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Physical Therapy in Sport 14 (2013) 35e43

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Physical Therapy in Sport

journal homepage: www.elsevier .com/ptsp

Original research

Influence of a preventive training program on lower limb kinematicsand vertical jump height of male volleyball athletes

Gustavo Leporace a,b,c, Jomilto Praxedes a,d, Glauber Ribeiro Pereira a,c, Sérgio Medeiros Pinto a,Daniel Chagas a,e, Leonardo Metsavaht b, Flávio Chame a, Luiz Alberto Batista a,e,*

a Laboratory of Biomechanics and Motor Behavior, State University of Rio de Janeiro, Brazilb Institute Brazil of Health Technologies, Rio de Janeiro, BrazilcBiomedical Engineering Program, COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, Brazild Post Graduation Program on Mechanical Engineering, State University of São Paulo, São Paulo, Brazile Post Graduation Program on Medical Sciences, State University of Rio de Janeiro, Rio de Janeiro, Brazil

a r t i c l e i n f o

Article history:Received 25 March 2011Received in revised form16 February 2012Accepted 21 February 2012

Keywords:ACLBiomechanicsTrainingInjury preventionSport performance

* Corresponding author. Laboratory of BiomechaniUniversity of Rio de Janeiro, Rua São Francisco Xavier,8122, P.O. Box 20550-900, Rio de Janeiro, Brazil. Tel.:

E-mail addresses: guleporace@yahoo.com.br (G.com, bmc_ef@yahoo.com.br (L.A. Batista).

1466-853X/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.ptsp.2012.02.005

a b s t r a c t

Objective: To examine the influence of a preventative training program (PTP) on sagittal plane kinematicsduring different landing tasks and vertical jump height (VJH) in males.Design: Six weeks prospective exercise intervention.Participants: Fifteen male volleyball athletes (13 � 0.7 years, 1.70 � 0.12 m, 60 � 12 kg).Interventions: PTP consisting of plyometric, balance and core stability exercises three times per week forsix weeks. Bilateral vertical jumps with double leg (DL) and single leg (SL) landings were performed tomeasure the effects of training.Main outcome measurements: Kinematics of the knee and hip before and after training and VJH attainedduring both tasks after training. The hypothesis was that the PTP would produce improvements in VJH,but would not generate great changes in biomechanical behavior.Results: The only change identified for the SL was the longest duration of landing, which represents thetime spent from initial ground contact to maximum knee flexion, after training, while increased angulardisplacement of the knee was observed during DL. The training did not significantly alter the VJH ineither the SL (difference: 2.7 cm) or the DL conditions (difference: 3.5 cm).Conclusions: Despite the PTP’s effectiveness in inducing some changes in kinematics, the changes werespecific for each task, which highlights the importance of the specificity and individuality in selectingprevention injury exercises. Despite the absence of significant increases in the VJH, the absolute differ-ences after training showed increases corroborating with the findings of statistically powerful studiesthat compared the results with control groups. The results suggest that short-term PTPs in low riskyoung male volleyball athletes may enhance performance and induce changes in some kinematicparameters.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Currently, decision making is an important task in managinga program of sports training. Among other things coaches shoulddecide what physical exercise should be used to improve athleticperformance most efficiently. From this point of view, selecting

cs and Motor Behavior, State524, Maracanã, 8� floor, roomþ55 21 2334 0592.Leporace), batista.l.a@gmail.

All rights reserved.

exercises that simultaneously act positively on more than oneperformance variable, seems an interesting choice.

There is evidence that training programs, including plyometric,balance and lumbar-pelvic stability exercises, can contribute toreduce the incidence of ACL injuries in female athletes (Heidt,Sweeterman, Carlonas, Traub, & Tekulve, 2000). Biomechanically,it is believed that these exercises propitiate changes in the kine-matic and kinetic lower-limb behaviours related to the mechanismof this type of injury, such as dynamic knee valgus displacement,maximum knee flexion and peak ground reaction forces (Hewett,Stroupe, Nance, & Noyes, 1996; Irmischer et al., 2004; Myer, Ford,Brent, & Hewett, 2006; Myer, Ford, McLean, & Hewett, 2006;Myer, Ford, Palumbo, & Hewett, 2005). Moreover, there is

G. Leporace et al. / Physical Therapy in Sport 14 (2013) 35e4336

evidence that such training programs can also result in directimprovement of athletic performance variables, mainly thoserelated to vertical jump height (VJH), power and agility. Therefore,it is reasonable to assume that such programs can be used for thepurpose of improving the overall performance of athletes (DiStefano et al., 2010; Luebbers et al., 2003; Myer et al., 2005;Myklebust, Maehlum, Holm, & Bahr, 1998; Newton, Kraemer, &Hakkinen, 1999; Villareal, Gonzalez-Badillo, & Izquierdo, 2008).Nevertheless, although such information might be important forthe development of training strategies, its practical use and greatwidespread requires the overcoming of limitations not addressedin previous studies, particularly with regard to the preventivepractice.

One of the limitations previously mentioned is the epidemio-logical indicators associated with the gender of the subjectsinvestigated. In this case, females tend to bemore studied, probablybecause they are more susceptible to ACL injuries, although theprevalence is higher in the male population, likely because moremen participate in sports (Renstrom et al., 2008).

It seems that ACL injuries result from multi-planar movementsand it has been argued that both the risk factors and the injury-inducing mechanisms differ between genders (Hewett, Myer, &Ford, 2005; Krosshaug, Slauterbeck, Engebretsen, & Bahr, 2007;Zazulak, Hewett, Reeves, Goldberg, & Cholewicki, 2007). Inwomen, the injury is primarily associated with dislocations andmechanical load in the sagittal, frontal and transverse planes(Hewett et al., 2005), whereas injuries in men seem to beprimarily related to movements and loads in the sagittal plane(Quatman & Hewett, 2009). Specifically, kinematic risk factors infemales are supposed to be related to knee valgus and flexion,associated with tibial rotation and hip adduction (Hewett et al.,2005). Otherwise, the risk factors in male athletes seem to berelated mainly to decreased knee flexion (Quatman & Hewett,2009). Thus, although there are studies demonstrating thatmales can reduce their injury rates with an injury preventionprogram (Caraffa, Cerulli, Projetti, Aisa, & Rizzo, 1996; Junge,Rosch, Peterson, Graf-Baumann, & Dvorak, 2002), the differencesbetween sex-related mechanisms of injury lead us to supposeeven that certain injury prevention exercises are effective infemale populations, there is no guarantee that they will offer thesame benefit to similar male populations.

Another limitation of previous studies is related to the motortasks used in the experiments. Researchers tend to use double-leglandings (Irmischer et al., 2004;Myer et al., 2005) to evaluate injuryrisk, which may also limit the generalisation of results because insome sports, such as volleyball, single-leg landings are commonafter a jump (Tillman, Hass, Brunt, & Bennett, 2004). In addition,approximately 25% of ACL injuries occur after single-leg landings(Krosshaug et al., 2007). According to DiStefano et al. (2010) thespecificity of training should be considered carefully to improveboth biomechanics and performance. However, the effect ofa preventive training program on the kinematic of different motortasks is not known.

Finally, despite evidence of effectiveness of plyometric trainingto improve sports performance by increasing VJH, power and agility(Meylan & Malatesta, 2009; Newton et al., 1999; Villareal et al.,2008), as well as the potential effectiveness of PTPs in reducingthe risk factors for and incidence of ACL injuries (Irmischer et al.,2004; Myer, Ford, Brent et al., 2006, Myer, Ford, McLean et al.,2006, 2005; Grindstaff, Hamiill, Tuzson, & Hertel, 2006), thedegree of influence of PTPs on specific performance variables, suchas those listed above, remains unproven. To our knowledge, onlyone study, by Myer et al. (2005), has evaluated the influence ofPTPs in improving sports performance as well as reducing riskfactors for ACL injury. The authors demonstrated the efficacy of

neuromuscular training in increasing athletic performance andreducing ACL injury risk in female athletes.

Kilding, Tunstall, and Kuzmic (2008) and DiStefano et al. (2010)also demonstrated the influence of preventive programs on athleticperformance, but they did not examine biomechanical aspects.DiStefano et al. (2010) suggested that future studies shouldexamine performance and lower-limb biomechanical behaviours toidentify the effects of PTPs.

Therefore, the aim of this study was to examine the effects ofa neuromuscular training program on lower-limb kinematicsduring single-leg and double-leg landings and vertical jump height.The experimental hypotheses were that: (i) the training-relatedchanges would be specific to each landing; (ii) the trainingprogramwould improve vertical jump height; and (iii) the trainingprogram would induce kinematic changes, such as increased kneeand hip range of motion.

2. Methods

Fifteen male volleyball athletes from a regional team (age:13 � 0.7 years, height: 1.70 � 0.12 m and body mass 60 � 12 kg)with no history of lower-limb joint injuries participated in thisstudy. At the time of the intervention, all the athletes had at leastthree years’ experience in the sport and all of them were used toplaying in regional and national competitions. A parent/guardiansigned informed consent for each athlete and authorised hisparticipation in the study, which was approved by the StateUniversity of Rio de Janeiro Ethics Committee.

In the week before starting and in the week after finishingtraining, the subjects underwent tests to examine lower-limbkinematics during landing after vertical jumps and the maximumheight reached in these tasks. The training was performed in thebeginning of the season.

Two vertical jumps were used to induce the targeted kinematicbehaviour. For each sequence, the athletes performed the propul-sive phase of jumping twice with both legs, but in one they landedwith the dominant leg (SL) and in the other they landed with bothlegs (DL) (Fig. 1A and B). The dominant leg was defined as the legthe participant would use to kick a ball as far as possible (Myer,Ford, Brent et al., 2006, 2005).

Initially, the athletes familiarised themselves with the tasks toreduce the influence of a learning effect on the biomechanicalvariables. At least 5 min after the familiarisation session, three SLand three DL were filmed in the sagittal plane and stored ina computer for later analysis. A trial was considered successful ifthe athlete could land without losing balance. No athlete hadmore than two unsuccessful trials. The landing tasks were per-formed in a random sequence to minimise the possible effects offatigue, beginning with SL or DL. A one-minute rest interval wasallowed between attempts. The subjects performed all tests andtraining with the shoes they used to play regularly. The onlyinstruction provided to the athletes was to jump as high aspossible and land in the specific condition (double leg orsingle leg).

The execution of landing tasks was filmed with a camera (SonyDCR HC 46) positioned in the sagittal plane 2 m away from theplace of execution, with the optical axis projected onto the centre ofthe capture area perpendicular to the vertical and horizontalorientations and a sample frequency of 30 Hz, which allowed theinterlaced processing of 60 frames per second.

Six spherical, 20-mm reflective markers were positioned at eachathlete’s iliac crest, greater trocanter, lateral condyle of the femur,lateral malleolus, lateral calcaneus and fifth metatarsal to allow usto examine the angular behaviour of the hip and knee in the sagittalplane (Fig. 2). The metatarsal point was used only to generate the

Fig. 1. Description of landings. In the two tasks, the athletes performed a bilateral vertical jump with a single-leg landing (SL) and a double-leg landing (DL).

G. Leporace et al. / Physical Therapy in Sport 14 (2013) 35e43 37

corporal model. To assure marker placement reliability, the sameresearcher, with great experience in palpatory anatomy, positionedall markers in all subjects.

A static trial was collected by asking the subject to stand stillwhile he was aligned with the laboratory (global) coordinatesystem. This measurement was used to define each subject’sneutral (zero) alignment, with subsequent dynamic kinematicmeasures quantified relative to this position. To calibrate theimages, four non-collinear points were positioned at the vertices ofa 50-cm-sided cube positioned parallel to the capture plane andlocated in the area where the landing tasks were executed(Robertson & Caldwell, 2004). The data for all subjects werecaptured in the same environment. Therefore, the devices were notrelocated between data collection events.

After capture, the images were transferred to a personalcomputer. The raw coordinates of the markers were transformedinto global coordinates (Abdel-Aziz & Karara, 1971, p.1) and pro-cessed through the Skill Spector software (Geeware, Version 1.2.4,USA), according to the protocol validated by McLean et al. (2005).They found a high correlation between a single camera 2D and a 3Dmovement analysis system for inter-subject difference for coronalplane knee kinematics (r: 0.76e0.80). The between day intra-testerreliability of this systemwasmeasured by Button, van Deursen, andPrice (2008). They found an ICC ranging from 0.75 to 0.87 for hip,knee and ankle sagittal plane kinematics.

We used a low-pass, fourth order Butterworth filter in theforward and reverse directions to prevent phase distortions, witha cut-off frequency of 6 Hz to smooth the kinematic signal.

Fig. 2. Positions of the six markers on the dominant lower-limb, used for kinematicalanalysis.

G. Leporace et al. / Physical Therapy in Sport 14 (2013) 35e4338

To determine the instant of ground contact, we used a custom-made footswitch transducer (FootPress LaBiCoM�) set in the sole ofthe subjects’ shoes in the region of the first metatarsal. When theelectrical circuit of the footswitch was triggered, a light-emittingdiode (LED) positioned in the camera’s capture field was acti-vated, indicating the presence or absence of ground contact.

The signal produced by the footswitch was also used to calculatethe time of the flight phase. This variable was used to estimate themaximum vertical height according to the formula 0.5*g*(t/2)2,where g is the acceleration of gravity (9.81 m/s2), and t is the flightphase duration, measured between the instants of loss andresumption of ground contact, according to the strategy validatedby Leard et al. (2007). The largest vertical displacement achieved byathletes in three trials was considered the maximum height, asMoir, Shastri, and Connaboy (2008) have demonstrated that thisstrategy has an excellent reliability level for males (ICC > 0.9) andshould be used instead of the arithmetic mean. MATLAB version 6.5(The Mathworks, USA) was used for signal processing.

2.1. Preventive training program

The frequency of training was three times/week for six weeks,45e60min per session. Participants were monitored during training

by a team of four or five physical education coaches and studentstrained to correct execution errors and to facilitate performanceimprovements, which provided an average of one coach for everythree athletes, allowing effective and individualised control of thetraining process. The training program was implemented in thefinal half of the competitive season.

The PTP used in this study consisted of a compilation of strat-egies used in previous studies (DiStefano et al., 2010; Grindstaffet al., 2006; Heidt et al., 2000; Hewett, Lindenfeld, Riccobene, &Noyes, 1999; Hewett et al., 1996; Irmischer et al., 2004; Meylanet al., 2009; Myer, Ford, Brent et al., 2006, 2005; Myer, Ford,McLean et al., 2006; Paterno, Myer, Ford, & Hewett, 2004; Pollard,Sigward, Ota, Langford, & Powers, 2006). The exercises aimed toallow a higher degree of specificity between training and volleyballperformance. Plyometric, balance and core stability exercises wereused (Appendix 1 and 2). After each PTP session, the athletes per-formed their technical training routine.

The PTP was divided into three phases according to Hewett et al.(1996): 1) The technique phase, focused mainly on basic aspectssuch as correct posture, body alignment throughout the jump, softlandings and instant recoil preparation for the subsequent jump; 2)The fundamentals phase, focused on proper technique to increasepower and ability and 3) The performance phase, focused onachieving maximal vertical jump height through improvedtechnique.

The training programwas modified every twoweeks to increasethe difficulty of the exercises because the Principle of Overloadindicates that a two-week period is long enough to allow athletes toassimilate the difficulties of previous exercises (Hewett et al., 1996).The degree of difficulty was increased through the use of single legexercises, increased repetitions and intensity and the use ofunstable surfaces with eyes open and closed during specificvolleyball techniques (dual task). In each session, approximatelyeight or nine plyometric exercises, four or five core stability exer-cises and four or five balance exercises were conducted. A 2-minrest interval was allowed between exercises.

2.2. Statistical analysis

The vertical jump height and angular and temporal lower-limbkinematics related to two landing tasks were compared duringbilateral vertical jumps (single-leg landings (SL) and double-leglandings (DL)) (Fig. 1A and B) between before and after the appli-cation of a preventive training program (PTP).

The influence of the PTP was measured in the following vari-ables: vertical jump height (VJH); angular position of the hip andknee upon ground contact after the flight phase (IAPH and IAPK,respectively); angular position of the hip and knee measured whenthe body’s centre of gravity reached its lowest position of verticaldisplacement (MAPH and MAPK, respectively); hip and knee rangeof motion (HRoM and KRoM) from the initial contact to themaximum angle of each joint; and the time of landing (TL), desig-nated as the time, in seconds, from the initial contact to the lowestposition of vertical displacement. These biomechanical variableswere selected on the basis of studies that proposed a relationshipbetween the behaviour of these variables and ACL injury riskfactors in men (Quatman & Hewett, 2009; Renstrom et al., 2008)and on the relationship between VJH and performance in volleyballplayers (Barnes et al., 2007).

The reliability of all dependent measures among three attemptsfor each landing task before and after training was determinedusing the intraclass correlation coefficient (ICC2,1) and standarderror of measurement (SEM) (Weir, 2005). To examine the influ-ence of the training programme on the tested variables, the pre-testand post-test values were compared using the Wilcoxon ranked

Table 1Inter-trial reliability, measured by intraclass correlation coefficient (ICC2,1) andstandard error of measurement (SEM), of the dependent variables among the threeattempts in each landing task.

Single-leg landing Double-leg landing

Pre-training Post-training Pre-training Post-training

ICC2,1 SEM ICC2,1 SEM ICC2,1 SEM ICC2,1 SEM

VJH 0.933 0.009 m 0.816 0.088 m 0.947 0.015 m 0.796 0.069 mIAPH 0.892 1.9� 0.881 1.5� 0.885 3.7� 0.820 3.3�

MAPH 0.870 3.5� 0.827 3.5� 0.960 2.6� 0.925 2.9�

HRoM 0.860 2.7� 0.854 3.0� 0.972 3.3� 0.925 3.3�

IAPK 0.869 1.8� 0.846 2.6� 0.961 2.3� 0.919 1.8�

MAPK 0.820 3.3� 0.798 2.1� 0.867 3.1� 0.898 2.7�

KRoM 0.803 2.0� 0.838 2.6� 0.923 3.0� 0.807 3.0�

TL 0.970 0.053 s 0.917 0.076 s 0.982 0.048 s 0.924 0.079 s

G. Leporace et al. / Physical Therapy in Sport 14 (2013) 35e43 39

test, with a significance level of 5%. We used a non-parametric testdue to the limited sample size and in consideration of theKolmogoroveSmirnov test results, which suggested a non-Gaussian distribution. The statistical analysis was performedusing GraphPad Prism, Version 5.00 for Windows (GraphPad Soft-ware, San Diego, California, USA).

3. Results

All individuals completed at least 80% of the training sessionsand performed the tests before and after training, with a mean of89% (16 from 18 total sessions) of compliance. No athlete wasinjured or had musculoskeletal pain during or after the trainingperiod.

Table 2Temporal and angular values of the dependent variables in the single-leg landing task. Thepre-training and the post-training values, respectively. The results are expressed as themethe pre- and post-training phases. In that column, values in parentheses represent the p

Single-leg landing

Pre-training Post-training

VJH (cm) 26.9(1.6) [23.5e30.4] 30.3(1.9) [26.1e34.5]IAPH (�) 23.6(2.1) [18.8e28.5] 23.6(0.9) [21.5e25.8]MAPH (�) 47.9(3.4) [40.1e55.7] 49.8(1.4) [46.6e53]HRoM (�) 24.2(2.2) [19.1e29.3] 26.2(1.4) [23e29.3]IAPK (�) 9.9(1.6) [6.3e13.5] 9.1(1.6) [6.4e11.9]MAPK (�) 57.3(1.8) [53.2e61.4] 59.3(1.3) [57e61.5]KRoM (�) 47.3(1.3) [44.3e50.4] 50.1(1.2) [48.1e52.1]TL (s)a 0.30(0.02) [0.24e0.35] 0.38(0.03) [0.32e0.44]

a Significant differences between pre- and post-training (p < 0.05).

Table 3Temporal and angular values of the dependent variables in the double-leg landing task. Thpre-training and the post-training values, respectively. The results are expressed as themethe pre- and post-training phases. In that column, values in parentheses represent the p

Double-leg landing

Pre-training Post-training

VJH (cm) 31.2(2.0) [26.9e35.5] 35.4(2.4) [30.1e40.7]IAPH (�) 23.6(2.6) [17.8e29.5] 22(2.4) [16.4e27.5]MAPH (�) 73.3(4.3) [63.4e83.1] 83.5(3.3) [76e90.9]HRoM (�) 49.6(6.2) [35.5e63.8] 61.5(2.5) [55.8e67.2]IAPK (�) 21.1(3.3) [13.6e28.6] 15.1(2.0) [10.5e19.6]MAPK (�) 70.9(2.2) [65.9e75.9] 71.7(2.0) [67.2e76.2]KRoM (�)a 47.8(3.3) [42.1e57.4] 56.6(1.8) [52.5e60.8]TL (s) 0.37(0.05) [0.26e0.49] 0.49(0.06) [0.35e0.63]

a Significant differences between pre- and post-training (p < 0.05).

3.1. Reliability analysis

The ICC2,1 of the dependent variables achieved in three attemptsat each landing before and after training are shown in Table 1.Excellent reliability values (>0.8) were found for all the situationsin both landings.

3.2. Vertical jump height

No statistically significant differences were found between thepre- and post-training in relation to the VJH values with SL or DL(Tables 2 and 3).

The means, standard errors, confidence intervals and p values ofSL and DL, the dependent variables of this study, are presented inTables 2 and 3, respectively, and described below.

3.3. Single-leg landing

There were no significant differences between training periodsfor IAPH, IAPK, MAPH, MAPK, HRoM and KRoM. However, aftertraining, athletes showed significantly longer TLs compared to thepre-training period.

3.4. Double-leg landing

There were no significant differences between trainingperiods for IAPH, IAPK, MAPH, MAPK, HRoM and TL. Aftertraining, the athletes showed a statistically significant increase inKRoM.

first column lists the dependent variables, the second and the third columns list thean (standard error) [95% CI]. The fourth column lists the average differences betweenercentage differences.

Mean of differences P value Effect size

2.7 (10%) 0.3054 0.520.01 (0.03%) 0.5566 0.012.0 (4.1%) 0.3223 0.231.9 (8%) 0.3750 0.33

�0.8 (�8.3%) 0.9219 0.162.0 (3.6%) 0.1934 0.412.9 (6%) 0.1309 0.72

0.08 (28.1%) 0.0273 1.06

e first column lists the dependent variables, the second and the third columns list thean (standard error) [95% CI]. The fourth column lists the average differences betweenercentage differences.

Mean of differences P value Effect size

3.5 (11.3%) 0.3635 0.52�1.7 (�7.1%) 0.5566 0.2110.2 (13.9%) 0.0645 0.8311.9 (23.9%) 0.1602 0.78�6.1 (�28.7%) 0.0840 0.700.8 (1.1%) 0.6250 0.126.1 (13.8%) 0.0371 0.80

0.12 (30.7%) 0.1533 0.64

G. Leporace et al. / Physical Therapy in Sport 14 (2013) 35e4340

4. Discussion

Improving the overall athletic performance involves, amongothers things, acting on several variables specific to each situation,seeking that athletes achieve better performance with a low injuryrisk. In this context, an important goal of preventive exercise is todevelop the ability to perform movements with less aggressivemechanical characteristics.

In this study, male volleyball athletes participated in a PTP, andtheir subsequent performance was examined to determine thePTP’s influence on athletic performance and lower-limb kinematicsin two landing tasks with different constraints.

To our knowledge, this is the first study to examine theinfluence of preventive training on both the athletic ability andlower-limb kinematics of young male athletes during differenttypes of landings. Other investigations that have addressed thistype of training focused on its influence on women (Myer et al.,2005) or on athletic performance alone in young people of bothsexes or just males (DiStefano et al., 2010; Irmischer et al., 2004).As has been already described, the risk factors and mechanismsof injury, although not proved by a prospective study, seem tobe different between genders, therefore, the inference of theresults obtained from female to male athletes should be donecarefully.

The results obtained in a previous investigation with the samepopulation (Leporace et al., 2010) showed that the valgus angle ofthe examined group during landings tasks are regarded as beingat low risk for ACL injuries (Hewett, Myer, & Ford, 2004; Schmitz,Kulas, Perrin, Riemann, & Shultz, 2007; Swartz, Decoster, Russell,& Croce, 2005; Yu et al., 2005). Although the knee flexion couldbe considered low, the adequate alignment in frontal plane maybe related to low risk for ACL injury. Myer, Ford, Brent, andHewett (2007) showed that youths with low susceptibility tothis injury are less sensitive to preventive training and havea tendency to make minor biomechanical adaptations to exercise-induced stimuli. Thus, it was generally expected the subjects inthe present study to have a low response to training, which doesnot imply that this training program does not generate anincrease in sports performance or that the results would besimilar for both types of landings, which in fact could beconfirmed in this study.

It was verified for SL that, despite the increase in the angularrange of motion of the hip and knee, the training effect on thekinematics wasminimal. This low expression for the angular lower-limb kinematics during unilateral landing may be partiallyexplained by the characteristics of the chosen exercises of the PTP(Tables 1 and 2). The programs have higher number of bilateralplyometric exercises than unilateral, and the stability exercisesrequired no deep knee flexion and no pronounced anterior trunkdisplacement. Paterno et al. (2004) used plyometric and stabilityexercises with backward-forward displacement for 6 weeks, whichresulted in improved control of body centre of gravity (CoG)displacement in the anterioreposterior direction, although noimprovement in medialelateral control occurred. Louw, Grimmer,and Vaughan (2006), who used a training protocol consisting ofunilateral plyometric exercises in a population similar to that of thepresent study, found significant changes in knee-joint displace-ment after training. This set of findings highlights the importance ofrespecting the Principal of Specificity when selecting an exercisetraining program.

It is possible that the athletes presented a greater landingduration after training to compensate for their inability to stabilisethe knee-joint in deep flexions during the SL. In general, thisstrategy may have been adopted to reduce the mechanical loadsaround a joint, given that with more time, the mechanical impulse

generated is more likely to be distributed with lower peak forces,leading to greater absorption of the energy generated by a task. Wesuggest that exercise programs with longer durations and morespecific exercises be conducted to evaluate the kinematic andkinetic adaptations in this population.

Regarding the maximum flexion of the hip in DL, althoughthe p value did not reach significance, there was a strong trend(p ¼ 0.0645) and a large effect size (Cohen’s d ¼ 0.83). The meandifference was approximately 10�, which resulted in a joint ROMincrease of approximately 12� in the post-training (Cohen’sd ¼ 0.78), mainly due to anterior trunk inclination. This kine-matic behaviour suggests the CoG moving to coordinates inwhich it remains horizontally aligned with the axis of kneemotion. This indicates decreasing extensor torque in this joint,which also reduces the ACL strain (Blackburn & Padua, 2008).This result may be related to the individual’s increased ability tocontrol the CoG projection over the support basis, which istypically obtained as an effect of neuromuscular trainingprograms (Paterno et al., 2004). The knee movement, regardedas a hip angular behaviour-associated movement, showeda significant difference (p ¼ 0.0371, Cohen’s d ¼ 0.80) ofapproximately 6� in the flexion displacement, which, accordingto Blackburn and Padua (2008), corroborates the ACL tensionreduction. However, this alteration was probably due to thedecrease in the knee flexion at initial contact, which mayactually increase injury risk (Hewett et al., 2005; Quatman &Hewett, 2009). Future studies are needed to discuss the effectof the paradigm related to the increase of knee displacement asa result of decreased knee flexion at initial contact.

Although other studies have suggested that neuromusculartraining induces changes in athletic performance (Bobbert, 1990;Kilding et al., 2008; Myer et al., 2005; Villareal et al., 2008),we found no statistically significant difference when comparingpre- and post-training, either for SL or DL. It is possible that 6 weeksof neuromuscular training is insufficient to produce changes inthese sports performance aspects in young male athletes. However,previous studies have reported that increases of approximately 10%in jump height with countermovement are associated withimprovements in sports performance (Bobert, 1990; Markovic,2007; Villareal et al., 2008).

In this study, based on the evaluation of differences in pre-and post-training absolute values and on the medium effect size(Cohen’s d ¼ 0.52), we observed that improvements, althoughnot statistically significant, in both landings types werecompatible and, in some cases, even higher than the values thatwere associated in the literature with a positive impact onathletic performance. Villareal et al. (2008) demonstrated ina meta-analysis that plyometric training programs generallygenerate increases of approximately 7%, corresponding to 3.9 cmin jump height after about 10 weeks. Regarding the preventiveprograms, Myer et al. (2005) showed improvements of about3.3 cm (8.3%) after six weeks of training; Kilding et al. (2008)reported improvements of 2 cm (6%) and Di Stefano et al.(2010) reported 1.7-cm improvements (6.9%) after nine weeksof training. The present study found differences of 2.7 cm (10%)and 3.5 cm (11.3%) for the vertical jumps with SL and DL landings,respectively. Despite the absence of statistical differences inthe pre- and post-training values, the training programemployed in this study increased the athletic performance ofyoung male volleyball athletes in a way similar to that reportedin the literature after a training period of only 6 weeks. Moreover,as described earlier the PTP was implemented in the final halfof the season. The focus of the training at this stage was ontactical aspects of volleyball while technical aspects related toperformance variables were already finished. This emphasizes

Exercises Weeks

1 2Wall jumps 20 s 25 sTuck jumps 20 s 25 sBroad jumps stick land 5 reps 10 repsSquat jumps 10 reps 15 repsDouble leg cone jumps (anteroeposterior) 30 s 30 sDouble leg cone jumps (medialelateral) 30 s 30 s180� jumps 20 s 25 sBounding in place 20 s 25 s

3 4Wall jumps 30 s 30 sTuck jumps 30 s 30 sJump, jump, jump, vert jump 5 reps 8 repsSquat Jumps 10 reps 15 repsBounding for distance 1 run 2 runsDouble leg cone jumps (anteroeposterior) 30 s 30 sDouble leg cone jumps (medialelateral) 30 s 30 sScissor jump 30 s 30 sHop, hop, sitck 5 reps/leg 5 reps/leg

5 6Wall jumps 30 s 30 sStep, jump up, down, vertical 5 reps 10 repsMattress jumps (anteroeposterior) 30 s 30 sMattress jumps (medialelateral) 30 s 30 sSingle legged jumps distance 5 reps/leg 5 reps/legSquat jumps 25 s 25 sJump into bounding 3 runs 4 runsSingle-legged hop, hop stick 5 reps/leg 5 reps/leg

G. Leporace et al. / Physical Therapy in Sport 14 (2013) 35e43 41

the importance of implementation of PTP despite the cycleof periodization of the training seeking also performanceimprovement.

The duration of neuromuscular training seems to be a deci-sive factor, given the significant changes in the young athletes’performance. Thus, further studies using longer training periodsare encouraged to determine the course of the changes inathletic performance over time and establish the minimumperiod necessary to achieve changes in biomechanical behav-iours associated with performance improvements at differentstages of training.

Unlike other explanations presented in the literature, it can besuggested that the lack of statistical differences for performance inthe vertical jump height could be associated with the researchdesign. The majority of previous studies used pre-post-test differ-ences to compare the training effect between intervention andcontrol groups (DiStefano et al., 2010; Kilding et al., 2008; Myeret al., 2005). In contrast, this study adopted the strategy ofmatching samples. It is suggested the implementation of random-ized controlled trials to test the hypothesis of an increase of verticaljump height after the performance of PTPs.

Another interesting finding of this study is related to the reli-ability of the jump height in the three attempts with both landingconditions. Although the findings for the DL jump support theliterature (Moir et al., 2008), no studies have examined the reli-ability of DL vertical jumps with successive SL landing, which iscommon in volleyball practice (Tillman et al., 2004). In this sense,our results suggest that both landing forms started from jumps ofbilateral propulsion can be employed in training programs toevaluate maximal jump height, due to the high reliability valuefound.

Althoughmuch of our findings obtained in this investigation areconsistent with the literature, three facts must be re-examinedcarefully. One, which is a limitation of this study, concerns thetraining time used. Because of technical staff planning and thesports calendar, it was only possible to apply the preventativetraining for a sixeweek period. Although several studies intendingto change movement patterns related to ACL injury risk have useda same time interval, the literature recommends that trainingprograms with PTP features should be conducted throughout thesports season with adequate periodization (Grindstaff et al., 2006).

The second aspect is related to the absence of a controlgroup. Thus, it is unclear whether the observed improvementsare due to the results of the applied training or other, uncon-trolled factors. Additional, controlled studies aimed atmeasuring the effect of neuromuscular training on lower-limbkinematics and kinetics after different periods of practice arerecommended to determine the time required to obtainincreased training efficiency without causing excessive stress onjoints.

The third aspect is related to the possible clinical significanceof the results. Despite the present study’s values for the angularvariables and height of jumps changes, alterations presented byathletes suggest that the exercise program employed hasa mechano-inductive capacity, even in limited magnitude.However, it was also possible to verify that the exercises tendedto induce more important developments in a type of landingthat corroborates with the proposition that the biomechanicalchanges in different motor behaviours after a neuromusculartraining program are specific to a given stimuli (DiStefano et al.,2010; Myer, Ford, McLean et al., 2006; Myer et al., 2005). Thus,a training program can be effective in inducing changes in motorbehaviour related to preventing the incidence of ACL injuriesand athletic performance in one motor conduct and not foranother.

5. Conclusion

The PTP seems to induce changes to the kinematic behaviour ofthe lower limbs. In the single leg condition, the time of landing wasincreased, while in the double leg condition the knee range ofmotion was improved. This highlights the importance of theselection of the tasks used to compare the kinematics of lowerlimbs. Also, although the vertical jump height was not statisticallydifferent, the improvement of 10% in both jumps tasks agrees withthe values described in the literature and is an important perfor-mance improvement in volleyball. Therefore, the results allow us toconclude that PTP may induce specific changes on lower limbskinematics and improve variables related to sport performance.This supports the idea that training programs similar to the oneused in this study should be used in sports; however, they must beapplied for longer periods, with adequate periodization during theseason.

Conflict of interestNone declared.

Ethical approvalThis studywas approved by the State University of Rio de Janeiro

Ethics Committee.

FundingThis study was partially supported by the Brazilian Research

Council (CNPq), Carlos Chagas Filho Foundation for ResearchSupport of Rio de Janeiro (FAPERJ) and Coordination for theImprovement of Higher Level Education (CAPES).

Appendix 1

Plyometric exercises performed by athletes in the three phases(six weeks) of training.

G. Leporace et al. / Physical Therapy in Sport 14 (2013) 35e4342

Appendix 2

Balance and core stability exercises performed by athletes in thethree phases (six weeks) of training.

Exercises Weeks

1 2Bird dog ($) 8 reps 12 repsFrontal bridge 15 s 20 sSide bridge 15 s 20 sCrunch 12 reps 15 repsDiagonal crunch 10 reps 12 repsSingle-leg balance with multi-planar movements (%) 2x 30 s 2x 30 sDouble leg balance (anterioreposterior) (%) (1) 2x 25 s 2x 35 sDouble leg balance (medialelateral) (2) 2x 25 s 2x 35 sDouble leg balance (#) (1) 2x 25 s 2x 35 s

3 4Double crunch 12 reps 15 repsDouble leg back bridge. Holding 3e5 s 10 reps 15 repsFrontal bridge 30 s 35 sSide bridge 25 s 30 sSingle-leg balance with multi-planar movements (%) 30 s 40 sDouble leg balance (%) (2) 30 s 40 sDouble leg balance. Knee flexed (medialelateral) (#) (2) 10 reps 20 repsDouble leg balance. Knee flexed (anterioreposterior)

(#) (1)10 reps 20 reps

Double leg balance (medialelateral) (�) (2) 10 reps 20 reps5 6

Lumbar extension 10 reps 12 repsSingle leg back bridge. Holding 3e5 s 10 reps 12 repsDouble crunch 15 reps 20 repsBalance with knee support (2) 30 s 40 sSingle leg balance (%) (1) 30 s 40 sSingle leg balance. Knee flexed (�) (1) 10 reps 20 repsDouble leg balance. Knee flexed (anterioreposterior)

(�) (2)10 reps 20 reps

Double leg balance. Knee flexed (anterioreposterior)(�) (2)

10 reps 20 reps

Single leg balance. Knee flexed (#) (1) 10 reps 20 reps

�: Performing volleyball skills (setting, passing, hitting).%: Eyes closed.#: Medicine ball catch.(1): Uniplanar wood instability device.(2): Rubber instability device.

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