Sport specificity background affects the principal component structure of vertical squat jump performance of young adult female athletes

Vassilios Panoutsakopoulos,Nikolaos Papachatzis,Iraklis A.Kollias

Biomechanics Laboratory,Department of Physical Education and Sports Sciences,Aristotle University of Thessaloniki,Thessaloniki 54121,Greece

Sport specificity background affects the principal component structure of vertical squat jump performance of young adult female athletes

Vassilios Panoutsakopoulos*,Nikolaos Papachatzis,Iraklis A.Kollias

Biomechanics Laboratory,Department of Physical Education and Sports Sciences,Aristotle University of Thessaloniki,Thessaloniki 54121,Greece

Purpose:Long-term training specificity is thought to alter performance in tests evaluating strength and power production capability.The aim of the presentstudy was to provide additional information to the lim ited existing know ledge concerning the possible differences of the force/time profi le of squat jumping among different groups of young female athletes.

Methods:One hundred and seventy-three adultwomen(20.1±2.8 years,1.71±0.09 m,65.6±10.3 kg,mean±SD forage,height,and mass, respectively)engaged in track and field(TF),volleyball(VO),handball(HA),basketball(BA),and physical education students(PE)executed maximal squat jumps(SQJ)on a force plate.Pearson’s correlation was used to identify the relationship between SQJ performance,the anthropometric characteristics and the biomechanical parameters.Differences concerning the biomechanical parameters among groups were investigated w ith analysis of variance,while the force-(FPD)or time-(TPD)dependency of SQJ execution was exam ined using principal components analysis(PCA).

Results:SQJwas unrelated to body heightbutsignificantly correlated w ith body mass(r=-0.26,p=0.001).TF jumped higherand produced largerpeak body power outputcompared to all the othergroups(p＜0.05).A llathletes were superior to PE since they performed the SQJ w ith a longer(p＜0.05)vertical body center of mass trajectory during the propulsion phase.PCA results revealed that TF significantly differentiated than the other groups by relying on FPD.

Conclusion:Various differentprofi les of FPD and TPD were detected due to differentsporting background in young female athletes.Since TF superiority in SQJ was relied on the larger power production and a greater FPD,female indoor team sport athletes are suggested to execute jumping exercises adopting the jumping strategies utilized by TF.

CopyrightⒸ2014,Shanghai University of Sport.Production and hosting by Elsevier B.V.All rights reserved.

Gender differences;Performance assessment;Power output;Principal components analysis;Rate of force development

1.Introduction

The ability to jump high is w idely considered a fundamental physical ability demand in the majority of sporting activities.Vertical jumping performance and the ability to generate the acquired impulse for the take-off is depended on a variety of factors such as the ratio of fast and slow tw itch muscle fibers,1,2the activation of the lower extrem ity muscles3,4and the coordinated energy transfer of the produced joint power in a proximal to distal sequence.5—9In the case of the vertical squat jump,performance(i.e.,the jumping height),is greatly depended upon the muscular strength of the leg extensor muscles.10However,the whole body peak mechanical power output has been found to be the most important factor regarding vertical jumping performance.2,11—14

The long-term training specificity is considered to have an effect on the strength and power production capabilities ofindividuals involved in sporting activities of different discipline.15—17Specifically,the training background is a factor that modifies the parameters defining vertical jumping performance among athletes of different sporting activities.12,15,18—21A more sophisticated investigation w ith the use of principal component analysis(PCA),a method that extracts a fewer number of factors from interrelated parameters that assess vertical jump performance,22revealed that athletes of different sporting background tend to achieve higher vertical jumps by utilizing the force and temporal parameters in a sport-background based combination.22—26The results of those studies agree that power-trained athletes(i.e.,volleyball players(VO)and track and field athletes (TF)) perform better in vertical jumping tests. Additionally,the findings of the above mentioned PCA studies converge to the fact that TF rely mainly on a forcedom inant pattern when aim ing for maximum jumping height, whereas handball(HA)and basketball(BA)players show an ineffective utilization of force parameters.

Despite the research conducted concerning the principal component structure of vertical jumping in male athletes, no studies addressing this issue in female athletes have been found.It is well documented that vertical jumping performance is signifi cantly different between males and females due to the existing gender differences concerning the strength and power production abilities.27—29Furthermore,it has been reported that although the temporal parameters are not different,signifi cant gender differences exist concerning the magnitude of the force dom inancy of maximal vertical squat jum p(SQJ)performance in untrained young adult males and females.30Since previous studies have reported differences concerning the principal component structure of vertical jumping only for male athletes of various sport-specifi c backgrounds,22—26it is of interest to exam ine the effect of sport specifi city on the maximal SQJ performance indices in female athletes.The purpose of the present study was to investigate the possibility thatyoung adult female athletes from differentsports utilize a force-and time-dependency pattern representative of their sporting background when executing a vertical SQJ.Itwas of interest to exam ine if female TF and VO rely more on a tendency of force dom inance opposed to HA and BA players,as previously shown for male athletes of the same sports.

2.M aterials and methods

2.1.Participants

A hundred and seventy-three women(20.1±2.8 years, 1.71±0.09 m,65.6±10.3 kg,mean±SD for age,height, and mass,respectively)volunteered for the study.In detail, 136 of the participants were athletes(Table 1)and were evaluated at the beginning of their competitive season,51 were national level TF(sprinters,jumpers,and throwers),48 were VO,19 were HA,and 18 were BA,all competing in top leagues of their respective sport.Inclusion to the study required athletes to constantly participate in systematic training programs for a period of at least 8 years.The sample also included 37 females who were physical education students (PE)and did notparticipate,besides theiracademic courses,in a systematic training program for at least 2 years prior to the study.No previous severe lower extrem ity injury was reported from the participants who gave their informed consent for participation in the study,which was accomplished according to the Institutional Research Ethics Code for the use of human subjects.

2.2.Procedure

Prior to the actual testing,the participants’anthropometric data(body height,body mass,and body fat composition)were collected.31Before testing,participants performed a 10-min cycling session at a constant pedaling velocity of 5.5 m/s w ith no additional load for warm-up,followed by a 10-m in flexibility program.A fterward,the participants executed three bare footed maximal SQJ on a force-plate w ithout the sw ing of the arms.At the starting position for the execution of the SQJ,the arms were placed on the hips,the feetwere in full contact w ith the force-plate and the knee joint was in an approximate 90°angle.The 90°angle of the knee joint was controlled by video-recording the SQJ attempt with a JVC GR-D720E video camera(Victor Company of Japan Ltd., Yokohama,Japan)which was connected to a PC through an IEEE 1394 interface(Texas Instruments Inc.,Dallas,TX, USA).The camera was fixed on a stationary tripod placed ata heightof 1.2 m and ata distance of 7 m from the participants. The optical axis of the camera was perpendicular to the sagittal plane of the participants.The recorded video wasdisplayed simultaneously on the capture screen of the Kinovea 0.8.15 software(Joan Charmant&Contributors,Bordeaux, France).This enabled to project a right angle mark on the displayed video,which helped the researchers to guide the participants in order to acquire the initial squatting position. When the desired 90°knee angle was obtained,the participants were instructed to“jump as high and as fast as possible w ithouta countermovementor the use of an arm-sw ing”.This instruction was provided because the arm sw ing and the countermovement have independent effects on lower extremity work and their combined effect produce greater jump height by enabling mechanisms other than the concentric strength of the leg extensor muscles which is assessed by the SQJ test.10,32,33A couple of trials were allowed for fam iliarization.Foran SQJto be considered valid,the participants had to land on the force-plate and had to avoid any downward movement of the body.The latter was evaluated immediately using the time history curve of the recorded vertical ground reaction force(vGRF).If the vGRF curve progressed lower than the line representing the body mass at the initial stages of the propulsion phase,the attemptwas notconsidered valid and itwas repeated.The progression of the vGRF curve below the line representing the body mass indicates a downward movement of the body which is caused by a countermovement.As mentioned above,the validity of the SQJ test requires the absence of a countermovement,because it allows muscles to be activated in a higher level and thus a greater amount of force is produced compared to the concentric contraction of the leg extensor muscles.33In all cases,a m inimum of 1-m in interval was perm itted between the executions of the SQJ in order to avoid fatigue.Only the bestattempt,as indicated by the height of the jump achieved,was selected for further analysis.

Table 1 Participant characteristics(mean±SD).

2.3.Instrumentation and data acquisition

The values of the anthropometric characteristics of the participants were collected using a Laffayette skinfold caliper (Laffayette InstrumentCo,Laffayette,IN,USA)and an SECA 220 scale w ith telescopic measuring rod(Seca Deutschland, Hamburg,Germany).Warm-up was conducted on a Monark 817E cycle ergometer(Exercise AB,Vansbro,Sweden).An AMTI OR6-5-1 force-plate(AMTI,New ton,MA,USA)was used to record the vGRF,which was sampled at a nominal frequency of 500 Hz.The signal from the force-plate was simultaneously stored in a Pentium II personal computer after being digitally converted using a PC-LabCard PCL-812PG (Advantech Co.,Taiwan,China)12 bit analogue-to-digital converter.

2.4.Data analysis

Custom designed software was used to extract the biomechanical parameters that define SQJ performance(achieved jump height,hjump)from the recorded vGRF-time curve.hjumpwas extracted using the body center of mass(BCM)vertical take-off velocity which was derived through the integration of the net vGRF.The analysis included only the best attempt,as indicated w ith the adoption of the criterion described above.

According to relative studies,22—24,26,30selected force and spatio-temporal parameters are included in PCA based on the fact that these parameters were found to represent the tendency of force-or time-dependency of SQJperformance.PCA is a mathematicalprocedure that investigates the variances ofa set of variables and it is used as a descriptive tool.34PCA converts a large number of highly intercorrelated variables into a smaller number of linearly combined uncorrelated(i.e.,“orthogonal”)computed factors named principal components. If a substantial correlation exists among the initial variables, the fi rst principal components w ill account for most(approximately 70%—90%)of the variation of the original variables.34Thus,the derived principal components preserve most of the information given by the initial variables.This procedure extracts a factor pattern matrix,in which the number of principal components is defined by the number of eigenvalues larger than 1.This is adopted because a principal component w ith a variance less than the above mentioned value contains less information than of the original variance(Kaiser’s rule).34In order to rationalize the identification of the extracted factors, the factor pattern matrix is rotated using specifi c criterions (i.e.,the loadings of the variables on the extracted factor)and a numberof iterations of the procedure in a way that the original variables are eventually strongly related to one of the extracted principal components.The use of PCA assists the acquisition of information about the force-or time-dependency of an individual’s jumping profi le by reducing the large number of biomechanical parameters needed to express vertical jumping performance into the coordinates of the factor scores(the plot of the individual scores on the rotated principal components).22Under this perspective,the follow ing force and spatio-temporalparameters were calculated(Fig.1):peak vGRF relative to body mass(FZbm),peak power relative to body mass(Pbm),maximum rate of force development(RFDmax), impulse time(tC),time to achieve peak force(tFZmax),and vertical BCM trajectory during the propulsion phase(SBCM). RFDmaxwas directly extracted as the fi rst time derivative of the recorded vGRF.Pbmwas obtained by multiplying the vGRF by the vertical BCM velocity during the propulsive phase and divided by the participant’s body mass.SBCM,from the initial starting position described in Section 2.2 to the instant of take-off,was extracted through integration of the vertical BCM velocity.

2.5.Statistical analysis

Fig.1.A typicalverticalground reaction force curve(upper leftplot,force)and the parameters calculated from it(rate of force development(RFD),work,power, velocity,position of the body centerofmass).Values next to the curve legends represent the m inimum and maximum values of each parameter during the push-off. The lower right section provides the details concerning the selected time instances of achieving maximum and m inimum values during the jump.

Data were presented as mean±SD and differences concerning the anthropometric data and the biomechanical parameters were identified w ith a one-way analysis of variance (ANOVA).A Scheffepost-hocanalysis w ith Bonferroni adjustment was conducted to detect differences among groups.Two-tailed Pearson correlation was used to detect the relationships among the anthropometric data andhjump.A PCA utilizing a Varimax rotation w ith Kaiser normalization on the data from the 173 participants was executed to exam ine the individual tendency toward force-or timedependency for the achievement of maximum SQJ performance.The number of principal components in the extracted factor matrix was determined by the number of eigenvalues larger than one.Crombach’sαwas used to test the reliability of the extracted rotated principal components.Differentiations among athletes of different sports concerning the tendency for force-or time-dependency were searched by plotting the individual factor regression scores on the rotated principal components and by performing an one-way ANOVA and Scheffepost-hocanalysis w ith Bonferroni adjustment on the extracted individual factor regression scores.The level of significance was set atp=0.05 for all statistical procedures.SPSS 10.0.1 software(SPSS Inc., Chicago,IL,USA)was used for the execution of the statistical tests.

3.Results

3.1.Anthropometric data

The comparison of anthropometric data revealed that V O were taller(p＜0.05)compared to HA,TF,and PE(Table 1).HA were also signifi cantly shorter(p＜0.05)than BA. Additionally,PE were signifi cantly lighter than VO and BA and also had lower lean body mass compared to TF,VO, and BA(p＜0.05).HA had the largest body mass index (BM I),which was significantly larger compared to VO (p＜ 0.05).

3.2.Group results for the examined SQJ biomechanical parameters

Results indicated that participants executed the SQJ in a consistent manner(intraclass correlation coefficient:0.95, coefficient of variation:2.9% ±2.2%),but the values of the biomechanical parameters were significantly different (p＜0.05)among the exam ined groups(Table 2).In detail, thepost-hocanalysis revealed that TF achieved the highesthjump(p＜0.05)after producing the largestPbm(p＜0.05) compared to the rest of the participants.Furthermore,TF was observed to have applied significantly higherFZbm(p＜0.05) than VO,HA,and PE.Significantly fastertCandtFZmax(p＜0.05)was noted for TF compared to VO and HA,who both in turn were significantly slower(p＜0.05)in the above mentioned parameters than BA and PE.Lower value forRFDmaxwas recorded for VO compared to TF(p＜0.05). Finally,PE had the shortest SBCMcompared to the examined groups of athletes(p＜0.05).

Table 2 Group results for the biomechanical parameters of the squat jump(mean±SD).

3.3.Results ofcorrelation and principalcomponents analysis

hjumpwas found to be negatively correlated w ith body mass (r=-0.26,p=0.001)but not w ith lean body mass (r=-0.11,p＞0.05)orbody height(r=0.04,p＞0.05).The force parameters exam ined(FZbm,Pbm,and RFDmax)were significantly(p＜0.001)correlated to each other,w ith correlation coefficients(r)ranging from 0.32 to 0.73(Table 3). Lower,yet significant,correlation coefficients were observed among the spatio-temporal parameters(tC,tFZmax,andSBCM) as well(p＜0.01).W ith the exception ofPbm,negative correlations were detected between the spatio-temporal and the force parameters.hjumpwas highly correlated w ithPbm(r=0.70,p＜0.001).

The correlation analysis revealed that it was valid to conduct the PCA because significant intercorrelations were detected among the tested variables.PCA revealed the existence of two principal components thatexplained 69.1%of the variance of the exam ined biomechanical parameters.The variable scores of the two extracted principal components are presented in Fig.2.The fi rst rotated principal component, which accounted for 40.2%of the variance,was interpreted to be associated w ith the time characteristics of SQJ(eigenvalue: 2.41)since it was linked w ith the spatio-temporal parameters (SBCM,tC,tFZmax).In detail,SBCM,tC,tFZmaxwere highly and positively loaded on this factor(loadings:0.60—0.93;commonalities:0.36—0.88;α=0.65).These loadings suggest that longtCis combined w ith largerSBCMand slowertFZmax. Negative relationships on this principal component(individuals spotted in sections A and C,Fig.3)indicate,w ith respect to force application,fast athletes,while positive relationships represent slow athletes(sections B and D).The second rotated principal component accounted for 28.9%of the variance and was related w ith the force characteristics (FZbm,Pbm,and RFDmax)of SQJ(eigenvalue:1.73).In specific,FZbm,Pbm,and RFDmaxhad high positive loadings of 0.92,0.89,and 0.59 respectively on this factor(commonalities:0.36—0.87;α=0.72).These loadings suggest thathighFZbmwas achieved through high RFDmaxand thus resulted in largePbm.Positive relationships on this principal component (individuals spotted in sections A and B,Fig.3)suggeststrong athletes,while negative relationships are interpreted to represent weak athletes(sections C and D).

The individual regression scores on the two principal components of the examined athletes for SQJ are plotted in Fig.3.The horizontal axis corresponded to the component identified as time-dependent,while the vertical axis was suggested to represent force-dependency.In general,the regression scores seem to be concentrated on the horizontalaxis.As mentioned above,athletes w ith high positive loadings on the second principal component and high negative loadings on the fi rst principal component are more likely to produce larger peak force and power outputs in a shorter duration of impulse.Thus,“fast and strong”(i.e.,powerful,since power=force×velocity)athletes are marked in the upper left section of the plot,a section mostly marked by TF.On the other hand,the vast majority of PE was in the bottom left section.This could be interpreted thatPE,despite having a fasttC,failed to produce largeFZbmandPbmto accelerate and to raise their BCM during the propulsion phase resulting in their poor SQJ performance.These two distinct patterns of the utilization of the biomechanical parameters for maximizing SQJperformance exhibited by TF and PE were verified by the analysis of variance of the regression scores on the verticaland horizontal axes,respectively.Furthermore,BA were linked more to a“fast”profi le compared to HA and VO(p＜0.05),despite the fact that these groups showed the same forcedependent profi le.

Table 3 Correlation matrix presenting the relationship among the examined biomechanical parameters and the squat jump height(n=173).

Fig.2.The extracted principal components and factor loadings.Based on the plotting of the factor loadings of the initialvariables,the extracted components were defined to represent the“time”(on the horizontalaxis;SBCM:the vertical body center of mass trajectory during the propulsion phase;tC:impulse time;tFZmax:the time to achieve peak vGRF)and the“force”(on the vertical axis;FZbm:peak vGRF relative to body mass;Pbm:peak power relative to body mass;RFDmax:the maximum rate of vGRF development)factors.In further detail,section(A)represents“strong and fast”athletes,section(B)represents“strong and slow”athletes,section(C)represents“weak and fast”athletes,and section(D)represents“weak and slow”athletes.

The different force/time-dependentprofi les indicated by the individual regression scores on the two principal components could be used to better interpret the initial vGRF,Pbm,and vertical BCM velocity curves.Fig.4 presents two cases on the opposite ends of the plots:a sprinter(TF)from the“fast andstrong”section and a goalkeeper(HA)from the“weak and slow”section.TF has a steeper ascentand a higher peak in all three curves compared to HA,thus justifying their positioning on the plot.

Fig.3.Individual regression factor scores on the two rotated principal components.Section(A)represents“strong and fast”athletes;section(B)represents“strong and slow”athletes;section(C)represents“weak and fast”athletes;and section(D)represents“weak and slow”athletes(see text for further details).Abbreviations: PE = physical education students; BA=basketball players;HA=handball players;VO=volleyball players; TF=track and field athletes.

Fig.4.Representative vertical ground reaction force(A),whole body power output(B)and vertical body center of mass velocity(C)curves of an sprinter (solid line)from the“strong and fast”section and a handball player(dotted line)from the“weak and slow”section of the plotof the individual regression scores on the extracted principal components.A ll curves are normalized w ith respect to impulse time(tC).vGRF=vertical ground reaction force.

4.Discussion

Results indicated that the sportspecific background had an effecton the biomechanical parameters thatdefine the vertical SQJ performance in young adult female athletes from different sports,since differences concerning the force-and timedependency were observed among the exam ined groups.In detail,TF achieved the highesthjumpand the largestPbmamong the participants,and alterations were observed among the indoor team sport athletes concerningtCandtFZmax. Despite being the fi rst(to the best of our know ledge)research dealing w ith the principal component structure of SQJ for female athletes,the present results verified previous findings concerning the importance of power on vertical jumping ability2,11—14,27,35and the differentiations of jumping ability parameters among different groups of athletes.15,18,19,22—26

Alterations in ability is believed to be characterized by particular,well distinguished anthropometric and biomotor profi les for each sport from the early stages of participation.36—38The present findings suggested that body height and lean body mass were found to be unrelated to the values of the biomechanical parameters andhjump.Additionally,the intra-group comparisons of the anthropometric parameters were in agreement with previous findings.31,35,39—44In particular,the participants w ith the higher lean body mass (mainly TF and VO)had the better SQJ performance.This could be interpreted under the perspective that body mass has been found to be a predictor of vertical jumping height.44,45However,the total body mass was found to be negatively related tohjump.This was a result of the fact that the heavier team sport athletes(BA and HA)had the lowest SQJ performance.It could be suggested from the present results that the produced whole body power output for the heavier athletes was notefficientenough for accelerating the BCM during the propulsion.

Vertical jumping performance was found to be different among athletes from different sporting backgrounds,confi rm ing sim ilar comparisons.19,37This study reproduces the finding that female TF exert larger power outputs in shorter impulse times compared to other athletes.19This seems reasonable since the force parameters and power in particular has been found to be correlated w ith jumping heightand thus they are considered to defi ne jumping performance in women.37,41,44,46In the presentstudy,young adult female TF displayed a force-dependent SQJ execution compared to the other groups of athletes,since TF performed the SQJ using a“fast and strong”pattern.Sport specifi city of SQJ execution could be supported by the individual plotting.Based upon the participants’distribution in each section,TF are mainly at the“strong”,BA at the“fast”,PE at the“weak”,and HA at the“slow”section of the principal components plot.The present study reveals that female TF enabled a distinguished power pattern for executing the SQJ,confi rm ing previous findings for male TF.22,26An additional factor to support TF superiority inhjumpis thought to be connected w ith the finding that TF have a larger force production capacity of leg extensor muscles compared to otherathletes,17w ith the knee extensors to be suggested as the major contributors to double leg vertical jump performance from a standing position.1,47It was also confi rmed that VO adopted a jumping pattern emphasizing on longtCand lowFZbmas found elsewhere.26Being in agreement w ith the previous studies,22,26team sport athletes were characterized by a less effective utilization of the SQJ force parameters than TF.Sim ilar observations37have attributed this finding to the fact that TF use a larger portion of single over double legged stationary jumps in training contrarily to the other groups.This training modality was found to be effective for strength and concentric power production of the lower extrem ities47,48and it composes a factor that is suggested to distinguish the jumping ability among TF and team sport athletes.26In general,differences in vertical jumping ability among different group of athletes has being attributed to the fact that prolonged training in a specific sport causes the central nervous system to program the muscle coordination for the execution of the jump according to the demands of that sport.15

Despite the fact that previous PCA studies on vertical jumping accounted for a higher percentage of variance (ranging from 74.1%to 78.8%),22—24,26,30the reliability scores of the two extracted rotated principal components revealed the validity of the present findings.Additionally,as mentioned previously,the comparison of the biomechanical parameters among the exam ined groups was consistent w ith previous findings for female athletes.19,37However,hjumpachieved in the presentstudy seems to be lower than reported elsewhere for respective groups of female athletes.42,49—55Besides skill level,the experimental procedure to disallow the use of the arm sw ing for the jump seems to attribute to these alterations.53,54Another constrain was the instruction given to the participants to“jump as high and as fast as possible”.This is because temporal constrains are suggested to be a factor for the relevancy of RFD to achieve maximum jumping heights.21Additionally,the starting posture w ith the demand of full foot contact on the force-plate imposes a lim itation regarding the ankle flexion that differentiates SQJ performance,56,57particularly for females w ith lim ited ankle dorsi-flexion.58

The results of the present study converge to the finding that the factor that differentiated SQJ performance among groups of young female athletes w ith different sporting backgrounds was the whole body peak mechanical power output and the force/time structure of the jump.This fi nding relays on the fact that many sport jumps are time-restricted w ith a combined demand for a maxim ization of the propulsive impulse.59The achievement of such a performance is determ ined by maxim izing the capabilities of the lower limb neuromuscular system concerning its power output and by optim izing its force-velocity mechanical profi le.60Under this perspective,neuromuscular and power training is found to beeffective for enhancing vertical jump performance and is recommended for team sport athletes,49,51—54taking into consideration the player’s playing position and skill level.52,53

5.Conclusion

Based on the findings of the presentstudy,PCA is a suitable method to detect the reliance upon force-or time-dependency of vertical squat jump performance of young adult female athletes from different sports.Additionally,this method could be possibly used for talent identification and sportorientation of young female athletes on the basis of recognizing sportspecific force/time profi les of vertical squat jumping.For example,an individual’s jumping pattern characterized by long impulse time and low force application could be interpreted as volleyball rather than a track and field sportspecific skill.Furthermore,in the case of indoor team sport athletes, the need for larger jumping heights in lim ited time,as defined by the demands of their sporting activities,could be fulfi lled by adopting the power-specific jumping exercises and training modalities used by TF.

Acknow ledgment

The authors w ish to thank the two anonymous reviewers for their valuable feedback on earlier versions of the manuscript.

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Received 15 March 2013;revised 1 June 2013;accepted 28 August2013

*Corresponding author.

E-mail address:bpanouts@sch.gr(V.Panoutsakopoulos)

Peer review under responsibility of Shanghai University of Sport

2095-2546/$-see front matter CopyrightⒸ2014,Shanghai University of Sport.Production and hosting by Elsevier B.V.A ll rights reserved. http://dx.doi.org/10.1016/j.jshs.2013.08.003