Strength, power, and their derivatives (acceleration, sprinting, and jumping) all make important contributions to the 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 the distance covered during a match (29,32). Some 96% of sprints are shorter than 30 m, and 49% are <10 m (32). Thus, the performance over distances of 10 m or less, and the velocity attained during the 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 the soccer player, and high scores for the squat counter movement jumps (CMJs) are to be anticipated in top players (0.40 and 0.65 m, respectively, in 1 study ). 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 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 the soccer player from the alternative or supplementary tactic of plyometric training. Plyometric exercise involves stretching the muscle immediately before making a rapid concentric contraction. The combined action is commonly called a stretch-shortening cycle (SSC). Nevertheless, the 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 the 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 the vertical jump height (p < 0.05) of youth soccer players, but their 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 the 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 the contradictory nature of existing information on the efficacy of plyometric training (12), our aim was to examine the effect of adding a combined hurdle and depth jump program to the normal in-season regimen of experienced soccer players. We hypothesized that 8 weeks of biweekly plyometric training would enhance leg PP, jump height, and sprint running velocity relative to players who maintained their normal in-season regimen.
Experimental Approach to the Problem
This study addressed the question as to whether 8 weeks of biweekly in-season plyometric training would enhance the physical performance of soccer players relative to their 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 in-season regimen) (Gc; n = 11). Two weeks before definitive testing, 2 familiarizations sessions were held. Definitive measurements began 4 months into the competitive season. Data were collected before enhanced training, and after completing the 8-week trial. The protocol included a force-velocity test to evaluate leg PP, maximal pedaling velocity (V0) and maximal force (F0); SJ and CMJ to assess leg jump power, velocity and leg force; a 40-m sprint that evaluated velocity during the 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 the same time of the 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 the performance tests.
All procedures were approved by the Institutional Review Committee for the 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 the experimental protocol and its potential risks. Subjects were told that they could withdraw from the trial without penalty at any time. Twenty-three male players were drawn from a single regional soccer team (age 19 ± 0.7 years, body mass 70.5 ± 4.7 kg, height 1.75 ± 0.06 m, body fat 14.7 ± 2.6%). Their mean soccer experience was 7.2 ± 1.2 years. All 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. The 23 individuals were randomly assigned between 2 groups: plyometric training (Gex; n = 12; age 19.1 ± 0.7 years, body mass 70.3 ± 5.5 kg, height 1.76 ± 0.06 m, body fat 14.7 ± 3.2%) and control (Gc; n = 11; age 19.0 ± 0.8 years, body mass 70.6 ± 4.2 kg, height 1.74 ± 0.06 m, body fat 14.6 ± 1.7%).
The study was performed over an 8-week period, from January to March. All subjects had engaged in the standard training regimen from the beginning of the football season (September) until the end of the study (March). Before the competitive season (August), all subjects (Gex and Gc) were 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 (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 the period January through March, the experimental group supplemented this standard regimen by a specific plyometric program. All subjects also engaged in weekly school physical education sessions; these 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 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 measurements or 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 and sprint running were undertaken on day 2.
Squat and countermovement jumps: 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 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 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 the 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 (V0) and maximal force (F0) were calculated using an accepted regression equation (2,34).
Anthropometry: Standard equations were used to predict body fat from the biceps, triceps, subscapular, and suprailiac skinfolds (40). Muscle volumes for the thigh and the leg were estimated from multiple measurements of skinfolds, citcumferences, and diameters for the 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 the sprint from a standing position to allow estimation of velocity during the first step. Video cameras (Sony Handycam, DCR-PC105E, © 2003 Sony corporation, Tokyo, Japan) were placed perpendicular to the running lane. The first camera filmed the first 5 meters, and the second the phase of maximal running velocity (from 35 to 40 m) (9,10,23). Recorded data included the average velocity during the first step (VS), the first 5 m (V5m) and between 35 and 40 m (Vmax). Two trials were separated by at least 5 minutes, the highest of the paired values being retained. Software (Regavi & Regressi, Micrelec, Coulommiers, France) converted hip displacements to the corresponding velocities (VS, V5m, and Vmax). The technique, including the reliability of the camera and data processing software, has previously been detailed (9).
Details of plyometric training: Subjects in both experimental and control groups avoided any training other than that associated with the soccer team. Each Tuesday and Thursday for 8 weeks, Gex supplemented the standard regimen with plyometric training, performed immediately before their standard training sessions (Table 1). Plyometric sessions began with a 15-minute warm-up and lasted for some 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 the maximal possible height with a minimal ground contact time. 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 (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).
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 × 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 − pretraining value]/pre training value) × 100. Pearson product-moment correlations determined relationships between braking force and pedaling velocity. The reliabilities of sprint velocities (VS, V5m, 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, whether a positive or a negative difference was seen (i.e., a 2-tailed test was adopted).
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 (W·kg−1) (Table 4, p < 0.01); however, there 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 these 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 V5m and Vmax; p < 0.01 for VS) (Table 6).
The main finding from this study is that, in accordance with our hypothesis, the supplementary plyometric training program increased several measures of potential soccer playing performance, including the absolute (W) and the relative (W·kg−1) PP of the 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 these 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); they observed increases in both SJ and CMJ scores relative to controls (p < 0.05), but the PP of the SJ relative to body mass (W·kg−1) did not improve. Our results accord with these findings (Table 5). In contrast, the PP of the CMJ was significantly higher for Gex than for Gc. This may be because the CMJ involves an SSC and is thus very similar to the 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 the CMJ parameters (jump height, velocity, absolute [W], and relative power [W·kg−1]) tended to increase more than values for SJ (Table 5), again reflecting similarity between the CMJ and plyometric training. Both CMJ absolute (W) and relative (W·kg−1) power were significantly improved after plyometric training, but the peak force showed no statistically significant change; this implies that the 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 (W·kg−1) (4.9, vs. 8.3 and 9.3% W and W·kg−1, respectively) (Table 3). Likewise, the 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 W·kg−1, 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 the cycle ergometer data (Table 5) (PP gains of 4.6 and 6.2% for W and W·kg−1, respectively, dropping to 1.4% W/Lm); indeed, gains of leg muscle volume seem the main determinant of the gain in PP as assessed by the force-velocity test.
Several previous studies have suggested that plyometric training can enhance sprinting ability because it uses the SSC. Mero et al. (24) found a close relationship (p < 0.001) between the rise of the 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 the first study to investigate the effect of short-term supplementary plyometric training on the sprint performance of soccer players. However, previous research has demonstrated that the 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 the 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 they used a similar training program to us. These discrepancies may reflect differences in methodology or the fitness level of the subjects (physical education students vs soccer players). The meta-analysis of De Villarreal et al. (12) concluded that subjects with the most sport experience showed the greatest increases in vertical jump height. This could also be true of sprint performance, explaining some of the discrepant results. Differences in the training protocol may also be a factor. In the study of Herero et al. (16), training consisted of horizontal and drop jumps continued 2 d· wk−1 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 the fastest motor units, increased nerve conduction velocity, frequency or degree of muscle innervations, and increased ability to maintain muscle recruitment and rapid firing throughout the sprint (1). Wilkerson et al. (38) showed that 6 weeks of preseason plyometric jump training improved neuromuscular attributes in 11 basketball players. In the present study, we did not assess neuronal adaptations, but we can assume from earlier studies that the plyometric training induced neuromuscular adaptations and that these adaptations contributed to the observed gains in sprint performance.
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 in-season technical and tactical male soccer training sessions to enhance the 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 further checking). In addition to effects stemming from the observed increases in muscle volume, there are many potential neuromuscular explanations of the response to plyometric training, and these merit further investigation. As the mechanisms become more fully understood, even larger gains of performance may be realized for a similar increase of training volume.
The authors would like to thank the “Ministére de l'enseignement supérieur et de la Recherche Scientifique, Tunisia” for financial support.
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