Plyometric 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 performance of both the upper and the lower limbs after an 8-week 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 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.
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 anthropometrically from external dimensions, the thickness of overlying fat, and 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. Participants maintained their normal intake of food and fluids, but they abstained from physical exercise for 1 day, drank no caffeine-containing beverages for 4 hours, and ate no food for 2 hours before testing. 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 and international laws and regulations. Informed consent was gained 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. Parental/guardian consent was also obtained for all participants involved in this investigation. Twenty-seven male young athletes (age ranges 10 to 14 years old) were drawn from a single regional training center; all were preparing for track events, such as the 100-m sprint or running endurance, such as 3,000 m events. Their mean experience of training was 3.4 ± 0.6 years. All were examined by the regional center 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 = 14; age: 11.7 ± 1.0 years; body mass: 43.0 ± 16.6 kg; height: 1.58 ± 0.2 m; body fat: 12.4 ± 4.6%; C; n = 13; age: 12.1 ± 1.0 years; body mass: 38.1 ± 4.1 kg; height: 1.54 ± 0.03 m; body fat: 13.3 ± 4.3%). Inclusion criteria ensured the participants were male, free of any cardiovascular or musculoskeletal disease, and in pubertal stage 1 or 2 as judged by pubic hair growth and genital development (31). Drawings of the 5 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
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 month. The standard training sessions, lasting 50 minutes, included skill activities (agility and coordination exercises) at various intensities, continuous or intermittent running at 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 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.
Subjects were familiarized with circuit training for 2 weeks before beginning either the initial 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, CMJ, and DJ followed by the force-velocity test. Anthropometrical assessment, the multiple-5-bound test (MB5) and sprinting, were undertaken on day 2.
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 90°, avoiding any downward movement, and performed a vertical jump by pushing upward, keeping their legs straight throughout. The CMJ began from an upright position, with the subject making a rapid downward movement to a knee angle of ∼90° and simultaneously beginning to push off. During the DJ, subjects were asked to rebound from a 0.3-m height box, immediately making a vertical jump. During all 3 jumps, subjects kept their hands on their hips and jumped as high as possible. One minute of rest was allowed between trials, with the highest of 3 jumps on each test being used in 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·kg−1) were then estimated as follows (2):
where h is jump height; g is gravitational acceleration (9.81 m·s−2); 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).
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):
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:
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).
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 corresponding velocities (VS, V5m, and Vmax). The reliability of the camera and the data processing software has been reported previously (4).
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, Wednesday, and Friday for 10 weeks, the experimental subjects added a period of plyometric training before beginning their habitual training session with the control group. Plyometric sessions began with a 15-minute warm-up and lasted for some 20 minutes. Subjects were instructed to perform all exercises with maximal effort. Each jump was performed to reach the maximal possible height, with minimal ground contact time (bouncing jump). Both hurdle and DJ were performed with small angular knee movements; the ground was touched with the balls of the feet only, thereby specifically stressing the calf muscles (23). Each set of hurdle jumps consisted of 10 continuous jumps over hurdles spaced at 1-m intervals. Each set of DJ comprised 10 maximal rebounds after dropping from a 0.3-m box, with an interval of 5 seconds between rebounds (23).
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 analyses of variance with repeated measures (group × time). If a significant F value was observed, Scheffé's post hoc procedure was applied to locate pairwise differences. Statistical power values were calculated and ranged between 0.32 and 1. Percentage changes were calculated as ([posttraining value − pretraining value]/pretraining value) × 100. Comparisons between initial and final tests used nonpaired Student t-tests. Pearson product-moment correlations determined relationships between braking force and pedaling velocity. The reliabilities of sprint velocities (VS, V5m, and Vmax) and vertical jump height (SJ, CMJ, and DJ) measurements were assessed using intraclass correlation coefficients (33); 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).
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 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).
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 7-week 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 of the subjects reached their maximal velocities over the 20- to 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 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.
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 in-season program for young track athletes to enhance their performance potential and would recommend this approach to strength and conditioning professionals.
The authors would like to thank the “Ministry of Higher Education and Scientific Research, Tunis, Tunisia” for financial support.
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