Handball is an intermittent sport where physiological characteristics, particularly the ability to make and to repeat the explosive muscular contractions required for sprinting, jumping, turning, changing pace, and throwing a ball, are important to performance not only in adults but also in adolescent players (5,11,35). Single-bout explosive efforts are as important as maximal aerobic power (15); although high-speed sprinting accounts for only 11% of the total distance covered in junior handball games (35), vital moments such as winning possession of the ball and scoring goals depend on such actions (35). Initial acceleration (32), jumping, and the agility to change direction, start, and stop quickly (34) are all crucial elements of fast play. Game analyses have demonstrated a mean 10- to 12-m sprint time of 2.3 seconds (35), and a mean of 50 turns per game (5,35).
Such explosive actions should be developed independently of aerobic power (16). Strength training can increase force availability (17,18), but the high velocity training of plyometrics may improve the rate of force development relative to traditional weight training (19). Some studies of junior players have found that gains of strength-enhanced explosive actions (17,18,29), but others have seen no significant benefits from such a program (9), suggesting that strength training in itself lacks several elements important to the development of explosive movements, including a stimulation of neural and segmental coordination, specificity of joint angle and angular velocities, and eccentric overloading (9).
Plyometric training provides the required stimuli and can enhance explosive contractions in both pubertal (30) and prepubertal (27) populations. Such a regimen is natural to many sports, with its emphasis on jumping, throwing, hopping, and skipping, and it is particularly appropriate where there is a need to develop explosive movements and vertical jumping ability, as in the handball player. Concerns regarding the safety of plyometric training for young athletes can be minimized by combining a proper technique with a progressive program and close supervision of participants (25). Many plyometric training routines indeed mimic movements that are encountered in the normal play of children, and no specific strength level is required to begin such programs (30).
Previous studies of plyometric training in prepubertal and pubertal children (23) have added the regimen as a supplement to regular sports activities. Thus, it has remained unclear whether the resulting improvement in explosive performance was because of introduction of the new training regimen or whether it merely reflected the response to an additional training load. Our objective in the present study was to determine whether the substitution of a short-term plyometric program for some existing drills within a regular in-season handball training program would enhance the explosive movements of male adolescent handball players. Given existing information on the efficacy of plyometric training (28), we added a combined lower limb (hurdle and depth jump) and upper limb (push-up) program to the normal in-season regimen for 8 weeks, without increasing the total training time of the experimental group. We hypothesized that these changes would yield gains in the muscular power of both upper and lower limbs and increases of jump height, sprint running velocity, and throwing ball velocity relative to control players who maintained their standard in-season regimen.
This study examined whether an 8-week biweekly in-season plyometric program without increase of total training time would enhance selected aspects of performance in adolescent handball players relative to that of their peers who continued to follow their customary in-season training regimen. A group of 23 experienced adolescent handball players volunteered to be assigned randomly between a plyometric training group (E; n = 12) and a control group (standard in-season regimen) (C; n = 11). Two weeks before definitive testing, 2 familiarizations 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 8-week trial. The test protocol included cycle-ergometer force-velocity tests of peak power for the upper and lower limbs (Wupper peak and Wlower peak, respectively), maximal pedaling velocity (V0) and maximal braking force (F0); assessments of leg power by vertical countermovement jump (CMJ) and squat jumps (SJ), leg force, jump velocity, and jump height; a handball throwing test; and a sprint test that evaluated velocity during the first step (V1S), 5-m velocity (V5m), and maximal running velocity (Vmax). Muscle volumes of the upper and lower limbs were estimated anthropometrically from external dimensions, the thickness of overlying fat, and assessments of bone dimensions.
Initial and final test measurements were made at the same time of day and under the same experimental conditions, at least 3 days after the most recent competition. Players maintained their normal intake of food and fluids, but before testing, they abstained from physical exercise for 1 day, drank no caffeine-containing beverages for 4 hours, and ate no food for 2 hours. Verbal encouragement ensured maximal effort throughout all tests.
All procedures were approved by the University Institutional Review Committee for the ethical use of human subjects, according to current national laws and regulations. Informed consent from participants after receiving both a verbal and a written explanation of the experimental protocol and its potential risks and benefits. They were assured that they could withdraw from the trial without penalty at any time. A parental or guardian consent for all players involved in this investigation was obtained. Twenty-three male players were drawn from a single national junior handball team (age: 17.2 ± 0.4 years, body mass: 79.9 ± 11.5 kg, height: 1.79 ± 4 m, and body fat: 12.7 ± 1.6%). Their mean experience of handball competition was 7.2 ± 1.1 years. Subjects were examined by the team physician, with a particular focus on conditions that might preclude plyometric training, and all were found to be in good health. Subjects were randomly assigned between the 2 groups, which were well matched in terms of their initial characteristics: (E: n = 12; age: 17.1 ± 0.3 years, body mass: 80.1 ± 11.9 kg, height: 1.81 ± 6.4 m, body fat: 13.1 ± 2.1%; C: n = 11; age: 17.2 ± 0.4 years, body mass: 78.0 ± 11.4 kg, height: 1.77 ± 5.3 m, and body fat: 13.2 ± 1.1%).
Procedures and Evaluation
The study was performed from January to March (an 8-week period) in the middle of the playing season (from the 22nd to the 29th week, commencing in the second preparatory period after the traditional 8-day winter holiday). Before the competitive season (August), all subjects had engaged in a light resistance training program for both the upper and the lower limbs; twice-weekly sessions included exercises using the body weight as a resistance. During the competitive season, which began in September and finished in May, subjects trained 5 times a week and played 1 official game per week. The standard training sessions, lasting 90 minutes, included skill activities at various intensities, offensive and defensive tactics, and 30 minutes of continuous play. All subjects also engaged in once-weekly school physical education sessions; these lasted for 40 minutes and consisted mainly of ball games. In addition, from September to the end of December, subjects in both experimental and control groups participated in twice-weekly fitness sessions (Tuesdays and Thursdays). The first session each week was devoted to circuit training (push-ups, exercises for the stomach and dorsal muscles, skipping rope, and throwing a medicine ball). The second session each week involved strength training against the body weight and sometimes the use of light weights (to a maximum of 60% of 1-repetition maximum for both limbs). The control group maintained this same routine on 2 days per week during the period January to March, but the experimental group replaced a part of their 2 physical fitness sessions by a specific plyometric program, without increasing their total training time.
Evaluations of muscle function were completed in a fixed order for 2 consecutive days. Care was taken to ensure that those undertaking plyometric training were tested 5–9 days after their last plyometric session, to ensure adequate recovery from the acute effects of training.
Subjects were familiarized with circuit training for 2 weeks before beginning either the initial measurements or the formal training. Testing was integrated into the weekly training schedule. A standardized battery of warm-up exercises was performed before maximal efforts. On the first test day, subjects performed the SJ and CMJ, followed by the force-velocity test. Anthropometrical assessment, handball throwing, and sprint running were undertaken on day 2.
Squat Jump and Countermovement Jump
Characteristics of the SJ and the CMJ (jump height, maximal force before take-off, maximal velocity before take-off, and the average power of the jump) were determined using a force platform (Quattro Jump, version 1.04; Kistler Instrument AG, Winterthur, Switzerland). Jump height was determined as the center of mass displacement, calculated from the recorded force and body mass. Subjects began the SJ at a knee angle of 90°, avoiding any downward movement, and they performed a vertical jump by pushing upward, keeping their legs straight throughout. The CMJ was begun from an upright position, making a rapid downward movement to a knee angle of ∼90° and simultaneously beginning to push-off. One minute of rest was allowed between 3 trials of each test, the highest jump on each test being used in subsequent analyses.
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 (3). In brief, 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. Arm tests were made in similar fashion, using an appropriately modified cycle ergometer (2,6,17).
Measured parameters included the peak power output, maximal braking force, and maximal pedaling velocity for upper and lower limbs (6). Arm tests began with a braking force equal to 1.5% of the subject's body mass (2,6). After a 5-minute recovery, the braking was increased in sequence to 2, 3, 4, 5, 6, 7, 8, and 9% of body mass. When testing the lower limbs, 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. For more details of the force-velocity tests, refer to Chelly et al. (3,6).
Circumferences and skinfold thickness at appropriate levels in the thigh, calf, arm and forearm, limb lengths, and the breadth of the femoral and humeral condyles were measured to estimate the muscle volume of the upper and lower limbs, using standard formulas as described previously (2,6). The overall percentage of body fat was estimated from the biceps, triceps, subscapular, and suprailiac skinfolds, using the equation (41):
where ∑4 folds is the sum of the 4 skinfolds (in millimeters) and a and b are constants dependent on sex and age.
The mean value of thigh cross-sectional area (CSA) was estimated from the maximal and mid thigh circumferences.
Three types of overarm throw were made on an indoor handball court: a standing ([penalty] throw), a 3-step running throw, and a jump shot. The standing and 3-step throws have been described by Hermassi et al. (17). In the jump shot, players made a preparatory 3-step run before jumping vertically 9 m from the goal, releasing the ball while in the air. Throwing times were recorded by a digital video camera (Sony Handycam DCR-PC105E; Sony, Tokyo, Japan), positioned 3 m above and parallel to the player. Data processing software (Regavi & Regressi, Micrelec, Coulommiers, France) converted the duration of ball displacements to velocities. Throws with the greatest starting velocity were selected for further analysis. The reliability of the data processing software has been verified previously (3); timing was accurate to 0.001 seconds, and the test-retest coefficient of variation for throwing velocity was 1.9%.
After familiarization, subjects made a maximal 30-m sprint on an outdoor tartan surface. Body displacement was filmed by 2 cameras (Sony Handycam, DCR-PC105E; 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 a second camera monitored the sprint between 25 and 30 m. Participants performed 2 trials, separated by an interval of 5 minutes. Appropriate software (Regavi & Regressi; Micrelec) converted measurements of hip displacement to the corresponding velocities (V1S, V5m, and Vmax). Because handball players rarely run more than 30 m during a match, we considered the sprint phase between 25 and 30 m as indicating the individual's maximal running velocity. The reliability of the camera and the data processing software has been reported previously (3).
Plyometric Training Programs
All subjects avoided any training other than that associated with the handball team throughout the study. Every Tuesday and Thursday for 8 weeks, the experimental subjects replaced a part of their standard regimen with plyometric training (Table 1). Plyometric sessions began with a 15-minute warm-up and lasted for some 30 minutes. Subjects were instructed to perform all exercises with maximal effort.
Plyometric push-ups were completed from the kneeling position, with the knees and feet remaining in contact with the floor (39). Subjects started with the trunk vertical and their arms hanging relaxed at their sides. They then allowed themselves to fall forward, extending their arms forward with slight elbow flexion, in preparation for contact. Subjects gradually absorbed the force of the fall by further flexing the elbows, stopping the movement with the chest nearly touching the floor. They then immediately reversed the action by extending the arms rapidly, thus propelling the trunk back to its starting position. This sequence was repeated every 4 seconds until the assigned number of repetitions had been completed. If a player was unable to return to the starting position during ascent, he was allowed to break form at the highest return point and help himself back to the starting position by flexing the hips and going into a quadruped position. In this case, the plyometric push-up was performed with the goal of achieving a maximal height and developing the ability to return to the starting position as soon as possible. Approximately 50% of participants could not propel themselves back to the starting position on the first day of plyometric training, but all were able to do so consistently by the fifth session. Participants were instructed to perform each repetition with maximum effort, emphasizing a fast switch from trunk descent to trunk ascent.
Each jump was performed to reach the maximal possible height, with a minimal ground contact time (bouncing jump). Both hurdle and drop jumps were performed with small angular knee movements; the ground was touched with the balls of the feet only, thereby specifically stressing the calf muscles (28). Each set of hurdle jumps consisted of 10 continuous jumps over hurdles spaced at 1 m intervals. Each set of drop jumps comprised 10 maximal rebounds after dropping from a 0.4-m box, with an interval of 5 seconds between rebounds (28).
Means and SDs were calculated using standard statistical methods. Normality of all variables was tested using Kolmogorov-Smirnov test procedure. Levene's test was used to determine homogeneity of variance. Training-related effects were assessed by 2-way analyses of variance with repeated measures (group × time). If a significant F value was observed, Sheffé's post-hoc procedure was applied to locate pairwise differences. Statistical power test values were obtained and ranged between 0.67 and 1. Percentage changes were calculated as ([posttraining value – pretraining value]/pretraining value) × 100. Comparisons between initial and final tests used nonpaired Student’s t-tests. Pearson’s product-moment correlations determined relationships between braking force and pedaling velocity. The reliabilities of sprint velocities (VS, V5m, and, Vmax), vertical jump (SJ and CMJ) height, velocity, force, and average power measurements were assessed using intraclass correlation coefficients (37); reliability was acceptable for all of our measurements of track velocity and jumping tests (Table 2). We accepted p ≤ 0.05 as our criterion of statistical significance, whether a positive or a negative difference was seen (i.e., a 2-tailed test was adopted).
Controls showed no significant changes in anthropometric characteristics, power, or track performance over the 2 months of observation. In the experimental group, plyometric training induced a significant increase of thigh and total leg muscle volumes (p < 0.001) (Table 3). Force-velocity tests also showed increases in Wupper peak (p < 0.01) and Wlower peak (p < 0.001), whether expressed in absolute units (W) or relative to body mass (W·kg−1) (Table 4), and maximal pedaling velocities showed significant increases (p ≤ 0.05) for both upper and lower limbs, although the peak power per unit of limb muscle volume remained unchanged. Data for SJ and CMJ were in accordance with these findings (Table 5), with significant increases in SJ height and SJ power (p < 0.01), SJ force (p ≤ 0.05), CMJ height, and CMJ force (p < 0.01), but no significant changes in jump velocities. There were also significant increases of all running (p < 0.001) and throwing velocity tests (Table 6).
Plyometric programs are often implemented during the preseason to bring young players to an appropriate initial level of fitness. Such a preseason regimen may serve to reduce injuries and improve the athletic performance of adolescents, both by strengthening ligaments, tendons, and bones and by enhancing muscular strength, endurance, and power. However, in such circumstances, it is difficult to allocate any improvement in explosive ability between the effects of a return to activity after a period of detraining and specific benefits of plyometric training.
Previous authors (8,28,30) have recommended continuation of a plyometric training program into the handball season to maintain and increase explosive ability. The primary aims of our study were thus to determine whether adolescent male handball players could enhance muscle power by an in-season plyometric program and whether gains could be realized without detriment to other aspects of performance. The answer to both of these questions is strongly positive. Our results substantiate the hypothesis that substitution of other activities by biweekly periods of in-season plyometric training enhanced the peak power of both upper and lower limbs, whether assessed by jumping, sprinting, a cycle ergometer force-velocity test, or throwing velocities (Tables 5 and 6). In contrast, the control group who continued with the standard training program showed no changes in any of these variables.
The development of large forces is important for muscle hypertrophy and remodeling (11,21). Receptors are stimulated, membranes are sensitized, and growth factors are released, triggering protein turnover and accretion (4,11). The mild reversible tissue damage induced by active plyometric stretching could be an important stimulus to such hypertrophy. However, competitive performance in handball depends not only on strength but also on the ability to exert force at the speed required by this discipline.
Our experimental subjects showed gains of upper limb power, both absolute (Warm peak) (27.4%; p < 0.001) and relative to body mass (Warm peak·kg−1) (28.7%; p < 0.001), although there was no significant change if power was expressed per liter of muscle volume (Figure 1A). Others, also (17,33), have seen improvements in both the absolute and the relative power of the upper limbs, but no changes if power was expressed per liter of muscle volume. We also saw considerable gains of absolute and relative power in the lower limbs (12.6%; p < 0.001), and again these differences disappeared if power was expressed per unit of either total leg muscle or thigh muscle volume (Table 4). These results mirror previous investigations (4,18), suggesting that despite the relatively short 8-week period of plyometric training, gains were attributable largely to an increase of local muscle volume (17), a common response to plyometric training. However, muscle volumes in the upper limb did not increase significantly (Table 3), implying that the increase of power in the arms reflects mainly neuronal adaptations, also a well-accepted response to heavy resistance training (1,13). Rapid improvements in power output have been coupled with changes in muscle activation patterns (21), the initiation of force-feedback reflexes from the Golgi tendon organs, and an improved synchronization of motor unit firing (20).
During a plyometric movement, the muscles switch rapidly from an eccentric to a concentric phase of contraction. The decreased duration of the amortization phase exploits stored elastic energy and the stretch reflex, allowing a greater than normal release of power during the concentric phase of movement (8). Previous studies of plyometric training have generally demonstrated 5–15% improvements in vertical jump height (4,8,19,24,28), attributable to a combination of enhanced coordination and greater muscle power after training, although a few authors have seen no significant change in jumping ability (8,19). Our data showed gains in SJ and CMJ of 13.9 and 9.3%, respectively, with improved vertical jump power and vertical jump force.
Because of velocity-specific training effects, handball training should simulate sport movements as closely as possible. Plyometric training seems an appropriate component of overall preparation in this context. Mero et al. (31) found a close relationship (p < 0.001) between the rate of rise of the center of gravity and maximal running velocity. Perhaps because of increases in muscle power, our experimental group showed increases rather than decreases in all track velocities after plyometric training (Table 6 and Figure 2), in agreement with findings in a recent study of junior soccer players who followed a similar regimen (4).
Most previous investigations of plyometric training have also observed increased velocities; 10 weeks of such training improved speeds over distances of 0–30, 10–20, and 20–30 m (p ≤ 0.05) (40), 12 weeks of no-depth jump plyometrics improved the 25-m sprint of entry-level collegiate athletes by 9% (24), 6 weeks of plyometric training decreased 50-m sprint times in 9 adult male athletes and a group of basketball players (40), and 7 weeks of once-weekly plyometrics decreased 20-m sprint times (7). However, Herrero et al. (19) found no significant gains of SJ height, CMJ height, or 20-m sprint time with their plyometric program, and Markovic et al. (28) found no improvements in 20-m sprint times, even though they used a similar training program to us. These discrepancies may reflect differences in the fitness level of the study participants (physical education students vs. elite handball players). The meta-analysis of de Villarreal et al. (8) concluded that individuals with the most sport experience showed the greatest increases in vertical jump height with plyometric training; the same could also be true of sprint performance. Differences in training protocol may also be a factor. Herrero et al. (19) continued horizontal and drop jumps for only 4 weeks, whereas we used more intense training (hurdle and depth jumps) over a longer period (twice a week for 8 weeks).
de Villarreal et al. (7) found that plyometric training enhanced maximal strength. Drop jump training also enhances an individual's ability to develop force rapidly and improves the maximal rate of force development (26). The drop jump requires development of maximal force during the eccentric phase of motion. Horita et al. (22) demonstrated that maximal vertical ground reaction forces were 4.7 times body mass when landing from a height of 0.5 m. Thus, one can speculate that the stimulus experienced by our handball players during the drop jump was effective in developing maximal strength and sprint performance.
Our experimental group enhanced velocities in all 3 types of ball throw (Figure 2 and Table 6), with respective increases of 23.6 ± 10.7%, 21.0 ± 6.7%, and 20.0 ± 9.9% (p < 0.001). In contrast, the controls showed no change in throwing ability. An enhanced throwing velocity is thought of major importance to successful play, giving an estimated advantage of 8–9% in men (12) and 11% in women (14). Using an identical force-velocity protocol, Bouhlel et al. (2) noted significant correlations between javelin performance and the peak power output of both the upper and the lower limb muscles. Chelly et al. (6) also found significant relationships between handball throwing velocity and the power of the upper and lower limbs. Critical factors in throwing are a transfer of power from the lower to the upper body and then to the ball (27); this ability is closely correlated with competitive performance (42). According to van den Tillaar (36), the greatest part of the total impulse is derived from the lower limbs; this assumption is supported by an earlier study of Fleck et al. (10), who noted the greater distance thrown in a set shot, when the feet and the lower limbs could be used to increase throwing velocity.
Differences in study design, methods of measurement (photoelectric cells, radar, and cinematography) (11,17,36), age, body mass, skill level (amateur or professional), throwing technique (standing throw, 3-step running throw, and jump shot), and intensity of training limit the possibility of detailed comparisons between our data and the findings of earlier investigators. Some resistance training programs have also increased handball throwing velocity. Gorostiaga et al. (11) noted a significant enhancement (p < 0.001) of standing handball throwing velocity after a 6-week heavy resistance program (supine bench press, half squat, knee flexion curl, leg press, and pec-dec). A second 10-week heavy resistance program for the upper limbs (17) yielded a considerable gain in throwing velocities. A third study (18) found gains in all throwing velocities (jumping shot, 3-step running throw, and standing throw) after an 8-week heavy resistance program for both upper and lower limbs. However, our study seems the first to have examined the value of a combined plyometric program to enhance the performance of the upper and lower limbs in elite adolescent male handball players.
The increased throwing velocities that we observed could reflect the gains of peak power output in both upper and lower limbs. Possible causes include not only a selective increase in CSA of the fast-twitch fibers but also more effective neural activation, changes in intrinsic muscular properties, an increase in myosin-adenosine triphosphatase activity, better synchronization of motor units, and a higher firing frequency (11,36,42).
In accordance with our hypothesis, the replacement of a part of our standard handball training by an 8-week plyometric training program enhanced several characteristics of importance to handball performance, including the absolute and relative muscle power of the upper and lower limbs (p < 0.001), SJ and CMJ height, and power (Table 5 and Figures 3A, B). Plyometric training has been advocated for a number of years as a means of improving performance in sports where lower-body power is the key to success. The plyometric training program that we used here is similar to that proposed by Markovic et al. (28), and like us (Table 5) they observed increases in both SJ and CMJ height scores relative to controls (p ≤ 0.05). This may be because the CMJ involves a stretch-shortening cycle that is very similar to the plyometric exercises used in our study. The improvement in vertical jump performance could be explained by an increase of fiber length (the number of sarcomeres in-series) (38) or by the stress-related overload placed on the body during plyometric training. However, plyometric training is also likely to improve coordination (28) and thus to induce neuromuscular adaptations that augment power production (16). Many of the CMJ parameters (jump velocity, absolute, and relative power) tended to increase more than the values for the SJ, reflecting the similarity between the CMJ and plyometric training. Both lower limb and thigh muscle volume increased significantly after plyometric training (p < 0.001) (Table 3). The gain in average SJ power per unit of muscle volume (W·L−1) was much smaller than the gain relative to body mass (W·kg−1) (0.6 vs. 11.6%). Likewise, the average CMJ power per unit of muscle volume regressed by an average of 0.5% as compared with an increase of 10.2% when power was expressed per unit of body mass (W·kg−1). These results suggest that in addition to neuromuscular adaptations, our plyometric training increased performance power through an increase in leg muscle volume. The same conclusion can be drawn from the cycle ergometer data (Table 4); indeed, differences between the power of the upper and lower limbs (Wupper peak and Wlower peak) disappear if power is expressed relatively to local muscle volume (Table 4 and Figure 1).
Limitations of Data
Our findings were limited to one particular category of handball players—elite adolescent males. Future studies should extend these observations to women, to other age groups, and to other levels of competition. Furthermore, observations are also needed with differing intensities and volumes of plyometric training to determine their optimum dosage for this form of preparation.
This controlled study shows that top elite adolescent male handball players who are participating in a demanding training schedule and consider themselves well trained can enhance their muscular strength and power by replacing a part of their standard regimen with an in-season 8-week biweekly program of plyometric training for both the upper and the lower limbs. Such training does not seem to interfere with the development of speed; indeed, our data show not only an enhanced peak power output but also substantial gains in vertical jumping, sprint performance, and handball throwing velocity. Moreover, it has proven quite easy and practical to incorporate the proposed regimen into the traditional routine of technical and tactical training, without detriment to other aspects of performance.
The gains that we have observed should be of great interest for handball coaches because performance of this sport relies greatly on specific power, jump, sprint, and throwing abilities, all of which were enhanced by the plyometric training regimen. Previous authors have found a similar need for deliberate plyometric training in other sports, but this is the first objective demonstration of its value in elite handball players. We strongly recommend that handball coaches implement in-season plyometric training to enhance the performance of their players. We would also encourage further investigation of the neuromuscular mechanisms contributing to the observed gains in performance.
The authors would like to thank the “Ministère de l'enseignement supérieur et de la Recherche Scientifique, Tunisia” for financial support.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
force-velocity tests; jumping; muscle mass; running velocity; stretch-shortening cycle; throwing