. Trial-to-trial speed variability, lassitude of the

patient from repeated trotting and therefore test-retest reliability

may be of concern in data collection of overground motion analy-

sis. Factors influencing the GRF pattern critically, such as surface

condition, running velocity and duration of exercise, are strictly

controlled within this experimental set-up. In the different exper-

iments conducted, the coefficient of variance of the running

velocity was between 1.0 and 1.4%. The high standardisation and

reproducibility of the trial run, together with the low intra-indi-

vidual variability, allow reliable comparison between different sub-

jects and between measurements of the same individual at dif-

ferent times, as this presents a recognised difficulty in subjective

assessment. Inter-individual variability can be further controlled by

using the subject as its own control (e.g. before and after diag-

nostic anaesthesia), transforming key parameters into relative val-

ues (e.g. limb impulse relative to the total stride impulse; left-to-

right asymmetry index), and by correcting for characteristic con-

formational dimensions (e.g. height at the withers).

With the system presented here, minimal instrumentation inter-

fering with the horse was required. Recently, a new software ver-

sion has been developed which allows the calculation of the 4

force traces without the goniometer positioning system, using

instead the force distribution along the treadmill platform for the

positional data of the weightbearing limbs. It can be concluded,

that this novel kinetic gait analysis system meets the required

standards of precision for locomotor research and is easily opera-

tional in a clinical set-up. Because the treadmill belt dictates the

path of the limbs, horizontal forces (fore-aft and lateral) are not

measurable with this concept. This presents a certain drawback as

it was shown that horizontal decelerating and propulsive forces

also change selectively in relation to lameness96,100. Furthermore,

the application of specific gait analysis concepts, such as inverse

dynamic calculations is limited.

l 97G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

Compensatory mechanisms of weightbearing

lameness

In clinically sound horses at the trot, the left-to-right symmetry of

force, temporal, and spatial parameters is very high. The asym-

metry indices of the investigated parameters did not deviate sig-

nificantly from zero. Nevertheless, asymmetries between 1.8%

and 6.8% (95% reference interval of the asymmetry index) for

the key force and temporal parameters confirmed that sound

horses may show an individual, natural locomotion asymmetry at

the trot 47,48,99,111. Averaging GRF-time histories from a represen-

tative group of sound horses results in a standard to which the

GRF data of an arbitrary horse can be compared. However, it must

be emphasised, that comparisons are restricted to data recorded

at the same moving velocity and to horses of similar exterior con-

formation and wither heights.

Unilateral weightbearing forelimb lameness changed the load

distribution and timing between the 4 limbs in a characteristic

way. Four mechanisms could be identified, which contributed to

diminishing the structural stress at the affected site, i.e. to reduce

peak forces:

With increasing lameness, horses reduced the total vertical

impulse per stride. Because the trotting velocity on the treadmill

was held constant, this was achieved by increasing stride fre-

quency, which resulted in a more continuous support of the body

centre of mass by a larger number of strides per unit of time.

With increasing lameness, the rate of loading and unloading the

limb was reduced by prolonging stance duration. The distribution

of a given impulse over a longer time period likewise results in a

reduction of the peak force. This mechanism was not only

observed in the affected, but also in the contralateral, sound fore-

limb.

With increasing lameness, the diagonal impulse decreased

selectively in the lame diagonal, but remained unchanged in the

sound diagonal stance. In the time domain, this resulted in a

shortened contralateral step duration and suspension duration

after push-off of the lame diagonal limb pair. Supposing that the

vertical decelerative portion of the vertical impulse is determined

by the preceding suspension phase, i.e. of the extent of vertical

impulse of the contralateral diagonal stance, and that the propul-

sive portion of the vertical impulse determines the subsequent

suspension phase, it may be speculated that the reduced diago-

98 l G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

nal impulse of the lame diagonal is mainly caused by a weakened

active propulsive force.

With increasing lameness, load (impulse) distribution was

shifted within the lame diagonal to the diagonal hindlimb and in

the sound diagonal stance to the contralateral forelimb, i.e. in the

fore- as well in the hind quarter away from the affected side. Kine-

matic studies indicated that this load shifting strategy along the

longitudinal axis is predominantly the result of the prominent

compensatory movement of the head and neck21,136.

The consequence is remarkable: with these load shifting mech-

anisms, the relief of peak forces in the lame limb was not only

effective but even suppressed an equivalent compensatory over-

load situation in the other limbs. Only in the moderate lameness

condition, first indications of a compensatory increase of Fzpeak in

the diagonal hindlimb could be observed. However, in an even

more pronounced lameness, the horse will be forced to switch

completely to another compensatory strategy, reducing stance

duration on the affected limb, which will obviously disturb the

rhythm of the gait and may induce compensatory overload in the

diagonal and contralateral limbs.

During a complete stride cycle, the sum of time-integrated ver-

tical ground reaction forces (stride impulse) equals the gravita-

tional force throughout stride duration: IzSD = bwt g SD, where

‘IzSD’ is the total vertical stride impulse, ‘bwt’ the bodyweight of

the horse, ‘g’ the gravitational acceleration and ‘SD’ the stride

duration. If this term is not met, the body centre of mass would

have a net downward or upward displacement at the end of each

stride. Furthermore, this term clearly illustrates that the impulse

and the time during which the forces act, must relate propor-

tionally. Under the condition that the shape of the force curve

stays unaffected, changes in impulse must therefore result in pro-

portional changes in the time domain. The question, if the lame

horse initially changes its loading pattern, which then results in

the adjustments of cadence and interlimb timing or vice versa, can

only be answered speculatively. If the dictating principle is econ-

omy, i.e. preserving function without consuming more energy,

the tuned application of force may be the triggering factor and

temporal adjustments would result from it. Obviously, the per-

ception of pain, which initiates the compensatory mechanisms,

depends itself on principles, such as the state of the autonomic

nervous system.

l 99G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

Compensatory movements of supporting forelimb lameness can

be observed at the croup or tuber sacrale and those of support-

ing hindlimb lameness at the head. These movements may mis-

lead the visual gait assessment and suggest a false lameness in a

sound limb without structural or functional lesions. Contrarily to

a kinematic study, which postulated a false lameness in the diag-

onal hindlimb as a result of forelimb lameness133, the patterns of

load redistribution indicate an ipsilateral compensatory strategy.

Furthermore, this study showed that in more pronounced lame-

nesses, the load shift within the lame diagonal matched a distinct

lowering of the sacrum at midstance of the diagonal hindlimb,

which may be interpreted as transmitted weightbearing lameness

in the ipsilateral hindlimb.

For analytical purpose, the force parameters showed the high-

est specificity for identifying the affected limb and the highest

sensitivity for grading the lameness. From simultaneously deter-

mined force profiles of the 4 limbs, the affected limb can be iden-

tified unambiguously even if the lameness is subtle. The time

parameters, suspension duration and contralateral step duration,

could only discriminate between the lame and the sound diago-

nal stance phase. Whereas the basic temporal parameters stride

frequency, stance, and swing duration changed bilaterally or in all

limbs proportionally. In general, the clinical lameness score and

the asymmetry of limb loading measured by the force treadmill

showed a good correspondence. However, in exceptional cases,

particularly with subtle lamenesses, discrepancies between these

two assessment methods were noticed. This was reflected in a

missing correlation between the subjectively determined degree

of lameness and the degree of weightbearing asymmetry139.

Many common locomotor disorders, such as navicular disease

or spavin occur bilaterally, which presents a problem not only for

the traditional, visual lameness assessment but also for gait analy-

sis systems, which rely on the asymmetry of measurements

between contralateral limbs. In those cases, the comparison of

specific parameters (e.g. stride impulse) with their reference val-

ues may help to detect an abnormal gait pattern. This remains to

be evaluated.

The analysis of force and time data of horses with induced uni-

lateral weightbearing hindlimb lameness indicates very similar

compensatory mechanisms, i.e. a decrease of the total stride

impulse (Figure 21), a reduction of the loading rate in the affected

as well in the contralateral hindlimb (prolonged stance duration;

100 l G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

l 101

Figure 22), and an impulse redistribution predominantly to the

contralateral hindlimb (Figure 23) without increasing peak vertical

forces in other limbs (Figure 24)142. This observation leads to the

assumption, that, similar to the conclusions drawn from kinematic

lameness assessment24, horses redistribute load using a consistent

strategy to compensate for pain in a single limb.

The identification of the affected limb and quantification of

lameness by the use of a gait analysis system alone, presents no

problem; the precise localisation of the ailment, however, remains

still unsolved. A characteristic feature that unequivocally points to

a specific lameness has still to be found. A large data base of spe-

cific lamenesses has to be compiled to provide the required refer-

ence values. The combined analysis of time-locked kinetic and

kinematic data will be compulsory. The task of using gait analysis

to provide qualitative new information that is beyond the scope

of visual assessment by experienced veterinarians, remains a con-

siderable challenge for future gait analysis research. Presently, the

localisation of the aliment and the assessment of bilateral and

subclinical lamenesses still rely on the diagnostic nerve block tech-

nique but would definitely benefit from the quantitative feed-

back, especially in more complex cases.

G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

Figure 21. Changes in total stride impulse (IzSD; A) and stride fre-

quency (SF; B) with increasing left hindlimb lameness in horses at

the trot (n= 8 trials).

Data are presented as mean± sem. Percentage information as pro-

portion of the “sound” value.

‘sound’ = condition before inducing unilateral hindlimb lameness,

‘1’ = subtle lameness, ‘2’ = mild lameness, ‘3’ = moderate lameness.

a = significant difference (P<0.05) compared to the sound condition.

b = significant difference (P < 0.05) compared to the preceding

condition.

[Ns/

kg]

7.6

7.2

6.8

6.4

6.0 75

78

81

84

[1/m

in]

87

A B

7.24

100% 99% 99% 97%

81.3

100% 101% 101% 103%

7.19 7.17 7.02 82.0 82.3 84.1

The treadmill-integrated force measuring system proved to be a

reliable tool in quantifying weightbearing and temporal asymme-

tries and, therefore, effectively assisting a clinical orthopaedic

work-up. Within sports medical care and preventive medicine, a

general status of the locomotor performance can be assessed

periodically. Curative procedures and rehabilitative training can be

102 l G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

240

260

280

300

[ms]

320

a,b a

FL

a

FR

HL HR

240

260

280

300

[ms]

320

sound 1 2 3

a,b aa,b

a,b

299

100% 100% 99% 99%

300

100% 100% 100% 99%

269

100% 100% 101% 102%

267

100% 100% 102% 103%

300 296 295 301 298 296

269 272 275 268 271 275

Figure 22: Changes in stance duration (StD) with increasing left

hindlimb lameness in horses at the trot (n = 8 trials).

Data are presented as mean ± sem. Percentage information as pro-

portion of the “sound” value.

‘sound’ = condition before inducing unilateral hindlimb lameness,

‘1‘= subtle lameness, ‘2’= mild lameness, ‘3’= moderate lameness;

‘FL’= left forelimb, ‘FR’= right forelimb, ‘HL’= left hindlimb,

‘HR’= right hindlimb.

a = significant difference (P<0.05) compared to the sound condition.

b = significant difference (P < 0.05) compared to the preceding

condition.

monitored more closely and finally the effect of new lameness-

alleviating drugs can be tested quantitatively. Information on load

distribution and interlimb coordination creates novel perspectives

for assessing gait quality and efficiency. The influence of a rider

on the horse’s internal balance as well as the effect of velocity or

fatigue on the loading pattern are interesting aspects that can be

studied easily. Especially, the consequence of various head-neck

positions on the locomotion pattern of the horse is of current

interest12,120 and merits further and conclusive investigations.

l 103G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

1.2

1.4

1.6

1.8

2.0

94% 85%1.2

1.4

1.6

1.8

2.0

[Ns/

kg]

FL FR

HL HR

[Ns/

kg]

2.2

2.2

a,b

a,b

a,ba,b

a

a,b

1.61

100% 98%

2.03

100% 99% 99% 98%

2.00

100% 100% 100% 98%

1.59

100% 100% 103% 106%

2.02 2.02 1.99 2.00 2.00 1.97

1.58 1.51 1.37 1.60 1.64 1.68

sound 1 2 3

Figure 23: Changes in vertical impulse (Iz) with increasing left

hindlimb lameness in horses at the trot (n = 8 trials).

Refer to Figure 22 for shared caption and abbreviations.

104 l G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N

Figure 24: Changes in peak vertical force (Fzpeak) with increasing left

hindlimb lameness in horses at the trot (n = 8 trials).

Refer to Figure 22 for shared caption and abbreviations.

11.08

100% 99% 100% 99%

11.09

100% 99% 101% 100%7

8

9

10

11

9.85

100% 98% 93% 85%7

8

9

10

11

9.80

100% 100% 101% 101%

[N/k

g]

FL FR

HL HR

[N/k

g]

12

12

a,b

a,b

10.98 11.16 11.07 10.99 11.10 10.93

9.82 9.91 9.949.63 9.20 8.38

sound 1 2 3

l 105

References

l 107

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Instrumented treadmill for measuring vertical ground reaction forces in

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Summary

Summary

The first publication describes the development and validation of

a novel instrumented treadmill capable of determining vertical

ground reaction forces in horses of all 4 limbs simultaneously.

The system consists of a load-sensitive treadmill platform, a

positioning system to localise the force application points of the

hoofs on the treadmill platform, a force-transfer coefficient matrix

that represents the transfer characteristics from every possible

force application point to every sensor of the load-sensing tread-

mill platform, a measuring system for time-locked data acquisi-

tion, and a fast computer system for calculation of the 4 individ-

ual force traces of each limb. Each of the 18 sensor forces is equal

to the sum of the unknown hoof forces weighted with the trans-

fer coefficients of the corresponding force application points. The

4 force traces are calculated, solving at each time point the result-

ing equation system, using the Gaussian least-squares method.

The force resolution of the system is 6 N, time resolution 2.3 ms

and spatial resolution 0.01 m. The natural frequency of the sys-

tem is located between 40 and 60 Hz, which is 3 times higher than

the frequency components of the information signal (<15 Hz).

Comparison between static test loads and the treadmill forces

showed deviations of <1.4%. Force traces calculated with the

treadmill-integrated force measuring system and those recorded

simultaneously by use of force shoes were highly correlated

(r≥0.998).

The validation experiments demonstrated that the novel kinetic

gait analysis system meets the required standards of precision for

locomotor research and is easily operational in a clinical set-up.

The purpose of the second study was to verify the sensitivity of

two gait analysis methods in detecting subtle lameness and to

compare the results to the traditional orthopaedic evaluation.

Twenty two horses were evaluated subjectively by three differ-

ent experienced clinicians and objectively with synchronised

ground reaction force and accelerometric gait measurements on

a treadmill. For each of the 3 methods the horses were inde-

pendently assigned to one of the following three groups: sound,

lameness forelimb, lameness hindlimb. Additionally, for each

horse the affected limb and the degree of lameness were defined.

l 125S U M M A R Y

A significant correlation was found between the clinical group

assignment and the group assignment based on force (r=0.51,

P<0.05) and acceleration data (r= 0.47, P<0.05) respectively. A

significant correlation was found between the identification of

the affected limb based on clinical and ground reaction force

(r = 0.65, P < 0.05) assessment. No significant correlation was

found either for the group assignment between the two measur-

ing methods or for the degree of lameness between the three

assessment methods.

The study concludes that the measurements of kinetic and kine-

matic parameters represent a helpful complementary tool in the

assessment of subtle gait alterations. However, this information

needs to be carefully interpreted and always related to the clini-

cal observations.

The third study served to establish representative treadmill

ground reaction force and interlimb co-ordination time data of

clinically sound horses at the trot.

Ground reaction force-time histories were collected from 30

Warmblood riding horses with easy, wide natural gaits. A total of

912 stride cycles per limb were analysed for force, time and spa-

tial parameters and averaged.

Shape and amplitude of the treadmill force curves were very

similar to force traces recorded with a stationary force plate. The

horses showed a high degree of symmetry in all investigated

parameters (95 % reference interval of left-right asymmetry

±1.8–6.8%); no significant differences were found between left

and right mean values. Intra-individual coefficients of variance of

the various parameters did not exceed 2.7%. Inter-individual coef-

ficients of variance were 2.5–3.5 times larger than the respective

intra-individual coefficients.

The time-efficient data acquisition of all four feet simultane-

ously over successive strides and the high regularity of the horse’s

gait pattern at controlled velocities represents the decisive advan-

tage of an instrumented treadmill. The normative standards pro-

vide a reference data base against which lame horses can be com-

pared.

The final investigation was performed to obtain a detailed view

on the load and time shifting mechanisms of horses with an uni-

lateral weightbearing forelimb lameness.

Reversible lameness was induced in 12 clinically sound horses

applying the solar pressure model. After recording the reference

126 l S U M M A R Y

measurements, three lameness degrees (subtle, mild and moder-

ate) were induced and assessed.

Four compensatory mechanisms could be identified. (1) With

increasing lameness, horses reduced the total vertical impulse per

stride. This was achieved by increasing stride frequency, which

resulted in a more continuous support of the body centre of mass

by a larger number of strides per unit of time. (2) With increasing

lameness, the rate of loading and unloading the limb was reduced

by prolonging stance duration. The distribution of a given impulse

over a longer time period resulted likewise in a reduction of the

peak force. (3) With increasing lameness, the diagonal impulse

decreased selectively in the lame diagonal, but remained

unchanged in the sound diagonal stance. In the time domain, this

resulted in a shortened contralateral step and suspension duration

after push-off of the lame diagonal limb pair. (4) With increasing

lameness, load (impulse) distribution was shifted within the lame

diagonal to the diagonal hindlimb and in the sound diagonal

stance to the contralateral forelimb. Kinematic studies indicated

that this load shifting mechanism along the longitudinal axis is the

result of the prominent compensatory movement of the head and

neck.

These load shifting mechanisms were not only effective in

reducing peak forces in the lame limb but even suppressed an

equivalent compensatory overload situation in the other limbs.

The load shift within the lame diagonal was paired with a distinct

lowering of the sacrum at midstance of the diagonal hindlimb,

which may be interpreted as a transmitted weightbearing lame-

ness in the ipsilateral hindlimb.

Force-time histories of all 4 limbs allow the reliable identifica-

tion of the affected limb and quantification of a lameness. The

precise localisation of the problem by use of a gait analysis system

alone still remains unsolved. A characteristic feature that unequiv-

ocally points to a specific lameness has still to be found.

l 127S U M M A R Y

Scientific Presentations on the PhD-Topic

Lahmheitsuntersuchung auf dem Laufband:

kinetische Ganganalyse von Stützbeinlahmheiten

beim Pferd

M.A. Weishaupt

Biokema Equine Continuing Education Seminar

Zurich, Switzerland, 9. October 2002.

Application of kinetic gait analysis in lame horses

M.A. Weishaupt

Annual Meeting of the European College of Veterinary Surgeons

Vienna, Austria, 5.–7. July 2002.

Technique for measuring vertical ground reaction forces

of a horse on a treadmill

M.A. Weishaupt

XVIIIth Congress of the International Society of Biomechanics

Zurich, Switzerland, 8.–13. July 2001.

Assessment of gait irregularities in the horse:

eye versus gait analysis

M.A. Weishaupt

Th. Wiestner, H.P. Hogg, P. Jordan, J.A. Auer, E. Barrey

4th International Workshop on Animal Locomotion

Vienna, Austria, 24.–26. May 2000.

128 l S C I E N T I F I C P R E S E N TAT I O N S O N T H E P H D - T O P I C

PhD Courses and Exams

Biometrie, Prof. E. Eggenberger:

Course and exam completed 1999

Biomechanics, Prof. E. Stüssi:

(Studiengang für Bewegungs- & Sportwissenschaften, ETH Zurich)

Course and exam completed 2000

Sports Physiology, Prof. U. Boutellier:

(Studiengang für Bewegungs- & Sportwissenschaften, ETH Zurich)

Course and exam completed 2000

Cell Biology, PD Dr. M. Schweizer:

Course and exam completed 2000

Immunology, Prof. T. Jungi:

Course and exam completed 2001

l 129P H D - C O U R S E S A N D E X A M S

Acknowledgments

An idea for a project has first to be born and persistently pursued

– my deepest thanks go to my professor Jöra Auer for having this

vision, giving me the opportunity and background to perform this

study and for continuously supporting me during the last 10 years.

An idea has to be put into action – my biggest gratitude goes to

Hermann Hogg and Thomas Wiestner for their professional

support in respect to technical and analytical questions, but

especially for their personal friendship.

A project needs – from the planning to the interpretation of the

results – a critical look from different points of view – my thanks

include Jachen Denoth, Edgar Stüssi and Hans Hoppeler for sup-

porting me as co-supervisors.

A project needs good background players – thanks a lot to Vreni

Hänni and Patrick Jordan for their amity, loyalty and help around

the treadmill.

Every dry theory profits from an attractive appearance – my thanks

go to Marianne Mathys for the layout of the thesis and to

Matthias Haab for the flip-book enclosure.

My deepest gratitude goes to my parents and Moni – to my most

precious 3M's – for their inspiration and generous love.

130 l A C K N O W L E D G M E N T S

l 131C U R R I C U L U M V I TA E

Curriculum vitae

Michael Andreas Weishaupt

Born on the 30 th August 1965 in Bienne, Switzerland

1984 –1989 Studies of Veterinary Medicine, University of Berne

1990 –1993 Assistant at the Department of Large Animal Medicine,

University of Berne (Prof. R. Straub) and at the Swiss National Stud,

Avenches (Dr. med. vet. P.-A. Poncet).

1993 Dr. med. vet.- thesis at the Department of Large Animal Medicine,

University of Berne (Prof. R. Straub and Prof. H. Hoppeler):

«Relationship among local structural, biochemical and functional

variables describing muscle oxidative capacity in horses and steers».

1993 – 2000 Assistant at the Department of Veterinary Surgery, University of Zurich

(Prof. J.A. Auer), Head of the Equine Performance Centre.

Since November 2000 Senior Assistant

Special interests: – Equine Sports Medicine and Exercise Physiology

– Equine Orthopaedics and applied Biomechanics

– Upper airway diseases in horses

– Member of the Veterinary Commission of the Swiss Racing Association

– Member of the Licence Commission of the Swiss Racing Association

– Member of the Medication Control Commission of the Swiss Association

of Equine Practitioners

Michael Andreas Weishaupt, Dr. med. vet.

Equine Hospital, Equine Performance Centre

Vetsuisse Faculty University of Zurich

Winterthurerstrasse 260

CH-8057 Zurich

Switzerland

Phone ++41 44 635 84 34

Fax ++41 44 313 03 84

Email mweishaupt@vetclinics.unizh.ch

Zurich, 2004

Design by Marianne Mathys

ISBN 3-9521627-3-6