EFFECTS OF IN-SEASON SHORT-TERM PLYOMETRIC TRAINING PROGRAM ON LEG POWER, JUMP- AND SPRINT PERFORMANCE OF SOCCER PLAYERS MOHAMED SOUHAIEL CHELLY, 1,2 MOHAMED ALI GHENEM, 3 KHALIL ABID, 1 SOUHAIL HERMASSI, 1,2 ZOUHAIR TABKA, 3 AND ROY J. SHEPHARD 4 1 Research Unit Evaluation and analysis of factors influencing sport performance, Higher Institute of Sport and Physical Education of Ksar Said, Tunis, Tunisia; 2 Higher Institute of Sport and Physical Education of Ksar Said, Tunis, Tunisia; 3 Laboratory of Physiology, Faculty of Medicine Ibn-El-jazzar, Sousse, Tunisia; and 4 Faculty of Physical Education and Health, University of Toronto, Toronto, Ontario, Canada ABSTRACT Chelly, MS, Ghenem, MA, Abid, K, Hermassi, S, Tabka, Z, and Shephard, RJ. Effects of in-season short-term plyometric training program on leg power, jump- and sprint performance of soccer players. J Strength Cond Res 24(10): 2670 2676, 2010 Our hyposis was that addition of an 8-week lower limb plyometric training program (hurdle and depth jumping) to normal in-season conditioning would enhance measures of competitive potential (peak power output [PP], jump force, jump height, and lower limb muscle volume) in junior soccer players. The subjects (23 men, age 19 6 0.7 years, body mass 70.5 6 4.7 kg, height 1.75 6 0.06 m, body fat 14.7 6 2.6%) were randomly assigned to a control (normal training) group (Gc; n = 11) and an experimental group (Gex, n = 12) that also performed biweekly plyometric training. A force velocity ergometer test determined PP. Characteristics of squat jump (SJ) and countermovement jump (CMJ) (jump height, maximal force and velocity before take-off, and average power) were determined by force platform. Video-camera kinematic analyses over a 40-m sprint yielded running velocities for first step (V S ), first 5 m (V 5m ) and between 35 and 40 m (V max ). Leg muscle volume was estimated using a standard anthropometric kit. Gex showed gains relative to controls in PP (p, 0.01); SJ (height p, 0.01; velocity p, 0.001), CMJ (height p, 0.001; velocity p, 0.001, average power p, 0.01) and all sprint velocities (p, 0.001 for V 5m and V max, p, 0.01 for V S ). There was also a significant increase (p, 0.05) in thigh muscle volume, but leg muscle volume and mean thigh cross-sectional area remain unchanged. We conclude that Address correspondence to Dr. Mohamed Souhaiel Chelly, csouhaiel@ yahoo.fr. 24(10)/2670 2676 Ó 2010 National Strength and Conditioning Association biweekly plyometric training of junior soccer players (including adapted hurdle and depth jumps) improved important components of athletic performance relative to standard in-season training. Accordingly, such exercises are highly recommended as part of an annual soccer training program. KEY WORDS depth jump, running velocity, muscle volume, stretch-shortening cycle, jumping, force velocity test INTRODUCTION Strength, power, and ir derivatives (acceleration, sprinting, and jumping) all make important contributions to performance potential of soccer players (17). During a typical game, a 2- to 4-second sprint occurs every 90 seconds (4,29); sprinting occupies some 3% of playing time and accounts for 1 11% of distance covered during a match (29,32). Some 96% of sprints are shorter than 30 m, and 49% are,10 m (32). Thus, performance over distances of 10 m or less, and velocity attained during first step are key indicators of player potential (8,9). A soccer match also demands numerous explosive movements, including some 15 tackles, 10 headings, frequent kicking, and changes of pace (3,4,32,39). Jumping ability and anaerobic performance are critical to soccer player, and high scores for squat counter movement jumps (CMJs) are to be anticipated in top players (0.40 and 0.65 m, respectively, in 1 study [32]). Soccer is becoming progressively more athletic, and short-term muscle power has become crucial in many game situations. The power that an individual can develop depends on both force and velocity, as determined by friction-loaded ergometers (9,34). Both linear force and parabolic power velocity are increased slightly after 8 weeks of heavy resistance training (9). Strength training is thus popular as a means of augmenting muscular power and performance in soccer players (9,17,31,32). In 1 study, 7 weeks of lower limb strength training with external loads of 50 kg induced respective increases in 1 repetition maximum (1RM) half 2670
www.nsca-jscr.org squat, CMJ height, SJ height, peak power (PP), 0- to 10-m sprint time and 0- to 40-m sprint time of 26, 5, 7, 7.5, 1.7, and 1.3% (31). Likewise, Chelly et al. (9) reported that 8 weeks of half squat training with heavy loads enhanced SJ height (p, 0.05), lower limb PP (p, 0.05), 1RM half back squat (p, 0.001), and sprint velocities (p, 0.05). Less is known about benefits to soccer player from alternative or supplementary tactic of plyometric training. Plyometric exercise involves stretching muscle immediately before making a rapid concentric contraction. The combined action is commonly called a stretch-shortening cycle (SSC). Neverless, use of SSC seems a particularly appropriate regimen for soccer, where players must frequently jump, run, and sprint. Similar gains of maximal strength have been reported with traditional strength and plyometric training, but latter approach appears to induce greater gains in muscle power (36). Currently available findings regarding jump height and sprint performance are contradictory. In 1 study, 6 weeks of depth jump or CMJ training improved vertical jump height (p, 0.05) of youth soccer players, but ir sprint performance remained unchanged (p. 0.05) (33). Likewise, Markovic et al. (22) found that 10 weeks of plyometric training increased squat jump (SJ) and CMJ height and power, but 20-m sprint time remained unchanged. In contrast, Rimmer and Sleivert (30) found that 8 weeks of plyometric training improved 0- to 10-m and 0- to 40-m sprint times (p =0.001). Most previous plyometric investigations have been completed preseason. De Villarreal et al (11) recently noted significant decreases in 20-m sprint time and jump height (CMJ and drop jump) if a 7-week plyometric training program was followed by 7 weeks of detraining. Given contradictory nature of existing information on efficacy of plyometric training (12), our aim was to examine effect of adding a combined hurdle and depth jump program to normal in-season regimen of experienced soccer players. We hyposized that 8 weeks of biweekly plyometric training would enhance leg PP, jump height, and sprint running velocity relative to players who maintained ir normal in-season regimen. METHODS Experimental Approach to Problem This study addressed question as to wher 8 weeks of biweekly in-season plyometric training would enhance physical performance of soccer players relative to ir customary in-season training regimen. A team of experienced soccer players was divided randomly into a Plyometric training group (Gex; n = 12) and a control group (standard inseason regimen) (Gc; n = 11). Two weeks before definitive testing, 2 familiarizations sessions were held. Definitive measurements began 4 months into competitive season. Data were collected before enhanced training, and after completing 8-week trial. The protocol included a force velocity test to evaluate leg PP, maximal pedaling velocity (V 0 ) and maximal force (F 0 ); SJ and CMJ to assess leg jump power, velocity and leg force; a 40-m sprint that evaluated velocity during first step, 5-m velocity and maximal running velocity, and anthropometric assessments of lower limb muscle volumes. Initial and final tests were carried out at same time of day, and under same experimental conditions, at least 3 days after most recent competition. Players maintained ir normal intake of food and fluids, but before testing, y 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 performance tests. Subjects All procedures were approved by Institutional Review Committee for ethical use of human subjects, according to current national laws and regulations. Participants gave written informed consent after receiving both a verbal and a written explanation of experimental protocol and its potential risks. Subjects were told that y could withdraw from trial without penalty at any time. Twenty-three male players were drawn from a single regional soccer team (age 19 6 0.7 years, body mass 70.5 6 4.7 kg, height 1.75 6 0.06 m, body fat 14.7 6 2.6%). Their mean soccer experience was 7.2 6 1.2 years. All were examined by team physician, with a particular focus on conditions that might preclude plyometric training, and all were found to be in good health. The 23 individuals were randomly assigned between 2 groups: plyometric training (Gex; n = 12; age 19.1 6 0.7 years, body mass 70.3 6 5.5 kg, height 1.76 6 0.06 m, body fat 14.7 6 3.2%) and control (Gc; n = 11; age 19.0 6 0.8years,bodymass70.66 4.2 kg, height 1.74 6 0.06 m, body fat 14.6 6 1.7%). Procedures The study was performed over an 8-week period, from January to March. All subjects had engaged in standard training regimen from beginning of football season (September) until end of study (March). Before competitive season (August), all subjects (Gex and Gc) were engaged in a light resistance training program for both upper and lower limbs. Twice weekly sessions included exercises using body weight as a resistance. During competitive season (September to March), subjects trained 5 times a week and played one official game. The standard training sessions lasting 90 minutes included skill activities at various intensities, offensive and defensive tactics, and 30 minutes of continuous play. The control group maintained this pattern, but during period January through March, experimental group supplemented this standard regimen by a specific plyometric program. All subjects also engaged in weekly school physical education sessions; se lasted for 40 minutes and consisted mainly of ball games. Tests 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 ir last plyometric session to ensure adequate recovery from acute effects of training. VOLUME 24 NUMBER 10 OCTOBER 2010 2671
In-Season Plyometric Training in Male Soccer Players Testing Schedule Subjects were familiarized with circuit training for 2 weeks before beginning eir measurements or formal training. Testing was integrated into weekly training schedule. A standardized battery of warm-up exercises was performed before maximal efforts. On first test day, subjects performed SJ and CMJ, followed by force velocity test. Anthropometrical assessment and sprint running were undertaken on day 2. Day 1. Squat and countermovement jumps: Characteristics of SJ and CMJ (jump height, maximal force before take-off, maximal velocity before take-off, and average power of jump) were determined using a force platform (Quattro Jump, version 1.04, Kistler Instrument AG, Winterthur, Switzerland). Jump height was determined as center of mass displacement, calculated from recorded force and body mass. Subjects began SJ at a knee angle of 90, avoiding any downward movement, and y performed a vertical jump by pushing upward, keeping ir legs straight throughout. The CMJ was begun from an upright position, making a 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, largest jump being used in subsequent analyses. The force velocity test: The force velocity test was performed on a mechanically braked cycle ergometer (Monark 894 E Ergometer, Vansbro, Sweden). A familiarization session was conducted on a separate day. Individuals completed 5 short maximal sprints against braking forces corresponding to 2.5, 5, 7.5, 9, and 11.5% of individual s body mass, with rest intervals of at least 5 minutes between trials. Software allowed estimation of velocity, braking force, and power output during each trial. Peak power was judged to have been reached when additional loading induced a decrease in power output. Relationships between braking force and pedaling velocity were plotted for each individual. Maximal pedaling velocity (V 0 ) and maximal force (F 0 ) were calculated using an accepted regression equation (2,34). Day 2. Anthropometry: Standard equations were used to predict body fat from biceps, triceps, subscapular, and suprailiac skinfolds (40). Muscle volumes for thigh and leg were estimated from multiple measurements of skinfolds, citcumferences, and diameters for lower limbs, as detailed elsewhere (19). Sprint performance: Subjects sprinted on a grass track for 40 m. In normal play, a sprint usually starts from a standing or jogging position, but in our tests, subjects began sprint from a standing position to allow estimation of velocity during first step. Video cameras (Sony Handycam, DCR- PC105E, Ó 2003 Sony corporation, Tokyo, Japan) were placed perpendicular to running lane. The first camera filmed first 5 meters, and second phase of maximal running velocity (from 35 to 40 m) (9,10,23). Recorded data TABLE 1. Training program for plyometric group. Week Exercise 3 sets 3 reps 1 40-cm hurdle jump 3 5 3 10 2 40-cm hurdle jump 3 7 3 10 3 40-cm hurdle jump 3 10 3 10 4 60-cm hurdle jump 3 5 3 10 5 40-cm drop jumps 3 4 3 10 6 40-cm drop jumps 3 4 3 10 7 40-cm drop jumps 3 4 3 10 8 40-cm drop jumps 3 4 3 10 included average velocity during first step (V S ), first 5 m (V 5m ) and between 35 and 40 m (V max ). Two trials were separated by at least 5 minutes, highest of paired values being retained. Software (Regavi & Regressi, Micrelec, Coulommiers, France) converted hip displacements to corresponding velocities (V S, V 5m, and V max ). The technique, including reliability of camera and data processing software, has previously been detailed (9). Details of plyometric training: Subjects in both experimental and control groups avoided any training or than that associated with soccer team. Each Tuesday and Thursday for 8 weeks, Gex supplemented standard regimen with plyometric training, performed immediately before ir standard training sessions (Table 1). Plyometric sessions began with a 15-minute warm-up and lasted for some TABLE 2. Intraclass correlation coefficient showing acceptable reliability of track running velocities and jump tests.* ICC 95% CI Track running velocity V S (ms 21 ) 0.86 0.67 0.94 V 5 (ms 21 ) 0.90 0.77 0.96 V max (ms 21 ) 0.84 0.62 0.93 Jump tests SJ height (m) 0.97 0.92 0.99 SJ force (N) 0.86 0.54 0.96 SJ velocity (ms 21 ) 0.95 0.84 0.98 SJ power (Wkg 21 ) 0.85 0.48 0.95 CMJ (m) 0.98 0.95 0.99 CMJ force (N) 0.82 0.38 0.94 CMJ velocity (ms 21 ) 0.96 0.88 0.99 CMJ power (Wkg 21 ) 0.95 0.85 0.98 *ICC = intraclass correlation coefficient; CMJ = countermovement jump; SJ = squat jump; CI = confidence interval. 2672
www.nsca-jscr.org TABLE 3. Anthropometric parameters before and after plyometric training.* Total leg muscle volume (L) Thigh muscle volume (L) Mean thigh CSA (cm 2 ) Pre 8.7 6 0.8 9.6 6 1.0 Post 9.0 6 0.9 9.6 6 0.8 Pre 4.0 6 0.4 6.6 6 0.7 Post 4.1 6 0.5 6.7 6 0.5 Pre 176.4 6 13.8 194.9 6 25.3 Post 182.3 6 16.3 193.3 6 23.2 Gex = plyometric training group; Gc = control group; CSA ¼ cross sectional area. *p, 0.05. A 2-way analysis of variance with repeated measure (group 3 time) was used to assess statistical significance of training related effects. 30 minutes. Jumps were performed on a grass track. Subjects were instructed to perform all exercises with maximal effort. Each jump was performed to reach maximal possible height with a minimal ground contact time. Both hurdle and drop jumps were performed with small angular knee movements; ground was touched with balls of feet only, reby specifically stressing calf muscles (22). Each set of hurdles consisted of 10 continuous jumps over hurdles spaced at intervals of 1 m. Each set of drop jumps comprised 10 maximal rebounds after dropping from a 0.4-m box, with a pause of 5 seconds between each rebound (22). Statistical Analyses Means and SDs were calculated using standard statistical methods. Training related effects were assessed by a 2-way analysis of variance with repeated measure (group 3 time). If a significant F value was observed, Sheffé s post hoc procedure was applied to locate pairwise differences. Percentage changes were calculated as ([posttraining value 2 pretraining value]/pre training value) 3 100. Pearson product moment correlations determined relationships between braking force and pedaling velocity. The reliabilities of sprint velocities (V S, V 5m, and Vmax) and vertical jump (SJ and CMJ) height, velocity, force, and average power measurements were assessed using intraclass correlation coefficients (ICCs) (28). As a general rule, an ICC over 0.90 is considered to be high, between 0.80 and 0.90 moderate and below 0.80 to be insufficient for physiological field testing (35); ICCs showed an acceptable reliability for our measurements of track velocity and jump tests (Table 2). We accepted p # 0.05 as our criterion of statistical significance, wher a positive or a negative difference was seen (i.e., a 2-tailed test was adopted). RESULTS Plyometric training induced a significant increase in thigh muscle volume (p, 0.05); measures of total leg muscle and thigh cross sectional area (CSA) showed a similar directional trend, but because of greater variability were not statistically significant (Table 3). Force velocity test data also showed increases of absolute PP (W) and PP relative to body mass TABLE 4. Force velocity test calculated parameters before and after plyometric training.* Absolute power (W) Pre 711 6 84 680 6 128 Post 743 6 87 682 6 129 Power (Wkg 21 ) Pre 10.1 6 0.7 9.6 6 1.5 Post 10.7 6 1.0 9.6 6 1.6 Power (W/total leg muscle volume) Pre 81.8 6 6.8 71.0 6 12.8 Post 82.8 6 7.6 71.4 6 12.9 Power (W/thigh muscle volume) Pre 177.4 6 15.4 103.5 6 19.3 Post 181.2 6 16.8 101.7 6 17.5 Power (W/CSA) (Wcm 22 ) Pre 4.0 6 0.4 3.6 6 0.9 Post 4.1 6 0.4 3.6 6 1.0 Maximal pedaling velocity (rpm) Pre 176.1 6 10.3 170.2 6 14.7 Post 182.2 6 7.9 170.0 6 16.6 Maximal force (N) Pre 84.4 6 8.5 84.6 6 4.4 Post 83.0 6 6.1 k 84.5 6 3.7 *Gex = plyometric training group; Gc = control group. A 2-way analysis of variance with repeated measure (group 3 time) was used to assess statistical significance of training related effects. p, 0.01. k p, 0.05. VOLUME 24 NUMBER 10 OCTOBER 2010 2673
In-Season Plyometric Training in Male Soccer Players TABLE 5. Jumps test values before and after plyometric training.* SJ Height (m) Pre 0.36 6 0.03 0.37 6 0.02 Post 0.39 6 0.03 0.37 6 0.02 Velocity Pre 2.4 6 0.1 2.1 6 0.2 (ms 21 ) Post 2.5 6 0.4 k 2.1 6 0.1 Force (N) Pre 1,540 6 141 1,504 6 167 Post 1,590 6 168 1,589 6 160 Power (W) Pre 1,398 6 145 1,451 6 150 Post 1,511 6 197 1,464 6 161 Power Pre 19.9 6 2.1 20.5 6 1.4 (Wkg 21 ) Post 21.7 6 2.5 20.7 6 1.7 CMJ jump Height (m) Pre 0.40 6 0.03 0.39 6 0.02 Post 0.41 6 0.03 k 0.39 6 0.02 Velocity Pre 2.5 6 0.1 2.2 6 0.2 (ms 21 ) Post 2.6 6 0.1 k 2.2 6 0.1 Force (N) Pre 1,347 6 111 1,313 6 133 Post 1,372 6 108 1,345 6 147 Power (W) Pre 1,673 6 147 1,612 6 176 Post 1,753 6 170{ 1,612 6 161 Power (Wkg 21 ) Pre 23.9 6 2.2 22.8 6 1.6 Post 25.2 6 1.6 23.1 6 1.4 *Gex = plyometric training group; Gc = control group; CMJ = countermovement jump; SJ = squat jump. A 2-way analysis of variance with repeated measure (group 3 time) was used to assess statistical significance of training related effects. p, 0.01. k p, 0.001. {p, 0.05. TABLE 6. Sprint running velocities values before and after plyometric training.* V S (ms 21 ) Pre 2.2 6 0.3 2.0 6 0.2 Post 2.6 6 0.3 2.2 6 0.3 V 5 (ms 21 ) Pre 4.0 6 0.5 3.6 6 0.3 Post 4.4 6 0.4 k 3.7 6 0.3 Vmax (ms 21 ) Pre 8.2 6 0.2 7.8 6 0.3 Post 9.0 6 0.2 k 8.0 6 0.3 *Gex = plyometric training group; Gc = control group. A 2-way analysis of variance with repeated measure (group 3 time) was used to assess statistical significance of training related effects. p, 0.01. k p, 0.001. (Wkg 21 ) (Table 4, p, 0.01); however, re was no increase of PP per unit of muscle volume or thigh muscle volume (Table 4), and maximal force even showed a small decrease. Data for SJ and CMJ were in accordance with se findings (Table 5), with a significant increase in SJ height relative to Gc (p, 0.01), increases in CMJ height and average jump power (W), but no significant increase in force after plyometric training. The increase in jump test scores was accompanied by a significant increase of running velocities (p, 0.001 for both V 5m and V max ; p, 0.01 for V S ) (Table 6). DISCUSSION The main finding from this study is that, in accordance with our hyposis, supplementary plyometric training program increased several measures of potential soccer playing performance, including absolute (W) and relative (Wkg 21 ) PP of legs, as assessed by force velocity tests (p, 0.001, Table 4), SJ, CMJ, and sprint running, scores (Tables 5 and 6). Improvements of muscle power and vertical jump height with plyometric training have been described previously (7,18,21). A recent meta-analysis (12) found gains in jump height of 4.7 15% after plyometric training. Our results are consonant with se finding (SJ and CMJ scores were increased by 7.1 and 4.2%, respectively). The plyometric training program that we used is similar to that proposed by Markovic et al. (22); y observed increases in both SJ and CMJ scores relative to controls (p, 0.05), but PP of SJ relative to body mass (Wkg 21 ) did not improve. Our results accord with se findings (Table 5). In contrast, PP of CMJ was significantly higher for Gex than for Gc. This may be because CMJ involves an SSC and is thus very similar to plyometric exercises used in our study. Moreover, plyometric training is likely to improve coordination (12) and thus to induce a neuromuscular adaptation that augments power production (5). Behm et al. (5) suggested that any increase of leg PP induced by plyometric is essentially because of neuronal adaptations: selective activation of motors units, synchronization, selective activation of muscles, and increased recruitment of motor units. Many of CMJ parameters (jump height, velocity, absolute [W], and relative power [Wkg 21 ]) tended to increase more than values for SJ (Table 5), again reflecting similarity between CMJ and plyometric training. Both CMJ absolute (W) and relative (Wkg 21 ) power were significantly improved after plyometric training, but peak force showed no statistically significant change; this implies that improvement in CMJ power production was largely because of an increase in peak velocity. Lower limb muscle volumes tended to increase after plyometric training, significantly so for thigh muscle volume (Table 3, p, 0.05). The gain in SJ average power per unit of muscle volume (W/Lm) was much smaller than gains in absolute values (W) or relative to body mass (Wkg 21 ) (4.9, vs. 8.3 and 9.3% W and Wkg 21, respectively) (Table 3). Likewise, 2674
www.nsca-jscr.org gain in CMJ average power per unit of muscle volume was only 1.6%, as compared with 4.9 and 5.9% for W and Wkg 21, respectively. These results suggest that in addition to neuromuscular adaptations, our plyometric training induced an increase in leg muscle volume and average power production. The same conclusion can be drawn from cycle ergometer data (Table 5) (PP gains of 4.6 and 6.2% for W and Wkg 21, respectively, dropping to 1.4% W/Lm); indeed, gains of leg muscle volume seem main determinant of gain in PP as assessed by force velocity test. Several previous studies have suggested that plyometric training can enhance sprinting ability because it uses SSC. Mero et al. (24) found a close relationship (p, 0.001) between rise of center of gravity in a drop jump and maximal running velocity. Drop jump and CMJ performance also bear significant relationships to 30- and 40-m sprint times (15,26). Our investigation showed improved sprint speeds after plyometric training (Table 6). To our knowledge, this is first study to investigate effect of short-term supplementary plyometric training on sprint performance of soccer players. However, previous research has demonstrated that velocity over distances of 0 30, 10 20, and 20 30 m is increased significantly (p, 0.05) after 10 weeks of plyometric training (21). A 12-week period of nondepth jump plyometric exercise also improved 25-m sprint performance of entry-level collegiate athletes by 9% (25). Similarly, 6 weeks of plyometric training decreased 50-m sprint times in 9 adult male athletes and a group of basketball players (37). In contrast, Herrero et al. (16) found no significant gains of SJ height, CMJ height or 20-m sprint time with plyometric training, and Markovic et al. (22) found no improvements in 20-m sprint times, even though y used a similar training program to us. These discrepancies may reflect differences in methodology or fitness level of subjects (physical education students vs soccer players). The meta-analysis of De Villarreal et al. (12) concluded that subjects with most sport experience showed greatest increases in vertical jump height. This could also be true of sprint performance, explaining some of discrepant results. Differences in training protocol may also be a factor. In study of Herero et al. (16), training consisted of horizontal and drop jumps continued 2 d wk 21 for only 4 weeks; in our study, training was more intense (hurdle and depth jumps) and continued for longer (twice a week for 8 weeks). Maximal intensity sprinting necessitates extremely high levels of neuronal activation (13,27). Measurable neurological parameters such as nerve conduction velocity, maximum electromyogram, motor unit recruitment strategy, and Hoffman reflex (H-reflex) all alter in response to physical training (6,14,20). Potential mechanisms for improvements in sprint performance include changes in temporal sequencing of muscle activation for more efficient movement, preferential recruitment of fastest motor units, increased nerve conduction velocity, frequency or degree of muscle innervations, and increased ability to maintain muscle recruitment and rapid firing throughout sprint (1). Wilkerson et al. (38) showed that 6 weeks of preseason plyometric jump training improved neuromuscular attributes in 11 basketball players. In present study, we did not assess neuronal adaptations, but we can assume from earlier studies that plyometric training induced neuromuscular adaptations and that se adaptations contributed to observed gains in sprint performance. PRACTICAL APPLICATIONS The current study indicates that in male regional junior soccer players 8 weeks of supplementary biweekly in-season plyometric training with suitably adapted hurdle and depth jumps substantially enhances leg PP output, jump height, and sprint velocities over both acceleration (0 5 m) and maximal speed (0 40 m) phases. We found it quite practical to add this short-term plyometric training program to traditional inseason technical and tactical male soccer training sessions to enhance performance potential of our players. The gains that were realized seem greater than could have been anticipated from a corresponding extension of traditional training (although this point needs furr checking). In addition to effects stemming from observed increases in muscle volume, re are many potential neuromuscular explanations of response to plyometric training, and se merit furr investigation. 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