Inter Segmental Coordination Of Competitive Weightlifters ...projekter.aau.dk/projekter/files/260287570/17gr_10111_bedoemmelse.pdf · Inter Segmental Coordination of Competitive Weightlifters

• Inter Segmental Coordination of Competitive Weightlifters During Heavy Back Squatting

Inter Segmental Coordination Of CompetitiveWeightlifters During Heavy Back Squatting

Gorm H. F. Rasmussen & Markus E. Sloth*

Aalborg University, Denmark

Abstract

Aim: The purpose of this study was to investigate the coordination pattern and coordination variabilityduring heavy back squats Methods: 10 competitive weightlifters participated in a single laboratory sessionand performed a one repetition maximum test in the back squat, followed by six single repetition sets at90% of 1RM. Kinematic and kinetic data was recorded using a synchronized eight-camera movementanalysis system (Qualisys) and force plate (AMTI). Hip, knee and ankle joint centers of the right legwere digitalized and analyzed using Visual3D motion (Version 4.21, C-Motion Inc. USA). Coordinationpattern and coordination variability in the inter segmental coupling of the shank-thigh and thigh-pelvissegment pairs were evaluated by calculating continuous relative phase (CRP), individual mean averagecontinuous relative phase (MACRP) and individual deviation phase (DP). Results: Inter segmentalcoordination the shank-thigh and thigh-pelvis segment pairs was not in- or out-of-phase by a constantmagnitude but changed across the repetition cycle. This was uniform across subjects. Significant intersubject differences was observed in CRP of the shank-thigh (p≤0.05) and thigh-Pelvis (p≤0.05) segmentpairs. Qualitative analysis of individual DP revealed similar magnitude of intra individual variabilityin inter segmental coordination of segment pairs, ranging from 7.5-12.4 (shank-thigh) and 7.2-11.5(thigh-pelvis) respectively. Conclusion: A general movement pattern appear to exist during heavy backsquatting, but the specific pattern is highly individual and exhibit a certain amount of neuromuscularflexibility. Key Words: MOTOR CONTROL, BACK SQUAT, COORDINATION, CONTINUOUSRELATIVE PHASE, MOTION CAPTURE.

Introduction

The back squat exercise is considered an ef-fective mean to improve lower body strength[Comfort et al., 2014] and has become a stablein the strength and conditioning programs ofathletes in which strength and power of thelower limbs are important for sports perfor-mance [Chandler, Stone, 1991; Comfort, Kasim,2007]. It is also a complex motor skill, in-volving numerous interacting components anddegrees of freedom. Mastering these degreesof freedom can lead to a stable, coordinatedand skillful movement [Stergiou et al., 2001b],which is necessary for maximum voluntaryforce production [Sale, 2008]. Thus, coordina-

tion can be defined as the process by which thedegrees of freedom are organized in time andin sequence to produce a functional movementpattern [Bernstein, 1967; Scholz, 1990].

A contemporary approach to understand-ing the construction of, and subsequent changein, patterns of coordination comes from thedynamical systems theory (DST). According tothe principles of DST, coordinated patterns areconstructed out of the constraints applied tothe neuromuscular system. These constraintscome from the organism (e.g. joint flexibil-ity, perceptual abilities), the environment (e.g.squatting with different loads) and the task(e.g. squatting with different range of motion)[Higgins, 1985; Kugler, Turvey, 1987; Thelen

*With supervision of Mathias Kristiansen, Assistent Professor at Institute of Health Science and Technology, AAU &Michael Voigt, Professor at Institute of Health Science and Technology, AAU

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et al., 1987; Beek, Beek, 1988; Clark, 1989; The-len, 1989; Thelen et al., 1991]. The constraintseffectively reduce the number of degrees offreedom and simplify the management of theneuromuscular system [Stergiou et al., 2001a].Slight variations in the way these degrees offreedom are coupled together in the coordina-tive structure may be referred to as variability,identified as inherent within and between allbiological systems [Hamill et al., 1999; Irwin,Kerwin, 2007]. A key aspect of DST is thatsuch variability is suggested as necessary forthe neuromuscular system to adapt (i.e. neuro-muscular flexibility) to global and local pertur-bations in the movement pattern, and for theexploration of new neuromuscular solutionsfor constraining the degrees of freedom [Kelsoet al., 1991; Schmidt et al., 1992; Kelso, 1997].

Two general squat patterns have previouslybeen reported in the literature: i) a squat withhorizontal posterior displacement of the pelvisand greater hip extensor kinetics (i.e. net jointmoment and power) and ii) a squat using animmediate vertical downward displacement ofthe pelvis with less horizontal posterior pelvicdisplacement, generating higher knee exten-sor and ankle plantar flexor kinetics [Flanaganet al., 2003]. However, with reference to the the-oretical ideas of DST, and considering the con-straints imposed on the lifter by the environ-ment (e.g. load and equipment), and percep-tion of proper technique (i.e. advocated move-ment pattern), experienced lifters will likelydemonstrate unique and variable movement so-lutions to the squat pattern, acquired throughan individual-specific self-organization process[Bartlett et al., 2007]. Unfortunately, little infor-mation is available regarding such inter- andintra individual variability in the back squat ex-ercise as previous investigations have primarilybeen concerned with general descriptions ofthe movement.

DST emphasizes the importance of study-ing the relationship between either the move-ments of the limb segments of the same limb(i.e. intra-limb coordination), or the relation-ship between the movements of two differentlimbs (i.e. inter-limb coordination), which are

viewed as crucial aspects of human motor be-havior [Donker et al., 2001; Lee et al., 1995;Seifert et al., 2010; Van Emmerik, Wagenaar,1996; Swinnen et al., 1997]. However, althoughseveral studies have quantified joint moments[Ariel, 1974; Escamilla et al., 1998; Hay et al.,1983; Lander et al., 1986, 1990; McLaughin et al.,1978; Nisell, Ekholm, 1986; Russell, Phillips,1989; Wretenberg et al., 1993, 1996], bar speed[McLaughin et al., 1978; Escamilla et al., 2001;Hales et al., 2009; Van Den Tillaar et al., 2014],joint angular velocity [Escamilla et al., 2001;Donnelly et al., 2006; Kellis et al., 2005; Zinket al., 2001] and select joint and segment an-gles [Lander et al., 1986, 1990; McLaughlin,1977; Russell, Phillips, 1989], none have in-cluded analysis of the coordinated movementsbetween the lower limb or trunk segments.Therefore a quantification of the inter segmen-tal coordination and coordination variabilitymay provide new and additional insight in theorganization of the neuromuscular system dur-ing squatting.

The continuous relative phase (CRP) is acommon tool to characterize inter segmentalcoordination based on angular position andvelocity [Stergiou, 2004]. Hence, CRP containsboth spatial and temporal information and re-duces the number of degrees of freedom fromfour (i.e. distal and proximal angles and an-gular velocity) to one [Stergiou, 2004], and hasbeen employed to investigate inter segmentalcoordination and coordination variability inboth cyclic and discrete movements [Donkeret al., 2001; Van Emmerik, Wagenaar, 1996; Em-merik van, Wegen van, 2000; Hamill et al., 1999;Stergiou et al., 2001a,b; Seifert et al., 2011, 2014;Figueiredo et al., 2012; Seifert et al., 2010; Raf-falt et al., 2016; Chardonnens et al., 2013; Irwin,Kerwin, 2007].

Thus, the purpose of the present study wasto investigate the inter segmental coordinationand coordination variability (both intra-subjectand inter-subject variability) during heavy backsquatting. We hypothesized that different in-dividuals would demonstrate significant inter-and intra subject variability in the inter seg-mental coordination pattern of the back squat.

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Methods

Experimental approach

A cross sectional design was used to investi-gate the inter segmental coordination and co-ordination variability in the back squat. Thesubjects performed a 1 repetition maximum inthe back squat, followed by a total of six singlerepetition sets at 90% of 1RM. The intensity of90% was included to reflect the force - velocitycharacteristics of a 1RM as close as possible,while concurrently permitting the performanceof several lifts necessary to assess inter- andintra subject coordination variability.

Subjects

Ten competitive weightlifters volunteered toparticipate in this study (Mean±(SD) age: 26.5(±5.9) years; stature: 183.9 (±7.6) cm; mass:90.9 (±14.6) kg; squat 1RM: 165 (±27) kg; re-sistance training experience: 8.4 (±4.3) years).Each subject had at least four years of experi-ence with the back squat: 7 (±4.6) years). Allsubjects has the ability to squat with a range ofmotion that permitted the cease of hips to bebelow the top of the knee. Prior to experimen-tal testing, subjects were informed about theexperimental procedure and gave their writteninformed consent to participate in the study.

Procedures

Data were collected for each subject during asingle laboratory session which involved 1RMtesting in the back squat, followed by a totalof six trials at 90% of their newly established1RM.

All subjects had experience with perform-ing 1RM and were familiar with predictingtheir maximal strength. In accordance withBrown, Weir [2001], all subjects performed gen-eral warm up consisting of five minutes ofcontinuous ergometer rowing at a self-selectedintensity. Subjects then performed a specificwarm up consisting of no more than two setsof five repetitions with less than 50%, eightrepetitions at 50% and three repetitions at 70%,

followed by three single repetitions at 80, 90and 100% of their predicted 1RM in the squat.Subjects were allowed to re-evaluate their es-timated 1RM following the lift at 90%. Hencetheir estimated 1RM could be increased or de-creased according to their assessment. Duringthe rest periods subjects were asked to indicatetheir rating of perceived exertion (RPE) from 0-10 to indicate the severity of each lift accordingto the number of repetitions in reserve (RIR)where a score of 10 was equivalent to 0 RIR. Ifthe subjects predicted maximum was not per-ceived as RPE 10, the weight was increased by2.5-10kg subsequently. If the subjects failedto lift their predicted 1RM the weight was re-duced subsequently by 2.5-10kg until the true1RM was established.

The weight was recorded as 1RM when sub-jects perceived it as maximal and indicated 10on the RPE scale. This approach for validatingthe subjects 1RM was adopted to limit poten-tial risk of injury from lifting maximal loadsas experienced lifters have been demonstratedas highly accurate when rating a maximal liftaccording to RPE based on repetition in reserve[Zourdos et al., 2015]. Following the comple-tion of a successful 1RM, all subjects performedsix single repetition sets at 90% of the newlyrecorded 1RM. A four to seven minute rest pe-riods were provided between all sets to limitthe confounding influence of fatigue. All liftswere performed starting from an erect posi-tion from which the lifter descended until thecrease of the hip at the hip joint was below theknee to ensure similar relative range of motionbetween subjects. When the desired range ofmotion was achieved, the lifter reversed theaction and ascended back to the erect position.If squatting depth requirements were not met,the trial was repeated. In case of failure to liftthe load, spotters were positioned to catch thebarbell. Safety strap shackles were suspendedfrom the ceiling and attached to the barbell foradditional safety precautions. Using this setup,a total six of subjects reached failure in processof establishing a 1RM with no complications.

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Measurement of Kinematic and KineticVariables

All trials were performed with one foot ona force plate (Advanced Medical Technology,Inc., Watertown, MA, USA) and ground reac-tion forces were collected. Three-dimensionalkinematics was collected at 120 Hz using an 8-camera Qualisys motion analysis system (Qual-isys AB, GÃuteborg, Sweden). Using standardcalibration practices outlined by the Qualisysusers manual, system calibration was com-pleted prior to data acquisition. Calibrationwas accepted if average 3D residuals were esti-mated at under 1.0 mm. Data acquisition wasonly attempted after an adequate calibrationwas achieved.

Following the system calibration, subjectswore a set of 33 reflective spherical markers (11mm diameter) for the recording of a standingreference trial to determine the segmental co-ordinate systems and the joint axes. For thereference trial, markers were placed on theright heel, lateral and medial malleolus, lat-eral and medial metatarsal-phalangeal joints,the medial and lateral femoral epicondyles, theright greater trochanter, right and left anteriorsuperior iliac spines, posterior superior iliacspines, acromion and both ends of the barbellrespectively. Furthermore, semi-rigid plastic orcardboard plates with a cluster of three track-ing markers were secured to the shanks, thighs,sacrum and on the middle upper back. Prior toinitiating the squatting protocol, markers wereremoved from the medial malleolus, medialmetatarsal-phalangeal joint, the medial and lat-eral femoral epicondyles, greater trochanter,anterior superior iliac spines, posterior supe-rior iliac spines, lateral and medial acromionrespectively.

Data processing and Analysis

Visual3D motion (Version 4.21, C-Motion Inc.USA) was used to calculate the joint kinematicand kinetic data from the right side only asthe back squat has been demonstrated as asymmetrical movement [Escamilla et al., 2001].All motion and force plate data were filtered

using a Butterworth 4th order bi-directionallow-pass filter with cut-off frequencies of 2 Hzand 6 Hz, respectively. The following discretevariables were calculated for each trial: Centerof pressure (CoP) and vertical barbell velocity(VBarbell).

All relevant kinematic data were exportedto MATLAB (MathWorks Inc, Massachusetts,USA) for subsequent data processing. To cal-culate the segment angles approximately intheir anatomical sagittal planes (i.e. compen-sating for rotations out of the projected sagittalplane (lab x-z-plane)) the segment angles werederived through trigonometry. The length ofeach segment was utilized as hypotenuse andcomputed as the distance between the distaland proximal joint centre. The adjacent wasthen determined by projecting the joint centresto the floor and calculating the distance. Foreach trial both the descending and the ascend-ing phases were resampled to 50 points. In thisway the cycle time was normalized to 100%and equally divided between the two phases.

During a 1RM squat, the lifter squats downuntil the end of the descend after which thebarbell initially accelerates at the beginning ofthe ascent to a first peak velocity, then deceler-ates to a minimum velocity, accelerates againto a second peak velocity, and finally deceler-ates until the end of the ascent [McLaughinet al., 1978]. In accordance with Escamilla et al.[2001], the squat was therefore divided intofive lifting phases: a) descending phase (DEC)→ lift initiation to the onset of positive barvelocity; b) acceleration phase (AP) → onsetof positive bar velocity to first peak bar ve-locity; c) sticking region (SR)→ first peak barvelocity to minimum bar velocity; d) maximumstrength region (MSR)→ minimum bar veloc-ity to second peak bar velocity; and e) deceler-ation phase (DP) → second peak bar velocityto lift completion.

Coordination pattern & variability

To examine inter segmental coordination,phase portraits were created by plotting thenon-normalized segment angle on the horizon-

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tal axis with the non-normalized segment angu-lar velocity on the vertical axis. Because therewere little to no frequency differences betweenthe oscillating segments during the squat, wechose not to normalize the phase portraits topreserve the dynamic qualities of the signals[Kurz, Stergiou, 2002].

Figure 1: A graphical illustration of bodily segmentschosen for the coordinative analysis. The an-gular position of each segments was calculatedcounter clockwise. The lab coordinates are il-lustrated in the top left corner

The phase angle (φi) was defined as theangle between the right horizontal axis to thepair of coordinates (θi,ωi). The phase anglewas calculated for three of the four quadrantsin the range of 0− 270◦ (equation 1, 2 and 3)[Stergiou, 2004].

(1.quadrant) : φi = (arctanωiθi) · 180

π(1)

(2.quadrant) : φi = 180− |(arctanωiθi) · 180

π| (2)

(3.quadrant) : φi = 180 + |(arctanωiθi) · 180

π| (3)

Phase angles were calculated for flex-ion/extension of the shank, thigh and pelvissegments on the right leg. Because of the dis-tinct start- and endpoint of the squat [Frostet al., 2010], a phase angle of 0◦ representsa point at which the segment is at 0 veloc-ity and either minimum or maximum angular

displacement. Finally, the continuous relativephase angle (CPR) between two segments wascalculated by subtracting the phase angle ofproximal segment from the phase angle of thedistal segment (equation 4) [Stergiou, 2004; Raf-falt et al., 2016].

CRPi = φi(D) − φi(p) (4)

Where φi(p) is the ith point of the proximalsegment and φi(D) is the ith point of the distalsegment.

The continuous relative phase representsthe coordination between two interacting seg-ments and was calculated for the foot-shank,shank-thigh and thigh-pelvis segment pairs. ACRP close to 0◦ would indicate an in-phasecoupling between the respective segments andthat the segments are moving in a similar fash-ion, while a value close to ±180◦ would indi-cate that the segments are moving in oppositedirections and have an anti-phase relationship[Stergiou, 2004]. A positive relative phase valueindicates that the distal segment is ahead ofthe proximal segments in phase space and anegative value indicates that the proximal seg-ment is ahead in phase space (i.e. ahead inits respective oscillation cycle) [Clark, Phillips,1993; Barela et al., 2000]. Moreover, the slope ofthe continuous relative phase curve configura-tion indicates which segment is moving fasterduring periods of the repetition cycle.

To visualize the average individual coordi-nation pattern and coordination and coordina-tion variability the average continuous relativephase (ACRP) was calculated across trials (thejth trial of n number of trials) from the en-semble curve by averaging the values of theensemble curve points, (equation 5) [Raffaltet al., 2016].

ACRPi =1n

n

∑j=1

CRPi,j (5)

To quantify intra subject coordination vari-ability, the deviation phase (DP) was calculatedby averaging the standard deviations of the en-semble relative phase curve points, where Nis the number of points in the relative phase

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mean ensemble curve and SD is the standarddeviation at the ith point (equation 6).

DP =∑N

i=1 |SDi|N

(6)

A high DP value indicates a less stable(more variable) organization of the neuromus-cular system, whereas the opposite is true fora low value [Stergiou, 2004]. This parametercould not be tested statistically but was evalu-ated qualitatively.

To quantify the inter segmental coordina-tion pattern of each subject the mean averagecontinuous relative phase (MACRP) was calcu-lated across the repetition cycle (N) (equation7) [Raffalt et al., 2016].

MACRP =1N

N

∑i=1

ACRPi (7)

Statistics

Inter individual variability in the relative phas-ing relationship between segment pairs wasassessed by analysing potential subject differ-ences in MACRP through a one way analy-sis of variance (ANOVA) and by analysingthe relative phase curves of each segment pairthrough a one way ANOVA using the statisti-cal parametric mapping method (SPM). Signif-icant main differences were further analysedthrough pairwise comparisons. The magnitudeof relationship between CoP and DP were inter-preted using Pearson correlation coefficients,described as trivial (0.0-0.1), low (0.1-0.3), mod-erate (0.3-0.5), high (0.5-0.7), very high (0.7-0.9),or practically perfect (0.9-1.0) [Hopkins, 2002].The level of significance was set at α ≤ 0,05. Allstatistical procedures were performed in MAT-LAB R2015b (MathWorks Inc, Massachusetts,USA).

Results

The assumption of symmetrical movement dur-ing the squat was tested by averaging mini-mum and maximum lateral/medial displace-ment of the barbell for each trial. The grand

mean range of lateral/medial barbell displace-ment was 2.4 ± 0.99cm which was considerednegligible. To test the influence of normaliza-tion on segmental angular velocity and dis-placement, CRP was calculated for each seg-ment pair with and without recommended nor-malization procedures[Kurz, Stergiou, 2002].The results revealed only a minor amplitudedifference in CRP. As this had no practical sig-nificance subsequent data analysis proceededwithout normalization.

Descriptions of segmental phase por-traits

Prior to examining the coordinative relation-ship between the segment pairs, the trajecto-ries of the pelvis, thigh and shank segmentsare presented. In Figure 2, the average non-normalized phase portraits of all subjects forthe pelvis, thigh and shank motion are de-picted. Although the trajectories of the thigh-and shank segments differ geometrically inshape, they both display the characteristics ofa limit cycle attractor. This type of attractoris one that cycles periodically in a closed or-bit, seemingly attracted to the same regionsof state space with each oscillation. This isparticularly apparent in the thigh’s behaviorand not at all in the pelvis’ behavior. Fromthese phase portraits it is clear that the thighand shank segmental motions during squattingtake the form of a limit cycle attractor, whereasthe pelvis segmental motion do not.

Coordination pattern

Figure 3 display the ensemble average CRPfor the shank-thigh and thigh-pelvis segmentpairs across all subjects ± standard deviation(SD). Initial inspection immediately reveals thatthe relative phasing relationship between therespective segment pairs is clearly non-linear.Throughout the repetition cycle the relativephasing is not in- or out-of-phase by a con-stant magnitude. That is, the inter segmen-tal coordination between the shank-thigh andthigh-pelvis segment pairs is not fixed but

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changes across the repetition cycle. However,both segment-pairs generally displayed a morein-phase rather than anti phase relationship asindicated by the relative phase angles beingcloser to zero.

Shank-thigh segment pair As indicated bythe negative phase angle, the thigh was aheadin phase space during DEC, whereas the shankwas ahead in phase space during the ascentas indicated by the positive phase angle. Thenegative slope of the CRP curve indicate thatthe thigh was moving faster in phase spaceduring the initial portion of DEC, during SRand DP (i.e. first 10%, 58-75% and the final 5%of the repetition cycle). The positive slope ofthe CRP curve indicated that the shank wasmoving faster in phase space during most ofDEC, through SR phase and MSR (i.e. 10-58%and again from 75-95% of the repetition cycle).

Thigh-pelvis segment pair As indicated bythe positive CRP angle the thigh was aheadin phase space during DEC (i.e. 0-45% of therepetition cycle) and AP (i.e. 51-60% of the rep-etition cycle), whereas the pelvis was ahead inphase space from the end of AP and through-out the ascent (i.e. 61-100% of the repetitioncycle). As indicated by the positive slope ofthe CRP curve the thigh was moving faster inphase space during the initial portion of DEC,during AP and DP (i.e. the first 10%, 50-60%and the final 5-8% of the repetition cycle). Thenegative slope of the CRP curve indicate thatthe pelvis moved faster in phase space dur-ing most of DEC and most of the ascent (i.e.10-45% and 58-92% of the repetition cycle).

Coordination variability

Intra subject variability evaluated throughqualitative comparison of DP was similaracross subjects and between segment pairs,ranging from 7.5-12.4 (shank-thigh) and 7.2-11.5 (thigh-pelvis) respectively. Figure 4 illus-trates both the ensemble average VBarbell andCRP ± SD of the six single repetitions at 90%and VBarbell and CRP of 100% for the subjectswith the highest (subject 8) and lowest (sub-

ject 4) DP respectively. Subject 4 exhibit SDacross the entire repetition cycle whereas theSD of subject 8 increase during the maximumstrength and deceleration phases. CRP andVBarbell was more similar between 90 and 100%for subject 4 (low DP) compared to subject 8(high DP).

Inter subject variability quantified by col-lapsing the CRP curves and comparing theindividual MARCP showed no significant dif-ferences between subjects in the coordinationpattern of the shank-thigh(p=0.37) and thigh-pelvis(p=0.86) segment pairs. In contrast, intersubject coordination variability quantified bycomparing the CRP curves directly in a con-tinuous manner through SPM showed signifi-cant inter subject difference in the coupling ofboth segment pairs (p ≤ 0.05) (figure 3). Posthoc pairwise comparisons of the CRP curvesrevealed significant differences between eachsubject and three or more other subjects (p ≤0.05), but no interesting pattern was identified.

Center of Pressure

Center of pressure was included as Dolan et al.[2014] have suggested that less displacement inCoP in the squat exercise is indicative of a moremature and proficient movement pattern. Fig-ure 5 illustrates the displacement of COP in thex (anterior/posterior) and y (lateral/medial)coordinate. The mean range of displacementwas higher in anterior/posterior direction witha mean range of 0.116m compared to the Lat-eral/medial displacement where the subjectshad a mean range of displacement of 0.046m.The grand mean CoP is initially moved poste-riorly during the descent, but translated anteri-orly from approximately 10% of the repetitioncycle to 58% of the repetition cycle. From thispoint on, translation of CoP is directed poste-riorly until the completion of the lift. A lowcorrelation was observed between CoP and DPof the shank-thigh (r2=0,22) and thigh-pelvis(r2=0,03). This correlation was significantly dif-ferent from a correlation of zero for the shank-thigh (p≤0.001) but not for the thigh-pelvis(p=0.16).

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Figure 2: Non normalized phase plots illustrating the grand mean trajectory (black line with "o") and mean individual trajectories (colored lines) for the the pelvis, thighand shank segments. The black arrow denotes the first data point and red and black lines are drawn to the first and third data points to illustrate the trajectory.The horizontal dotted line illustrate the reference line for calculating the phase angle. All trajectories are normalized to 100 points.

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M {

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α =0.05, F* = 4.139

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Figure 3: Mean ensemble CRP across all subjects (black line) and the standard deviation between subjects (grey area) during the squat for the shank-thigh (left sidegraph) and thigh-pelvis (right side graph) coupling. Vertical lines denote the transition between lifting phases: DEC; Decent, AP, acceleration phase; SR,sticking region; MSR, maximum strength region; and D, deceleration phase. The vertical dotted lines denotes the standard deviation for the onset of therespective lifting phases. SPM denotes significant inter subject differences in CRP of the shank-thigh and thigh-pelvis segment pairs p ≤ 0.05 (N=10).

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Figure 4: Representative figure of vertical barbell velocity (top) and CRP for the shank-thigh (middle) and thigh-pelvis (bottom) segment pairs of the subjects with thehighest and lowest DP respectively. Right side figure: Ensemble average vertical bar velocity (black line) ± SD (grey area) for 90% and 100% (red line) andensemble average continuous relative phase (black line) ± SD (grey area) for 90% and 100% (red line) for subject 4 (low DP). Left side figure: Ensembleaverage vertical bar velocity (black line) ± SD (grey area) for 90% and 100% (red line) and ensemble average continuous relative phase (black line) ± SD(grey area) for 90% and 100% (red line) for subject 8 (high DP). Vertical lines denote the transition between lifting phases: DEC; Decent, AP, accelerationphase; SR, sticking region; MSR, maximum strength region; and D, deceleration phase. The vertical dotted lines denotes the standard deviation for the onset ofthe respective lifting phases.

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DEC AP SR MSR D

Figure 5: Left side graph: Grand mean center of pressure displacement (Black line) and individual mean center ofpressure displacement (Colored lines) in the x coordinate (anterior/posterior translation). Right side graph:Grand mean center of pressure displacement (Black line) and individual mean center of pressure displacement(Colored lines) in the y coordinate (lateral/medial translation). Vertical lines denote the transition betweenlifting phases: DEC; Decent, AP, acceleration phase; SR, sticking region; MSR, maximum strength region;and D, deceleration phase.

Discussion

To the best of our knowledge, this is the firststudy to investigate the inter segmental coor-dination and coordination variability of theback squat. A non linear, but relatively morein-phase relationship was observed for bothsegment pairs across all subjects. However, asexpected, significant inter subject differencesemerged in the CRP of the shank-thigh andthigh-pelvis segment pairs. Interestingly, mag-nitude of intra subject variability, as indicatedby individual DP, appeared to be similar be-tween the respective segment pairs. Althoughsubjects were not expected to produce invari-ant movement patterns, the similarity in DPfor the shank-thigh and thigh-pelvis segmentpairs is surprising. Finally, the present studyobserved a discrepancy between inter subjectcoordination variability assessed through com-parison of MACRP and SPM. This is importantbecause it may indicate that valuable informa-tion regarding inter subject variability could belost by collapsing the ensemble CRP curves to

a single value for comparison.

Kinematic & kinetic parameters

All subjects demonstrated a clear double peakin VBarbell , separated by point minimum inVBarbell (figure 4) which is similar to the pat-tern previously reported by McLaughin et al.[1978] and Escamilla et al. [2001]. Moreover,maximum VBarbell was generally higher duringthe second peak which is also comparable tothe results of McLaughin et al. [1978] and Es-camilla et al. [2001], in which most lifters alsoreached their maximum VBarbell at their secondpeak during a 1RM lift. Collectively, this sug-gest that data obtained at a load of 90% 1RMare representative to that of a load of 100%.From the onset of the repetition cycle until ap-proximately the SR subjects demonstrated anaverage anterior CoP displacement of 0,06m(Figure 5). This substantially greater than pre-viously reported for experienced lifters by Legget al. [2016] (0,06m vs. 0,01m). Consideringthat CoP is the reaction force to the accelera-tions and the gravitational force of the center

11


off mass of the system (lifter+barbell), the hor-izontal displacement pattern of CoP (Figure5) may be the product of a general movementstrategy employed by the subjects. That is, thesubjects may actively tend to move their centerof mass forward during the descent. Assumingthis is the case, this would also suggest thatthe subjects of this study utilized neither of theexact patterns previously reported by Flanaganet al. [2003], but employed a third and differentmovement strategy.

Coordination pattern

The coordination pattern, evaluated from CRPthe shank-thigh and thigh-pelvis segment pairsin figure (3), was rather uniform between sub-jects. During both the descending and ascend-ing phases of the squat the shank-thigh andthigh-pelvis segment pairs were dominated byin-phase coupling. The largest deviations fromthis general pattern were observed during theinitial and final portions of the lift, and to-wards the end of the acceleration phase. Allsegment pairs changed from a relatively moreanti-phase pattern initially and became pro-gressively more in-phase towards the end ofthe descent. This relatively more in-phase pat-tern was then maintained until the end of stick-ing region, following which it reversed andbecame progressively more anti-phase. Thisnon-linear phasing relationship is further em-phasized by the observation of two local mini-mas and maximas for both segment pairs dur-ing the repetition cycle (Figure 3). These pointsrepresent reversals in the coordination dynam-ics, characterizing changes in the inter segmen-tal relationship [Barela et al., 2000]. Theoreti-cally, this change in the phasing relationshipcould be due to either more or less changein the phase angle of each segment (or somecombination of the two) [Clark, Phillips, 1993].Inspection of the separate trajectories indicatethat this is predominantly due to changes inthe phase angle of the shank and thigh as littlemotion occur at the pelvis.

Intra subject variability

In the present study, inter segmental (shank-thigh and thigh-pelvis) coordination was quan-tified by continuous relative phase. As indi-cated by the similar DP range for shank-thigh(7.5-12.4) and thigh-pelvis (7.2-11.5), magni-tude of intra subject coordination variabilitywas similar between segment pairs and be-tween subjects. As variability in the inter seg-mental coordination could be interpreted asa less stable movement pattern across multi-ple single repetition sets, this initially suggesta similar level of stability in the coordinationpattern of each subject. However, when com-paring the most stable coordination pattern tothe least stable, certain differences appear (Fig-ure 4). Specifically, VBarbell and the CRP curvesof subject four (low DP) are very similar acrossthe entire repetition cycle, suggesting a highlystable coordination pattern. This stability isfurther emphasized by the similarity betweenVBarbell and CRP for 90% 1RM and 100% 1RMrespectively. In comparison, the CRP curves ofsubject eight (high DP) demonstrate a greatermagnitude of variability in the latter portionof the lift. Moreover, where VBarbell and CRPis similar during DEC for 90% 1RM and 100%1RM, they differ substantially during the as-cent. Coupled with the greater magnitude ofcoordination variability, this apparent sensitiv-ity to load increments may suggest a less stableorganization of the neuromuscular system.

Considering the limited movement andnon-sinusoidal behavior of the pelvis segment(Figure 2), this may suggest that coordinationof this segment during squatting is more ofa stabilizing character. That is, the movementcharacteristics of the pelvis may reflect a coordi-native response to changes in the shank-thighrelationship rather than a specific movementstrategy. In comparison, the trajectory of theshank and thigh segments clearly demonstratethe behaviour of a limit-cycle attractor. Hence,the majority of observed intra subject variabil-ity is most likely the product of local perturba-tions to the trajectory of these segments. Ac-knowledging the existence of neuronal "noise"

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within the central nervous system [Stein et al.,2005], it could be speculated that these pertur-bations stem from inherent variability in neuralactivation of the high number of both mono-and biarticular muscles involved in movingthe shank and thigh. On the other hand, theobserved variability may also be reflect an ex-ploration process, by which the subjects seekto exploit the numerous degrees of freedompresent in the system [Busquets et al., 2016].In any case, considering that the constraints ofthe lifter by external parameters (i.e. load andrequired execution execution) were constant,the intra subject variability may primarily havebeen related to intrinsic parameters and organ-ismic constraints [Newell, Vaillancourt, 2001;Raffalt et al., 2016].

Previous studies have argued that transla-tion magnitude of CoP may be an importantfactor for technical stability (i.e. low coordi-nation variability) during heavy squats [Dolanet al., 2014]. As such, stability of CoP shouldbe expected to correlate well with stability ofinter segmental coordination considering thatmovement in CoP is the product of movementin the bodily segments. Although the resultsof this study does demonstrate a significantrelationship between the stability of CoP andinter segmental coordination, correlation waslow and provides limited support to this idea.However, it could be speculated that these dataare slightly skewed and unique to this par-ticular group of weightlifters. Specifically, itmay be that the perception of proper techniqueand advocated movement pattern in the localweightlifting community is characterized bygreater anterior/posterior translation of CoP ingeneral. Hence, correlation might be strongerin a different sub group.

Inter subject variability

Initially, inter subject variability quantified byMACRP showed no significant differences be-tween subjects in the coordination pattern ofthe shank-thigh and thigh-pelvis segment pairsp ≥ 0.05. In contrast, SPM revealed signifi-cant inter subject differences in the continuous

relative phase of the shank-thigh and thigh-pelvis segment pairs p ≤ 0.05 (Figure 3). Thismay suggest a methodological limitation forMACRP as a measure of differences betweenCRP curves. Comparing continuous relativephase measures between individuals by com-paring the ensemble average would have leadus to believe that inter segmental coordinationwas similar between subjects. However, thiswould have been an erroneous conclusion asthe inter segmental coordination is significantlydifferent between individuals when comparingthe CRP curves directly.

The CRP curves demonstrate, that for allsubjects the inter segmental coordination ofthe shank-thigh and thigh-pelvis segments isnot fixed, but changes across the repetition cy-cle. Interestingly, the general pattern of thephasing relationship appear to be similar be-tween subjects. Hence, the inter individualdifferences arise from differences in the magni-tude by which the shank-thigh and thigh-pelvissegments are relatively in- or out-of-phase atany point during the squat. Thus, consideringthat the environmental factors (i.e. load andequipment), and task constraints (i.e. execu-tion requirements) where uniform for all sub-jects, the observed inter subject differences ininter segmental coordination is most likely theproduct of individual differences in organis-mic constraints (i.e. neural and morphological).This notion is supported by the different shankand thigh trajectories in Figure 2, as differencesin anthropometrics and ankle joint flexibilityhave a profound impact on shank and thigh dis-placement during squatting [Hemmerich et al.,2006].

Closer inspection of the individual meancontinuous relative phase measures in Figure 3further reveals that the magnitude of inter sub-ject variability differ across the repetition cycle.Specifically, the relative phase measures of eachsubject appear to be more similar for the thigh-pelvis segment pair during the sticking regionaround 58-74% of the repetition cycle. Accord-ing to Clark, Phillips [1993], phase locked orconverging points of coordination may be nec-essary to ensure the desired movement occurs

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whereas variability in other portions of themovement cycle would permit flexibility re-quired to meet the ever changing constraintsof the system. The sticking region has previ-ously been referred to as a part of the rangeof motion in which a disproportionately largeincrease in difficulty to continue the lift is expe-rienced [Kompf, Arandjelovic, 2016]. Hence, itcould be speculated that reduced inter subjectvariability in this phase may reflect changesin the squat pattern that require a tighter cou-pling of the degrees of freedom for successfultask performance. As such, the sticking regionin the squat may simply reflect a convergingpoint in the movement pattern characterizedby limited neuromuscular flexibility to meetthe constraints of the system.

In conclusion, this study provides newand additional insight in the organization ofthe neuromuscular system during heavy backsquatting. Although the coordination patternwas surprisingly uniform, significant inter sub-ject differences in the coordinative character-

istics was observed. Furthermore, the similaramounts of intra subject variability indicate i)that the individual coordination patterns ex-hibited similar levels of stability and ii) a cer-tain amount of neuromuscular flexibility in thesquat, despite the constraint imposed on thesystem by a near maximal load. Collectively,the results of the present study demonstratethat competitive weightlifters utilize uniqueand variable movement solutions to heavy backsquatting, albeit within a general squat pattern.

Practical Applications

The present findings suggest it may be viablefor coaches and practitioners to utilize a gen-eral model when teaching the squat. Appropri-ate task constraints may be applied to the backsquat in order to encourage safe and efficientexecution. However, within these constraints,we recommend that athletes are permitted toexplore the multiple degrees of freedom anddevelop their own unique solution to the squatpattern.

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