Effect of plyometric training on sand versus grass onmuscle soreness and jumping and sprinting ability insoccer players

F M Impellizzeri,1 E Rampinini,1 C Castagna,2 F Martino,3 S Fiorini,3 U Wisloff4,5

1 Human PerformanceLaboratory, Mapei SportResearch Center, Castellanza,Varese, Italy; 2 School of Sportand Exercise Sciences, Facultyof Medicine and Surgery,University of Rome Tor Vergata,Rome, Italy; 3 Facolta diMedicina e Chirurgia, Corso diLaurea Specialistica in Scienze eTecnica dello Sport, Universitadegli Studi di Firenze, Firenze,Italia; 4 Department ofCirculation and Medical Imaging,Norwegian University of Scienceand Technology, Faculty ofMedicine, Trondheim, Norway;5 Department of Cardiology, St.Olav’s Hospital, Trondheim,Norway

Correspondence to:Franco M Impellizzeri,Neuromuscular ResearchLaboratory, Schulthess Clinic,Lengghalde 2, 8008 Zurich,Switzerland; franco.impellizzeri@kws.ch

Accepted 14 May 2007Published Online First25 May 2007

ABSTRACTObjective: The lower impact on the musculoskeletalsystem induced by plyometric exercise on sand comparedto a firm surface might be useful to reduce the stress ofintensified training periods or during rehabilitation frominjury. The aim of this study was to compare the effectsof plyometric training on sand versus a grass surface onmuscle soreness, vertical jump height and sprinting ability.Design: Parallel two-group, randomised, longitudinal(pretest2post-test) study.Methods: After random allocation, 18 soccer playerscompleted 4 weeks of plyometric training on grass (grassgroup) and 19 players on sand (sand group). Before andafter plyometric training, 10 m and 20 m sprint time,squat jump (SJ), countermovement jump (CMJ), andeccentric utilization ratio (CMJ/SJ) were determined.Muscle soreness was measured using a Likert scale.Results: No training surface 6 time interactions werefound for sprint time (p.0.87), whereas a trend wasfound for SJ (p = 0.08), with both groups showing similarimprovements (p,0.001). On the other hand, the grassgroup improved their CMJ (p = 0.033) and CMJ/SJ(p = 0.005) significantly (p,0.001) more than players inthe sand group. In contrast, players in the sand groupexperienced less muscle soreness than those in the grassgroup (p,0.001).Conclusions: Plyometric training on sand improved bothjumping and sprinting ability and induced less musclesoreness. A grass surface seems to be superior inenhancing CMJ performance while the sand surfaceshowed a greater improvement in SJ. Therefore,plyometric training on different surfaces may beassociated with different training-induced effects on someneuromuscular factors related to the efficiency of thestretch-shortening cycle.

Soccer is a high-intensity, intermittent activityduring which players are required to repeatedlyperform striding, turning, sprinting and jumping,which place considerable demands on the neuro-muscular system.1 2 Therefore, the ability of thelower limb muscles to produce power is animportant fitness component for soccer players.3

Plyometric training has been shown to be aneffective method for the improvement of bothsprinting and jumping ability4–6 and it has also beenreported to improve running economy,7 jointstability and reduce the severity of knee injuries.8 9

For these reasons, soccer training commonlyincludes this form of muscular power training.

Plyometric training typically consists of stretch-shortening cycle exercises characterized by multi-joint actions, rapid eccentric phases and explosive

concentric muscular contractions potentiated bythe stretch reflex.6 The rapid eccentric muscleactions induce muscle damage and, consequently,muscular soreness and a transient decrease instrength.10 11 Plyometric training is commonlyperformed on firm surfaces (eg grass and wood),but a more recent study has shown that dropjumps on sand induce less muscle damage whencompared to a firm surface.12 Indeed, Miyama andNosaka12 found a greater reduction in maximalisometric force following 100 consecutive dropjumps on a firm compared to sand surface, whilemuscle soreness and creatine kinase concentrationwere lower on sand. However, jumping on sandcauses a lower reuse of elastic energy and energyloss due to feet slipping during the concentricaction.12 13 This might induce different trainingeffects compared to training on a firm surface. Onthe other hand, the lower stress on the muscu-loskeletal system may decrease the risk of injuriesand the overall physical strain of training ses-sions.14 This may be particularly helpful during thepreseason training period where a high incidence ofoveruse injuries has frequently been reported insoccer players.15 16 This could be a consequence offactors such as hard surface17 18 and the hightraining load.19 Furthermore, the lower impact onthe musculoskeletal system induced by plyometrictraining on sand might be useful during rehabilita-tion programmes. According to Mihama andNosaka,12 less muscle soreness and muscle damagedoes not necessarily imply that plyometric trainingon sand is preferable to that on grass. Indeed,possible differences in the training effects inducedby sand or firm surfaces should be taken intoaccount. However, to the best of our knowledge,no studies have examined the effect of plyometrictraining completed on different surfaces.

Therefore, the aim of this study was to comparethe effects of plyometric training on musclesoreness, vertical jump height and sprinting abilitywhen performed on either a sand or grass surface.

METHODS

Study design and randomisationA parallel two-group, randomised, longitudinal(pretest2post-test) design was used. After baselinemeasurements, subjects were randomly allocatedto two intervention groups: plyometric training onsand (sand group) and plyometric training on grass(grass group). Balanced restricted randomizationwas obtained using blocks (block sizes of 4 and 6)with allocation ratio of one-to-one.20 As theindependent variable was training surface, no

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control group was used. Baseline measurements were completed1 week before the commencement of the training period, whilepostintervention measurements were carried out 4 weeks afterthe end of the experimental training period. This taperingduration (4 weeks) was necessary and corresponded to therecovery period needed to maximize and highlight the training-induced changes after 4 weeks of high-intensity plyometrictraining.21 Furthermore, in the current study we adopted thetraining programme used by Luebbers et al.21 to ensureimprovement in jumping ability, with the slight modificationthat set 6 repetitions instead of set 6 distance was used.

ParticipantsForty-four amateur soccer players (mean (SD) body mass 74 kg(7), height 178 cm (7) and age 25 years (4)) were involved in thestudy. Written informed consent was obtained after verbal andwritten explanation of the experimental procedures. The studywas approved by an independent institutional review boardaccording to the guidelines and recommendations for Europeanethics committees. All subjects were familiarized with alltesting procedures prior to the commencement of the studyand had already had experience of plyometric training.

Plyometric training programmeAs the preseason training period lasted 4 weeks, we employed a4-week plyometric training programme similar to that used byLuebbers et al.21 that has been shown to be effective inenhancing lower limb muscular anaerobic power (table 1).Training was completed on a grass pitch or on a 0.2 m-deep drysand surface. The characteristics of the sand (ie grain diameter)resulted in a harder sand surface compared to beach sand. Theplyometric training sessions were completed 3 times a week inaddition to the usual training consisting of generic and specificaerobic interval training and technical-tactical training.Participants were asked to exert a maximal intensity duringall the training sessions. Recovery time between repetitions andsets were 15–30 s and 1–2 min, respectively. During the 4-weekrecovery period corresponding to the start of the competitiveseason, soccer players trained three times a week and competedonce a week in an official match, but no plyometric exercises orsprint training were completed.

Training outcomesJump testsSoccer players completed the jump test sessions after a 15 minwarm-up consisting of low-intensity running, striding and self-administered submaximal jumps performed as practice andspecific additional warm-up. All jumps were performed on aphotocell mat (Optojump, Microgate, Bolzano, Italy) in arandom order with at least five trials of each type of jump test:squat jump (SJ) and countermovement jump (CMJ). All verticaljumps were completed keeping the hands on the iliac crest to

avoid an influence of the upper limbs on jump performance.Players performed the SJ starting from a standing position,bending the knees to 90u, stopping for 3 s, and then jumping ashigh as possible trying to avoid any knee or trunk counter-movement. CMJs were performed starting from a standingposition after which players were asked to jump as high aspossible with a rapid, preparatory downward eccentric action.Jump height was estimated from flight time calculated as thetime between take-off and subsequent landing.22 The eccentricutilization ratio was calculated as CMJ/SJ.23 The best result foreach jump type was used for the analysis.

Sprinting testsThe 10 m and 20 m sprint tests were performed 30 min afterthe jump test session and following a further warm-upconsisting of low-intensity running and striding. Two trials ofeach test, with a full recovery between sprints, were completed.Sprint time was recorded with two telemetric photocells(Microgate, Bolzano, Italy) placed at the start and at the finishlines of the two sprint distances. The best sprint time for eachdistance was used for the analysis.

Muscle sorenessMuscle soreness was assessed each day of the training periodand before the post-test testing session using an Italian versionof the 7-point Likert scale of muscle soreness24(table 2). A meanvalue for the training period for each training group wascalculated.

Statistical analysisData are presented as mean (SD). We tested the null hypothesisof no difference between groups in all baseline measures usingmultiple unpaired Student t tests. A two-way mixed analysis ofvariance (ANOVA) was used on each continuous dependentvariable. The independent variables included one betweensubjects factor, plyometric training surface, with two levels(grass and sand), and one within subject factor, time, with twolevels (pretest and post-test). We used these ANOVAs to testthe null hypothesis of no different change over time betweenthe grass and the sand group (plyometric training surface6timeinteraction) and no different change over time in response toplyometric training intervention. When a significant F valuewas found, Bonferroni’s post hoc test was applied. Effect sizes(g2) were also calculated, and values of 0.01, 0.06 and above 0.15were considered small, medium and large, respectively.25 Thelevel of statistical significance was set at p,0.05.

RESULTSThirty-seven players completed the study. No differences werefound between groups in baseline values. No significantinteractions were found for the 10 m sprint time and 20 msprint time, while significant interactions were found for CMJ

Table 1 Four-week plyometric training programme* modified fromLuebbers et al21

Exercise

Number of sets (number of repetitions)

Week 1 Week 2 Week 3 Week 4

Vertical jumping 15 (10) 20 (10) 25 (10) 25 (10)

Bounding 3 (10) 4 (10) 5 (10) 5 (10)

Broad jumping 5 (8) 5 (10) 7 (10) 8 (10)

Drop jump 3 (5) 5 (9) 6 (15) 6 (15)

*See text for details.

Table 2 Likert scale of muscle soreness from Vickers24

Value Description

0 A complete absence of soreness

1 A light pain felt only when touched/a vague ache

2 A moderate pain felt only when touched/a slight persistent pain

3 A light pain when walking up or down stairs

4 A light pain when walking on a flat surface/painful

5 A moderate pain, stiffness or weakness when walking/very painful

6 A severe pain that limits my ability to move

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and eccentric utilization ratio (table 3). The interaction for SJwas close to significance (p = 0.08; medium effect size). Indeed,SJ improved by 3.4 cm in the sand group (90% CI 1.2 to 5.6 cm),while in the grass group it increased by 1.8 cm (90% CI 0 to3.6 cm). The main effect for time was significant for the 10 msprint time, which decreased from pretest (1.88 s (SD 0.09)) topost-test (1.81 s (SD 0.09)) by 0.07 s (90% CI 20.10 to 20.04 s).Similarly, the main effect for time was significant for the 20 msprint time, which decreased from pretest (3.21 s (SD 0.12)) topost-test (3.12 s (SD 0.10)) by 0.09 s (90% CI 20.13 to 20.05 s).Post hoc analysis for CMJ showed an increase of 5.5 cm (90% CI2.3 to 8.2 cm) in the grass group and 2.4 cm (90% CI 20.27 to5.0 cm) in the sand group. The main effect for time wassignificant for SJ height, which increased from 34.2 cm (SD 3.9)to 36.8 cm (SD 3.8) during the intervention period (2.6 cm: 90%CI 1.3 to 4.0 cm).

The changes in muscle soreness during the training periodwere similar between groups. Indeed, no significant plyometrictraining surface 6 time interaction was found for musclesoreness as measured by the Likert scale during the 4-weektraining period (p = 0.28; g2 = 0.03), while the main effect fortime was significant (p,0.0001; g2 = 0.48). However, theaverage value calculated for the whole training period of thesand group was lower than that of the grass group (ie significantbetween-subjects effect, p,0.001) (fig 1). This indicates that themuscle soreness experienced by the sand group was system-atically lower than that of the grass group. Muscle sorenessvalues corresponding to the day of the post-training testing

session were similar between groups (0.4 (SD 0.6) for the sandgroup vs 0.6 (SD 0.5) for the grass group).

DISCUSSIONThe main findings in the present study were that plyometrictraining performed on sand gives a similar improvement insprint performance, a different training response on jumpingabilities, but induced less muscle soreness than when performedon grass. These data suggest different training-induced effectson neuromuscular factors related to the stretch-shortening cycledependent upon the surface on which the training is performed.

Plyometric training is commonly performed on firm surfacesto create high stretch loads that will store elastic energy,increase the precontraction activation state and greatly activatethe stretch-reflex to produce explosive concentric musclecontractions.6 These characteristics have been suggested toinduce neuromuscular adaptations leading to improvement inperformances requiring the stretch-shortening cycle such asjumping and sprinting.4 6 26 However, this training modalityplaces considerable demands on the leg muscles, Achilles andpatellar tendons, and muscle2tendon unit.12 27 These factors, inaddition to the fatigue caused by increased training load duringthe preseason training period, may increase the injury risk.19 28 29

On the other hand, the use of a softer surface may be useful inthe rehabilitation phase or during intensified training periods toreduce the stress on the musculoskeletal system and hence therisk of injuries or re-injuries. The results of the present studyshowed that during the 4-week training period, the sand group

Table 3 Effect of 4 weeks of plyometric training on a grass versus sand surface on sprint and vertical jump performance

Grass group Sand group

Interaction

Effect size (g2)

Pretest(mean (SD))

Post-test(mean (SD))

Pretest(mean (SD))

Post-test(mean (SD)) Value Descriptor

10 m sprint (s) 1.89 (0.08) 1.82 (0.07) 1.88 (0.09) 1.80 (0.11)* p = 0.88 0.001 Small

20 m sprint (s) 3.23 (0.08) 3.14 (0.09) 3.19 (0.15) 3.11 (0.11)* p = 0.87 0.001 Small

SJ (cm) 34.0 (3.2) 35.8 (3.3) 34.3 (4.5) 37.8 (4.0)* p = 0.08 0.085 Medium

CMJ (cm) 37.8 (3.6) 43.3 (5.9){ 37.2 (3.6) 39.6 (5.5){ p = 0.03 0.124 Medium

CMJ/SJ 1.11 (0.02) 1.21 (0.03) 1.09 (0.02) 1.05 (0.03)* p = 0.005 0.206 Large

*p,0.001, significant main effect for time.{p,0.01, significant difference from pretest (post hoc analysis after ANOVA).{p,0.001, significant difference from pretest (post hoc analysis after ANOVA).CMJ, countermovement jump; CMJ/SJ, eccentric utilization ratio; DJ, drop jump; SJ, squat jump.

Figure 1 Mean weekly values of musclesoreness during the training period forgrass and sand group, and box-whiskerplot representing the mean values for thewhole training period of the two groups(black squares indicate the 5th and 95thpercentiles). *p,0.001, significant maineffect for time; {p,0.001, significantbetween-subjects effect.

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experienced less muscle soreness. Indeed, using the 7-pointLikert scale, the mean value of muscle soreness of the sandgroup was 0.6 lower than the grass group, with individualmaximal values reached in the first week of training of 5 for thesand group and 6 for the grass group. This result confirms thefinding of Miyama and Nosaka12 who showed that plyometrictraining on sand induced less muscle soreness than jumping on afirm surface. Muscle soreness decreased during the trainingperiod for both groups indicating a repeated bout effect despitethe fact that the exercise intensity was increased each week.Although we did not measure other indirect indices of muscledamage, it is likely that the lower muscle soreness was related toless muscle damage and hence less stress on the musculoskeletalsystem as shown by Mihama and Nosaka.12

A significant effect of training surface was found in somejump characteristics related to the efficiency of the stretch-shortening cycle. During SJ, no prestretch actions are involvedand therefore this type of jump test is commonly considered ameasure of pure concentric strength.23 30 Although both groupsincreased their SJ performance, a trend for greater improvementin the sand group was found. This may suggest that jumping onsand required a more intense concentric push-off phase,probably to compensate for the degradation of elastic energypotentiation caused by sand absorption and the difficulty of theankle to push along the vertical axis.13 On the other hand,performance in CMJ is enhanced by the effects of prestretchaugmentation.31 32 The grass group showed a greater improve-ment in CMJ than the sand group. Similarly, the eccentricutilization ratio (CMJ/SJ) increased in the grass group only. Asthis index has been suggested to provide information on slowstretch-shortening cycle performance,23 this finding seems toindicate a greater effectiveness of plyometric training on grasson performances requiring slow stretch-shortening cycleactions. It has previously been reported that vertical jumpheights during SJ and CMJ are lower on sand than on a firmsurface.12 13 33 The lower jump height on sand has beenassociated with a lower reuse of the stored elastic energy aswell as energy loss due to feet slipping during the push-offphase.12 13 34 These characteristics may potentially reduce themechanical load on the musculoskeletal system and hence thetraining effect on the efficiency of the muscle2tendoncomplex.35 However, the other performances requiring faststretch-shortening cycle actions increased similarly in bothgroups. Indeed, no surface effects were found for sprintperformance. The positive effects of plyometric training onsprint ability has already been reported.4 However, as in thepresent investigation no control group was included, we cannotexclude that the sprint improvements found after plyometrictraining were determined by other physical training exercisesused during the study period (eg generic and specific aerobicinterval training and technical and tactical training).Nevertheless, the two plyometric training surfaces did notaffect the improvement in sprint performance for distancestypical of soccer.2 The improvement in sprint performancefound in this study may be very useful for soccer playersbecause it would correspond to an advantage during sprint-duels that may allow players to reach the ball before theopponent.1 However, future studies using biomechanicalanalyses are needed to investigate more fully the underlyingmechanisms behind the effect of plyometric training surface onjumping ability.

Overall, the results of this study suggest that plyometrictraining on sand may be incorporated into soccer training whena reduction of stress on the musculoskeletal system is desired,

for example, during a period of intensified training or duringsport rehabilitation (eg before the inclusion of plyometricexercises on firm surfaces36). Indeed, no effects of plyometrictraining surface were found in performances requiring faststretch-shortening cycle actions such as sprinting ability.However, a grass surface seems to be superior in enhancingCMJ performance while a sand surface appears to induce greaterimprovement in SJ. Therefore, the results of the present studysuggest that plyometric training on different surfaces may beassociated with different training-induced effects on neuromus-cular factors related to the efficiency of the stretch-shorteningcycle. This must be taken into account when using plyometrictraining on sand in athletes. Although the preliminary results ofthe present study suggest that plyometric training on sand maybe potentially useful and effective for soccer players, thebiomechanical characteristics of jumping on sand may increasethe risk of overuse injuries to the low back.13 37 38 Therefore,future studies should also investigate the safety of prolongedplyometric training on sand.

Acknowledgements: We are grateful to the soccer players who participated in thestudy. We thank Fabio Barducci and Duccio Ferrari Bravo for their valuable support.The authors would also like to thank David Bishop and Nicola Maffiuletti for their expertassistance.

Competing interests: None declared.

REFERENCES1. Stolen T, Chamari K, Castagna C, et al. Physiology of soccer: an update. Sports Med

2005;35:501–36.2. Spencer M, Bishop D, Dawson B, et al. Physiological and metabolic responses of

repeated-sprint activities: specific to field-based team sports. Sports Med2005;35:1025–44.

3. Wisloff U, Castagna C, Helgerud J, et al. Strong correlation of maximal squatstrength with sprint performance and vertical jump height in elite soccer players.Br J Sports Med 2004;38:285–8.

4. Rimmer E, Sleivert G. Effects of a plyometric intervention program on sprintperformance. J Strength Cond Res 2000;14:295–301.

5. Diallo O, Dore E, Duche P, et al. Effects of plyometric training followed by a reducedtraining programme on physical performance in prepubescent soccer players. J SportsMed Phys Fitness 2001;41:342–8.

6. Bobbert MF. Drop jumping as a training method for jumping ability. Sports Med1990;9:7–22.

7. Turner AM, Owings M, Schwane JA. Improvement in running economy after6 weeks of plyometric training. J Strength Cond Res 2003;17:60–7.

8. Heidt RS Jr, Sweeterman LM, Carlonas RL, et al. Avoidance of soccer injuries withpreseason conditioning. Am J Sports Med 2000;28:659–62.

What is already known on this topic

c Plyometric training on sand induces less muscle soreness anddamage compared with a firm surface.

c No previous studies have investigated the effect of plyometrictraining on jumping and sprinting ability performed on sandcompared to grass.

What this study adds

c Both sand and grass surfaces resulted in positive changes insprinting and jumping ability.

c Plyometric training on sand produced less muscle sorenessthan that on grass during the whole training period.

c A grass surface was superior in enhancing CMJ performancewhile a sand surface tended to induce greater improvement inSJ.

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9. Chimera NJ, Swanik KA, Swanik CB, et al. Effects of plyometric training on muscle-activation strategies and performance in female athletes. J Athl Train 2004;39:24–31.

10. Marginson V, Rowlands AV, Gleeson NP, et al. Comparison of the symptoms ofexercise-induced muscle damage after an initial and repeated bout of plyometricexercise in men and boys. J Appl Physiol 2005;99:1174–81.

11. McHugh MP. Recent advances in the understanding of the repeated bout effect: theprotective effect against muscle damage from a single bout of eccentric exercise.Scand J Med Sci Sports 2003;13:88–97.

12. Miyama M, Nosaka K. Influence of surface on muscle damage and soreness inducedby consecutive drop jumps. J Strength Cond Res 2004;18:206–11.

13. Giatsis G, Kollias I, Panoutsakopoulos V, et al. Biomechanical differences in elitebeach-volleyball players in vertical squat jump on rigid and sand surface. SportsBiomech 2004;3:145–58.

14. Almeida SA, Williams KM, Shaffer RA, et al. Epidemiological patterns ofmusculoskeletal injuries and physical training. Med Sci Sports Exerc 1999;31:1176–82.

15. Woods C, Hawkins R, Hulse M, et al. The Football Association Medical ResearchProgramme: an audit of injuries in professional football—analysis of preseasoninjuries. Br J Sports Med 2002;36:436–41; discussion 441.

16. Walden M, Hagglund M, Ekstrand J. Injuries in Swedish elite football—aprospective study on injury definitions, risk for injury and injury pattern during 2001.Scand J Med Sci Sports 2005;15:118–25.

17. Inklaar H. Soccer injuries. II: aetiology and prevention. Sports Med 1994;18:81–93.18. Ekstrand J, Nigg BM. Surface-related injuries in soccer. Sports Med 1989;8:56–62.19. Ekstrand J, Hilding J. The incidence and differential diagnosis of acute groin injuries

in male soccer players. Scand J Med Sci Sports 1999;9:98–103.20. Schulz KF, Grimes DA. Generation of allocation sequences in randomised trials:

chance, not choice. Lancet 2002;359:515–9.21. Luebbers PE, Potteiger JA, Hulver MW, et al. Effects of plyometric training and

recovery on vertical jump performance and anaerobic power. J Strength Cond Res2003;17:704–9.

22. Bagger M, Petersen PH, Pedersen PK. Biological variation in variables associatedwith exercise training. Int J Sports Med 2003;24:433–40.

23. McGuigan MR, Doyle TL, Newton M, et al. Eccentric utilization ratio: effect of sportand phase of training. J Strength Cond Res 2006;20:992–5.

24. Vickers AJ. Time course of muscle soreness following different types of exercise.BMC Musculoskelet Disord 2001;2:5.

25. Ary D, Cheser Jacobs L, Razavieh A, et al. Introduction to research in education.Belmont, CA: Wadsworth, 2006:155–7.

26. Young WB, Wilson GJ, Byrne C. A comparison of drop jump training methods:effects on leg extensor strength qualities and jumping performance. Int J Sports Med1999;20:295–303.

27. Jamurtas AZ, Fatouros IG, Buckenmeyer P, et al. Effects of plyometric exercise onmuscle soreness and plasma creatine kinase levels and its comparison with eccentricand concentric exercise. J Strength Cond Res 2000;14:68–74.

28. Moran KA, Marshall BM. Effect of fatigue on tibial impact accelerations and kneekinematics in drop jumps. Med Sci Sports Exerc 2006;38:1836–42.

29. Zatsiorsky VM. Science and practice of strength training. Champaign, IL: HumanKinetics, 1995:44.

30. Hasson CJ, Dugan EL, Doyle TL, et al. Neuromechanical strategies employed toincrease jump height during the initiation of the squat jump. J Electromyogr Kinesiol2004;14:515–21.

31. Cavagna GA, Dusman B, Margaria R. Positive work done by a previously stretchedmuscle. J Appl Physiol 1968;24:21–32.

32. Kubo K, Kawakami Y, Fukunaga T. Influence of elastic properties of tendon structureson jump performance in humans. J Appl Physiol 1999;87:2090–6.

33. Bishop D. A comparison between land and sand-based tests for beach volleyballassessment. J Sports Med Phys Fitness 2003;43:418–23.

34. Lejeune TM, Willems PA, Heglund NC. Mechanics and energetics of humanlocomotion on sand. J Exp Biol 1998;201:2071–80.

35. Kjaer M. Role of extracellular matrix in adaptation of tendon and skeletal muscle tomechanical loading. Physiol Rev 2004;84:649–98.

36. Chmielewski TL, Myer GD, Kauffman D, et al. Plyometric exercise in therehabilitation of athletes: physiological responses and clinical application. J OrthopSports Phys Ther 2006;36:308–19.

37. Bahr R, Reeser JC. Injuries among world-class professional beach volleyball players.The Federation Internationale de Volleyball beach volleyball injury study. Am J SportsMed 2003;31:119–25.

38. Lian O, Refsnes PE, Engebretsen L, et al. Performance characteristics of volleyballplayers with patellar tendinopathy. Am J Sports Med 2003;31:408–13.

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