effects of in-season short-term plyometric training

EFFECTS OF IN-SEASON SHORT-TERM PLYOMETRIC TRAINING PROGRAM ON SPRINT AND JUMP PERFORMANCE OF YOUNG MALE TRACK ATHLETES MOHAMED SOUHAIEL CHELLY,1,2 SOUHAIL HERMASSI,2

AND

ROY J. SHEPHARD3

1

Research Unit, Sport Performance & Health, High Institute of Sport and Physical Education, Ksar Saıˆd, University of “La Manouba,” Tunis, Tunisia; 2Department of Biological Sciences Applied to Physical Activities and Sport, High Institute of Sport and Physical Education, Ksar Said, University of “La Manouba,” Tunis, Tunisia; and 3Faculty of Kinesiology and Physical Education, University of Toronto, Toronto, Ontario, Canada ABSTRACT Chelly, MS, Hermassi, S, and Shephard, RJ. Effects of inseason short-term plyometric training program on sprint and jump performance of young male track athletes. J Strength Cond Res 29(8): 2128–2136, 2015—We studied the effect of supplementing normal in-season training by a 10-week lower limb plyometric training program (hurdle and depth jumping), examining measures of competitive potential (peak power output [PP], sprint running velocity, squat jump [SJ], countermovement jump [CMJ], drop jump [DJ], and lower limb muscle volume). The subjects (27 male track athletes, aged 11.9 6 1.0 years; body mass: 39.1 6 6.1 kg; height: 1.56 6 0.02 m; body fat: 12.8 6 4.4%) were randomly assigned between a control (normal training) group (C; n = 13) and an experimental group (E; n = 14) who also performed plyometric training 3 times per week. A force-velocity ergometer test determined PP and SJ, and an Optojump apparatus evaluated CMJ height and DJ (height and power). A multiple-5-bound test assessed horizontal jumping, and video-camera analyses over a 40-m sprint yielded velocities for the first step (VS), the first 5 m (V5m), and between 35 and 40 m (Vmax). Leg muscle volume was estimated anthropometrically. Experimental group showed gains relative to C in SJ height (p , 0.001); CMJ height (p , 0.01); DJ height and power relative to body mass (p , 0.01 for both); and all sprint velocities (p , 0.01 for VS and V5m, p # 0.05 for Vmax). There was also a significant increase (p , 0.01) in thigh muscle volume, but leg muscle volume, thigh cross-sectional area, and PP remained unchanged. We conclude that adding plyometric training improved important components of athletic

Address correspondence to Mohamed Souhaiel Chelly, csouhaiel@ yahoo.fr. 29(8)/2128–2136 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association

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performance relative to standard in-season training in young runners.

KEY WORDS children, depth jump, running velocity, stretchshortening cycle, jumping, force-velocity test

INTRODUCTION

P

lyometric training can enhance explosive contractions in both prepubertal (22) and pubertal (24) populations. Such a regimen is a natural preparation for many sports, with its emphasis on jumping, throwing, hopping, and skipping, and it is particularly appropriate in runners, where there is a need to develop explosive movements, such as sprint departure, sprint acceleration, and maximal running velocity. Concerns regarding the safety of plyometric training for young athletes are also minimal if a proper technique is combined with appropriate progression and close supervision of participants (20). Indeed, many plyometric training routines mimic movements that are encountered in the normal play of children, and no specific strength level is required to begin such programs (24). Plyometric exercise involves stretching the muscle immediately before making a rapid concentric contraction. The combined action is commonly called a stretch-shortening cycle. Such contractions are often made during the different phases of running. Although gains of maximal strength are similar with traditional strength and plyometric training, the latter approach seems to induce greater gains in muscle power (34). Currently available findings regarding the impact of plyometrics on running performance are contradictory. Some studies have suggested that plyometric training can improve vertical jump height or power without any increase in sprint running performance (23,32). In contrast, other investigations have found that plyometric training improved sprint performance (10,28). Chelly et al. (5) demonstrated that an 8-week plyometric training program yielded significant increases in 3 sprint running velocities: during the first step (VS); the first 5 m (V5m), and between 35 and 40 m (Vmax). Recently, Chelly et al. (6) also observed improvements in the muscular

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Journal of Strength and Conditioning Research performance of both the upper and the lower limbs after an 8week upper- and lower-body plyometric training program, with particularly noteworthy gains in leg peak power, jump height, and all 3 sprint velocities (VS, V5m, and Vmax). All of these various investigations were conducted either in adolescents or in adults. Current evidence suggests a large effect of plyometric training on jumping ability (12,19,26) and a more variable effect on running speed, with the greatest improvement seen over short distances (12,13,19,26). There is also some evidence of a beneficial effect on the power of the lower limbs (16). Less is known about the benefits of plyometric training in young children (17). There are distinctive differences between prepubertal, pubertal, and adult subjects in terms of muscle mass, muscle fiber distribution, and neuromuscular activity, and corresponding differences in training response might be anticipated. Thus, there is a need to examine the effects of plyometric training in young children. The objective of this study was to determine whether the integration of a short-term plyometric training program into a regular in-season training program for youth athletes would enhance leg power, explosive movements such as jumping (jump height and jump power) and sprint performance relative to standard conditioning procedures. Based on existing information concerning the efficacy of plyometric training in older individuals (23), we added a combined lower limb (hurdle and depth jump) program to the normal in-season regimen for 10 weeks. We hypothesized that these changes would

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augment the muscular power of the lower limbs, with increases of jump height and sprint velocity relative to control players who maintained their standard in-season regimen.

METHODS Experimental Approach to the Problem

This study examined whether a 10-week biweekly in-season plyometric program would enhance selected aspects of performance in young athletes relative to their peers who continued to follow their customary in-season training regimen. A group of 27 well-trained young athletes (see Procedures and Evaluation for details of training) volunteered to be assigned randomly between a plyometric training group (E; n = 14) and a control group (standard in-season regimen) (C; n = 13). Two weeks before definitive testing, 2 familiarization sessions were held. Definitive measurements began when all participants were 4 months into the competitive season. Data were collected before modification of training and after completing the 10-week trial. The test protocol included cycle ergometer force-velocity tests of peak power for the lower limbs, maximal pedaling velocity (V0), and maximal braking force (F0); assessments of vertical jump height by countermovement jump (CMJ) and squat jumps (SJs), lower limb power by Drop Jump (DJ); and a sprint test that evaluated velocities during the first step (VS), 5 m (V5m), and at maximal running speed (Vmax). The muscle volumes of the lower limbs were estimated

TABLE 1. Training programs for plyometric (E) and control (C) groups.* E (n = 14) Week 1 2 3 4 5 6 7 8 9 10

Exercise 10-m sprint 0.3-m hurdle 10-m sprint 0.3-m hurdle 20-m sprint 0.3-m hurdle 20-m sprint 0.4-m hurdle 30-m sprint 0.4-m hurdle 30-m sprint 0.4-m hurdle 40-m sprint 0.3-m DJ 40-m sprint 0.3-m DJ 50-m sprint 0.3-m DJ 50-m sprint 0.3-m DJ

jump jump jump jump jump jump

C (n = 13)

Reps

Sets

Exercise

Reps

Sets

3 10 3 10 3 10 3 10 3 10 3 10 3 10 3 10 3 10 3 10

3 5 4 7 3 10 4 5 3 7 4 10 3 4 4 4 3 4 4 4

10-m sprint

3

3

10-m sprint

3

4

20-m sprint

3

3

20-m sprint

3

4

30-m sprint

3

3

30-m sprint

3

4

40-m sprint

3

3

40-m sprint

3

4

50-m sprint

3

3

50-m sprint

3

4

*DJ = drop jump.

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Plyometric Training in Young Male Runners regulations. Informed consent was gained from participants TABLE 2. Intraclass correlation coefficients showing acceptable reliability of track after receiving both a verbal running velocities and jump tests.* and a written explanation of ICC 95% CI the experimental protocol and its potential risks and benefits. Track running velocity They were assured that they VS (m$s21) 0.80 0.52–0.91 could withdraw from the trial V5 (m$s21) 0.85 0.55–0.95 Vmax (m$s21) 0.85 0.62–0.94 without penalty at any time. Jump tests Parental/guardian consent was Squat jump height (m) 0.98 0.96–0.99 also obtained for all particiCountermovement jump height (m) 0.98 0.95–0.99 pants involved in this investiDrop jump height (m$s21) 0.96 0.92–0.98 gation. Twenty-seven male Multiple-5-bounds test 0.96 0.94–0.98 young athletes (age ranges 10 *ICC = intraclass correlation coefficient; CI = confidence interval; VS = first step; V5 = to 14 years old) were drawn first 5 m. from a single regional training center; all were preparing for track events, such as the 100-m anthropometrically from external dimensions, the thickness sprint or running endurance, such as 3,000 m events. Their of overlying fat, and bone dimensions. mean experience of training was 3.4 6 0.6 years. All were Initial and final test measurements were made at the same examined by the regional center physician, with a particular time of day, and under the same experimental conditions, at focus on conditions that might preclude plyometric training, least 3 days after the most recent competition. Participants and all were found to be in good health. Subjects were ranmaintained their normal intake of food and fluids, but they domly assigned between the 2 groups, which were well abstained from physical exercise for 1 day, drank no caffeinematched in terms of their initial characteristics: (E; n = 14; containing beverages for 4 hours, and ate no food for 2 hours age: 11.7 6 1.0 years; body mass: 43.0 6 16.6 kg; height: 1.58 before testing. Verbal encouragement ensured maximal effort 6 0.2 m; body fat: 12.4 6 4.6%; C; n = 13; age: 12.1 6 1.0 throughout all tests. years; body mass: 38.1 6 4.1 kg; height: 1.54 6 0.03 m; body fat: 13.3 6 4.3%). Inclusion criteria ensured the participants Subjects were male, free of any cardiovascular or musculoskeletal All procedures were approved by the University Institutional disease, and in pubertal stage 1 or 2 as judged by pubic hair Review Committee for the ethical use of human subjects, growth and genital development (31). Drawings of the 5 according to current national and international laws and stages of pubic hair and genital development were given to each participant and their parents for joint assessment of sexual maturity status. Procedures and Evaluation

Figure 1. Training-associated changes in velocities during the first step (VS), over 5 m (V5m), and between 25 and 30 m (Vmax). E = experimental group receiving plyometric training; C = control group continuing standard regimen. *Nonpaired Student t-test significantly different at p # 0.05. **Nonpaired Student t-test significantly different at p # 0.01.

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The study was performed from January to March (a 10-week period) in the middle of the competitive season. Before their competitive season (September), all subjects had engaged in a light resistance training program for both the upper and the lower limbs; twice weekly sessions included exercises that used the body weight as a resistance. During the competitive season, which began in November and finished in May, subjects trained 4 times a week and participated in 1 official competition per

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TABLE 3. Comparison of both vertical and horizontal jump test performance between plyometric training group (E) and control group (C) before and after 10-week trial.*† Test SJ height (m) CMJ height (m) DJ height (m) DJ power (W) DJ power (W$kg21) Multiple-5-bound test

Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post

E 0.21 0.24 0.23 0.25 0.22 0.25 1,143 1,350 28.5 33.3 8.2 8.9

6 6 6 6 6 6 6 6 6 6 6 6

C 2.8 0.03z 0.03 0.03§ 0.03 0.02§ 339 356§ 5.2 4.6§ 0.6 0.7z

0.20 0.21 0.21 0.22 0.20 0.20 946 975 24.8 25.5 8.0 8.1

6 6 6 6 6 6 6 6 6 6 6 6

0.02 0.02 0.03 0.03 0.02 0.02 152 194 4.2 4.3 0.6 0.6

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a well-trained state. However, none of the subjects had previously performed plyometric training with hurdle or DJ. Evaluations of muscle function were completed in a fixed order over 2 consecutive days. Care was taken to ensure that those undertaking plyometric training were tested 5–9 days after their last plyometric session, to allow adequate recovery from the acute effects of training. Testing Schedule

Subjects were familiarized with circuit training for 2 weeks *SJ = squat jump; CMJ = countermovement jump; DJ = drop jump; MB5 = multiple-5before beginning either the inibound test. †A 2-way analysis of variance (group 3 time) assessed the statistical significance of tial measurements or formal training-related effects. training. Testing was integrated zp # 0.001. §p # 0.01. into the weekly training schedule. A standardized battery of warm-up exercises was performed before maximal efforts. month. The standard training sessions, lasting 50 minutes, On the first test day, subjects performed the SJ, CMJ, and DJ followed by the force-velocity test. Anthropometrical assessincluded skill activities (agility and coordination exercises) at ment, the multiple-5-bound test (MB5) and sprinting, were various intensities, continuous or intermittent running at undertaken on day 2. various fractions of maximal velocity, sprinting over distances of 10, 15, and 20 m, and flexibility exercises. All subjects also engaged once weekly in school physical education sessions; these lasted for 40 minutes and consisted mainly of ball games. All subjects, thus, began the trial in

Day 1

Squat Jump, Countermovement Jump, and Drop Jump. The jump heights of the SJ, CMJ, and DJ were determined using Optojump photoelectric cells (Microgate, Bolzano, Italy). Subjects began the SJ at a knee angle of 908, avoiding any downward movement, and performed a vertical jump by TABLE 4. Force-velocity data for lower limbs compared between plyometric pushing upward, keeping their training group (E) and control group (C) before and after 10-week trial. legs straight throughout. The Test E C CMJ began from an upright position, with the subject makAbsolute power (W) Pre 284 6 79 238 6 61 ing a rapid downward movePost 310 6 84 262 6 69 21 ment to a knee angle of ;908 Pre 7.1 6 1.1 6.2 6 1.1 Power (W$kg ) Post 7.6 6 1.1 6.8 6 1.2 and simultaneously beginning Power (W per total leg muscle volume) Pre 75 6 14 67 6 14 to push off. During the DJ, subPost 71 6 9 67 6 12 jects were asked to rebound Power (W per thigh muscle volume) Pre 99 6 19 93 6 23 from a 0.3-m height box, Post 97 6 13 93 6 22 21 22 immediately making a vertical Power (W$CSA ) (W$cm ) Pre 2.7 6 0.5 2.4 6 0.5 Post 2.6 6 0.4 2.4 6 0.5 jump. During all 3 jumps, subMaximal pedaling velocity (rpm) Pre 160 6 21 181 6 21 jects kept their hands on their Post 158 6 18 171 6 19 hips and jumped as high as Maximal force (N) Pre 66.7 6 13.9 59.7 6 9.5 possible. One minute of rest Post 64.4 6 13.9 58.9 6 10.5 was allowed between trials, with the highest of 3 jumps on each test being used in VOLUME 29 | NUMBER 8 | AUGUST 2015 |

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Plyometric Training in Young Male Runners subsequent analyses. The Optojump photoelectric cells consist of 2 parallel bars (1 receiver and 1 transmitter unit); these were placed approximately 1 m apart and parallel to each other. The transmitter contains 32 light-emitting diodes, positioned 3 mm above ground level at 31.25-mm intervals. The Optojump bars were connected to a personal computer, and proprietary software (Optojump software; version 3.01.0001) that allowed quantification of jump heights. The Optojump system measured the flight time of vertical jumps with an accuracy of 1 millisecond (1 kHz). Jump height (for SJ, CMJ, and DJ) and DJ power expressed relatively to body mass (W$kg21) were then estimated as follows (2):

h¼

Power ¼

gtf2 8

g 2 3tf 3tt 43tc

Day 2

Anthropometry. Circumferences and skinfold thickness at appropriate levels in the thigh, calf, limb lengths, and the breadth of the femoral condyles were measured to estimate the muscle volume of the lower limbs, using standard formulas as described previously (4,5). The overall percentage of body fat was estimated from the biceps, triceps, subscapular, and suprailiac skinfolds, using the equation (36):

% Body fat ¼ a:log

X

4 folds 2b:;

P where 4 folds is the sum of the 4 skinfolds (in millimeters); a and b are constants dependent on sex and age. The mean thigh cross-sectional area (CSA) was estimated from the maximal and midthigh circumferences according to the following formulae:

;

Circumference ðC Þ ¼ 2p3Radius ðRÞ

where h is jump height; g is gravitational acceleration (9.81 m$s22); tf is flight time; tc is contact time; tt is total time of 1 jump = tf + tc. The Force-Velocity Test. Force-velocity measurements for the lower limbs were performed on a standard Monark cycle ergometer (model 894E; Monark Exercise AB, Vansbro, Sweden), as detailed elsewhere (4). In brief, subjects completed 5 short maximal sprints against consecutive braking forces of 2.5, 5, 7.5, 9, and 11.5% of the subject’s body mass. The instantaneous maximal pedaling velocity during a 7-second all-out sprint was used to calculate the maximal anaerobic power for each of the applied braking forces, and subjects were judged to have reached peak power if an additional load induced a decrease in power output. Measured parameters included the peak power output, maximal braking force, and maximal pedaling velocity (4). For more details of the force-velocity tests, see Chelly et al. (4).

R¼

C : 2p

Multiple-5-Bound Test. The MB5 began from a standing position, with subjects performing 5 forward jumps with alternative left- and right-leg contacts to cover the longest possible distance, measured by a tape to the nearest 5 mm (26).

Sprint Running. After familiarization, subjects made a maximal 40-m sprint on an outdoor tartan surface. Body displacement was filmed by 2 cameras (Sony Handycam; DCR-PC105E, Tokyo, Japan; 25 frames per second) placed at a distance of 10 m perpendicular to the running lane. The first camera filmed the individual over the first 5 m, and the second camera monitored the sprint between 35 and 40 m. Participants performed 2 trials, separated by an interval of 5 minutes. Appropriate software (Regavi & Regressi; Micrelec, Coulommiers, France) converted measurements of hip displacement to the correspondTABLE 5. Comparison of lower limb muscle volumes between plyometric training ing velocities (VS, V5m, and group (E) and control group (C) before and after 10-week trial.*† Vmax). The reliability of the camera and the data processing softTest E C ware has been reported Total leg muscle volume (L) Pre 3.8 6 0.8 3.6 6 0.5 previously (4).

Thigh muscle volume (L) Mean thigh CSA (cm2)

Post Pre Post Pre post

4.3 2.5 2.9 103 116

6 6 6 6 6

1.0 0.8 1.0 16 19z

3.9 2.4 2.6 98 109

6 6 6 6 6

0.6 0.7 0.8 14 17

*CSA = cross-sectional area. †A 2-way analysis of variance (group 3 time) assessed the statistical significance of

training-related effects. zp # 0.01.

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Plyometric Training Program

All subjects avoided any training other than that associated with the regional training center throughout the study. Details of the standard training session and the added plyometric training are given in Table 1. Every Monday,

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dropping from a 0.3-m box, with an interval of 5 seconds between rebounds (23). Statistical Analyses

All statistical analyses were performed using the SPSS 19.0 program for Windows (SPSS, Inc, Chicago, IL, USA). Mean and SD values were calculated, using standard statistical methods. Normality of all variables was tested, using the Kolmogorov-Smirnov test procedure. Levene’s test was used to determine homogeneity of variance. Training-related effects were assessed by 2-way Figure 2. Training-associated changes in vertical jump height assessed through squat jump (SJ), analyses of variance with countermovement jump (CMJ), drop jump (DJ), and multiple-5-bound test (MB5) over the experimental period. repeated measures (group 3 E = experimental group receiving plyometric training; C = control group continuing standard regimen. *Nonpaired Student t-test significantly different at p # 0.05. **Nonpaired Student t-test significantly different at p # 0.01. time). If a significant F value ***Nonpaired Student t-test significantly different at p # 0.001. was observed, Scheffe´’s post hoc procedure was applied to locate pairwise differences. Wednesday, and Friday for 10 weeks, the experimental subStatistical power values were calculated and ranged between jects added a period of plyometric training before beginning 0.32 and 1. Percentage changes were calculated as their habitual training session with the control group. Plyo([posttraining value 2 pretraining value]/pretraining value) 3 metric sessions began with a 15-minute warm-up and lasted 100. Comparisons between initial and final tests used nonfor some 20 minutes. Subjects were instructed to perform all paired Student t-tests. Pearson product-moment correlaexercises with maximal effort. Each jump was performed to tions determined relationships between braking force and reach the maximal possible height, with minimal ground conpedaling velocity. The reliabilities of sprint velocities (VS, V5m, and Vmax) and vertical jump height (SJ, CMJ, and DJ) tact time (bouncing jump). Both hurdle and DJ were permeasurements were assessed using intraclass correlation formed with small angular knee movements; the ground coefficients (33); reliability was acceptable for all of our was touched with the balls of the feet only, thereby specifimeasurements of track velocity and jumping tests (Table 2). cally stressing the calf muscles (23). Each set of hurdle jumps We accepted p # 0.05 as our criterion of statistical signifconsisted of 10 continuous jumps over hurdles spaced at 1-m icance, whether a positive or a negative difference was seen intervals. Each set of DJ comprised 10 maximal rebounds after (i.e., a 2-tailed test was adopted).

RESULTS

TABLE 6. Comparison of sprint running velocity between plyometric training group (E) and control group (C) before and after 10-week trial.*† Test VS (m$s21) V5m (m$s21) Vmax (m$s21)

Pre Post Pre Post Pre post

E 1.2 1.4 2.0 2.2 5.3 5.5

6 6 6 6 6 6

C 0.4 0.4z 0.5 0.5z 0.7 0.7§

1.4 1.5 2.3 2.4 5.0 5.1

6 6 6 6 6 6

0.4 0.5 0.6 0.5 0.5 0.5

*VS = first step; V5 = first 5 m. †A 2-way analysis of variance (group 3 time) assessed the statistical significance of

training-related effects. zp # 0.01. §p # 0.05.

Statistical analyses revealed that the experimental group increased all vertical (SJ and CMJ) and horizontal (MB5) jump performances relative to controls (Figure 1 and Table 3). In contrast, the force-velocity test parameters showed no intergroup differences after 10 weeks of plyometric training (Table 4). Neither group showed any significant changes in total leg and thigh muscle volumes. However, the mean VOLUME 29 | NUMBER 8 | AUGUST 2015 |

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Plyometric Training in Young Male Runners thigh CSA was statistically increased in the experimental group after the plyometric program (Table 5). Sprint velocities over short distances, such as VS and V5m, were markedly improved (p , 0.01), whereas Vmax showed smaller gains (p # 0.05) (Figure 2 and Table 6).

DISCUSSION This study indicates that 10 weeks of plyometric training enhanced the performance of early pubertal male runners relative to control subjects engaged in a standard conditioning program; the experimental group showed significant gains with respect to sprint velocities (VS, V5m, and Vmax; Table 6), vertical jump height and power (SJ, CMJ, and DJ), horizontal jump length (MB5; Table 3), and mean thigh CSA (Table 5). However, total leg and thigh muscle volumes, absolute and relative leg muscle power, maximal pedaling velocity, and maximal force remained unchanged after the training period. Sprinting can be divided into 2 main phases: an acceleration phase that typically continues to 30 m and a phase of maximal velocity reached shortly thereafter (7,25). However, the duration of acceleration depends on numerous factors including sex and performance level. In women of average ability, maximal velocity was observed between 25 and 35 m (29), whereas high-level sprinters can continue accelerating over up to 60 m (11). To the best of our knowledge, there is no information about the duration of the acceleration phase in an untrained prepubertal population. However, we have assumed that for this age group acceleration was complete after 35 m, and that the mean velocity between 35 and 40 m provided a good estimate of maximal velocity. To the best of our knowledge, this is the first investigation that has measured sprint velocities over distances as short distance as VS and V5m in prepubertal boys. All sprint velocities were significantly enhanced after the plyometric training (Table 6). It has generally proven more difficult to enhance initial acceleration than maximal velocity, probably because of the smaller margin for improvement (8,19). However, our investigation shows that it is possible to increase even the initial acceleration (VS) in prepubertal boys with 10 weeks of plyometric training. The improvement over a short distance (V5m) is in agreement with investigators such as Meylan & Malatesta (26), who found a significant decrease in 10-m sprint time after 8 weeks in-season plyometric program in young soccer players. Ingle et al. (16) found a significant decrease in 40-m sprint time after 12-weeks of combined strength and plyometric training, which seems in agreement with our enhancement in Vmax. In contrast, Christou et al. (8) found no improvement of 20- to 30-m sprint times, and Ramirez-Campillo et al. (27) found no significant enhancement of 20-m sprint performance after a 7week plyometric training program. However, we cannot compare our data too closely with these findings, because we measured Vmax over a 35- to 40-m distance, which is necessarily different from either a 20- to 30-m or a 20-m sprint time. It seems likely that in the earlier studies not all

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of the subjects reached their maximal velocities over the 20to 30-m distance. Moreover, our plyometric training program was conducted 3 times per week, whereas Christou et al. (8) limited their training sessions to twice weekly. Another finding of the present study was that plyometric training increased the vertical (SJ, CMJ, and DJ) and horizontal jump (MB5) performance. This issue has not been widely examined during early puberty, although most studies of plyometric training in young boys have found a significant increase in jump performance. Our plyometric training program induced an increase in SJ height performance, in agreement with previous investigations (8,12,14,16,27). Likewise, MB5 was strongly increased after our 10-week intervention (p , 0.001). All of these findings strongly demonstrate the effectiveness of plyometric training in improving jump performance in prepubertal and early pubertal boys. Enhancement of the vertical jump has been reported for early (8,12,16,26,27) and late puberty (5), as well as in adults (18,23). It seems that plyometric exercises can enhance jumping in the prepubertal population although their neuromuscular system is not yet completely matured (30), and that their elastic tissue is more compliant (21) than that of adults. One possible explanation for the gains in vertical jump enhancement could be a change in the rate of force development, as already reported in adults (1,31,35). However, this hypothesis needs further investigation for children. It is also possible that plyometric training may enhance the power transfer between concentric and eccentric phases of muscle action and confer a positive transfer of neuromuscular demands, resulting in improved coordination and synchronization of active muscle groups (9). According to our results, total leg and thigh muscle volumes remained unchanged after plyometric training. However, the cross-sectional area of the thigh was significantly increased (Table 5). Muscle force is normally proportional to cross-sectional area, and the gains in jump and sprinting performance increase could be explained by this muscle adaptation. However, the increased vertical and horizontal jump performance could also have been induced by various neuromuscular adaptations involving the stretch reflex and the storage of elastic energy: greater muscle stiffness at ground contact could result in a fast recoil of the muscle (26) and subsequent better use of elastic energy (3); muscle activity could be increased as a result of an earlier activation of the stretch reflex (3); and desensitization of the Golgi tendon organs might allow the elastic component of muscles to undergo greater stretch (15). Because no physiological measurements (e.g., electromyography, motor units activation, muscle stiffness) were taken in this study, the adaptations underlying the response to plyometric training remain uncertain. After plyometric training, absolute peak power increased by an average of 9.1%. However, the control group also increased their absolute peak power by 10.0%, suggesting that these gains were because of the standard training rather than the plyometrics. Likewise, absolute peak power, peak

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Journal of Strength and Conditioning Research power relative to body mass or to lower limb muscle volume, maximal pedaling velocity, and maximal force (Table 4) remained unchanged after plyometric training. This is somewhat surprising, because jump performance and sprinting velocities were enhanced, and these observations run contrary to findings in older adolescents and adults (5,6). One possible explanation is that the cycle ergometer test does not involve the stretch-shortening cycle, which is widely represented in the plyometric training program. In addition, maximal cycling is somewhat unfamiliar to young subjects. Our findings were limited to 1 category of young male track athletes, and these observations should be extended to girls. Furthermore, observations are needed with differing intensities and volumes of plyometric training to determine the optimum dosage for this form of in-season training. Finally, there is a need to confirm that such gains could not be achieved by an increase in the duration of normal training.

PRACTICAL APPLICATIONS This study indicates that in young track athletes, 10 weeks of supplementary biweekly in-season plyometric training with suitably adapted hurdle and depth jumps substantially enhances jumping performance (vertical and horizontal jumps) and track running velocities over both acceleration (0–5 m) and maximal speed (0–40 m) phases relative to the traditional regimen. We found it quite practical to add this short-term plyometric training program to traditional inseason program for young track athletes to enhance their performance potential and would recommend this approach to strength and conditioning professionals.

ACKNOWLEDGMENTS The authors would like to thank the “Ministry of Higher Education and Scientific Research, Tunis, Tunisia” for financial support.

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