Agility is a motor ability that is important to success in sports. It is defined as a high-speed action that involves a rapid change of direction (COD) in response to a stimulus (34). Effective agility performance is dependent on several factors and inputs including physical (strength and conditioning), cognitive (motor learning), and technical (biomechanics) elements (34). Change of direction ability refers to a movement where no immediate reaction to a stimulus is required. Therefore, the COD ability is preplanned, and is influenced by strength, jump, and sprint performance (4,37). Because of the strong association between these physical demands (i.e., strength, sprint, and jump performance) and COD ability (3,4,34), it seems that an improvement in these variables may enhance COD ability.
Plyometric training (PT) is a popular training method to enhance strength (33), power (30), sprint performance (31), and COD ability (23,26,35). It commonly includes quick and powerful movements involving the muscle stretch-shortening cycle (SSC) (12). The SSC entails the storage of elastic energy during the initial stretch, which contributes to a potentiation of force during the subsequent shortening of the muscle (12). The ability of athletes to use the SSC may positively affect sprint (31), strength (33), jump (30), and COD ability (3).
In a recent review of research on adult athletes (3), a 2–3 d·wk−1 of PT for 6–8 weeks with moderate-to-high intensity was recommended to induce meaningful gains in COD ability (effect size [ES] = 0.96). Plyometric training also may have positive effects on COD ability, in youths (2,6,7,9,10,17,21–27,32,35,36). For example, Ramirez-Campillo et al. (24) examined the effects of 7 weeks (2 d·wk−1) of depth jump (DJ) PT on COD ability (i.e., L-run) in soccer players with a mean age of 10 years and found meaningful improvements (ES = 1.03, −0.4 seconds). Recently, Hammami et al. (10) investigated the effects of 8 weeks (2 d·wk−1) of hurdle and DJ PT on repeated COD ability in soccer players with a mean age of 15 years and reported meaningful improvements (ES = 0.66, −1.5 seconds). With regard to training at different ages, it seems that maturation plays an important role in performance gains in response to training. Lloyd et al. (13) reported that youths between 10 and 11 years of age showed accelerated SSC development, a phenomenon that continued near the time of peak height velocity (PHV). However, the effects of intervention studies incorporating PT on COD ability in subjects in different age groups are unclear (16). Although the effects of PT on COD ability in youth athletes from 10 to 12.9 (PRE PHV), 13 to 15.9 (MID PHV), and 16 to 18 (POST PHV) years of age have been reported (2,6,7,9,10,17,21–27,32,35,36), comparisons across these age groups are scarce. Therefore, the purpose of this systematic review and meta-analysis was (a) to describe the effects of PT on COD ability and (b) to compare the effects of PT on COD ability in PRE, MID, and POST PHV youths.
Experimental Approach to the Problem
In this study, the meta-analysis was performed in different steps, grounded in previous recommendations (3,20).
This meta-analytical review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement (19) and was approved by the Institutional Review Board of the responsible Department, from Payame Noor University. Literature searches of PubMed, Google Scholar, MEDLINE, SPORTDiscus, Science Direct, and Web of Science databases were conducted in June 2016. In addition, manual searches were performed in journals that are relevant to sports science as well as references lists obtained from gathered articles. The search terms included “agility,” “agility performance,” “agility times,” “change of direction,” “plyometric training,” “plyometrics,” “neuromuscular training,” “explosive training,” “power training,’ “jump training,” “stretch shortening cycle,” “youth,” “young,” adolescent, “maturation,” “pubertal,” “trainability,” “children,” “pediatric,” and “age.” After eliminating duplicates, the search results were screened by 2 investigators against the inclusion criteria. After subsequent screening, 16 articles were obtained for inclusion in the final meta-analysis. Figure 1 presents the steps taken to eliminate inappropriate studies for a variety of reasons.
The inclusion criteria for studies in this review were as follows (5): (a) experimental trials published in English-language refereed journals with full-text availability; (b) healthy participants; (c) interventions that only used PT focused on lower limb exercises; (d) the inclusion of preintervention and postintervention measurements of COD ability, and (e) male participants aged 10 to 18 years.
The studies were read and coded by 2 of the researchers with a focus on the following variables: descriptive information (age, body mass, height, and group size); sport activity (physically active, soccer, rugby, basketball, tennis, and none); type of PT intervention (aquatic PT, land PT, mat PT, grass PT, sand PT, vertical PT, horizontal PT, bilateral PT, unilateral PT, progressive PT, and nonprogressive PT); type of plyometric drill (DJ, countermovement jump, vertical jump, standing long jump, hurdle jump, and mixed model [combination of different plyometric drills]); frequency of weekly sessions, program duration, outcome measurements of COD ability (i.e., T test, Illinois agility test, shuttle run, 505, L-run, 10 × 5, 10-m, and zigzag), and findings (i.e., magnitude of COD ability changes). Moreover, the studies were separated into 3 age-range categories: PRE, MID, and POST PHV, according to previous recommendations (13,20). Details about the coding of studies are presented in Table 1.
For each COD ability test, the ES was calculated using Hedges and Olkin's g (11), with the following formula (1): g = (Mpost−Mpre)/SDpooled, where Mpost is the mean at posttest, Mpre is the mean at pretest, and SDpooled is the pooled of the measurements (2) as recommended previously (3,30,31,33):
Effect size was the standardized value that indicated the magnitude of training effects between groups or experimental conditions in a study. It has been suggested (3,11) that ES should be corrected for the magnitude of the sample size of each study. Therefore, correction was performed using the following formula (3): 1−3/(4m−9), where m = n−1, as proposed by Hedges and Olkin (11).
Data are presented as the mean ± SDs. To determine the effects of PT on COD ability, the ES and time gain (TG) are reported. To compare the magnitude of improvements across groups an analysis of variance (ANOVA) was used (3,30,31,33). Pearson product-moment correlation coefficient (r) was used to determine the relationship between COD ability ESs and TG with training variables. The significant level of each test, α, was set at p ≤ 0.05. Threshold values for assessing the magnitudes of the ESs were ≤0.35, 0.36–0.80, 0.81–1.50, and >2.0 for trivial, small, moderate, and large, respectively (3,28).
Subjects' basal characteristics (mean ± SD) are presented in Table 2.
The effects of PT on COD ability ESs in PRE, MID, and POST age groups are presented in Figure 2.
When the PRE, MID, and POST age groups were combined (All), the experimental groups showed greater (p ≤ 0.05) improvements in COD ability (ES = 0.86; TG = −0.61 seconds) compared with the control groups (ES = −0.07; TG = 0.13 seconds). Similarly, in each age group, greater (p ≤ 0.05) improvements in COD ability were observed in the experimental groups compared with the control groups (Table 3). However, experimental PRE, MID, and POST age groups did not show differences in ES (p = 0.41) or TG (p = 0.11) after PT (Figure 3).
Results of the ANOVA indicated that no significant differences existed between any of the training variables across groups, except training frequency (POST vs. PRE, p = 0.041) (Table 4).
There were no relationships between training duration (weeks) (r = 0.118 and r = −0.74), training frequency (sessions per week) (r = 0.436 and r = −0.624), number of total jump (repetitions) (r = −0.304 and r = −0.010), rest between sets (s) (r = −0.030 and r = −0.105), and rest between exercise sessions (hours) (r = 0.064 and r = −0.180) with COD ability ESs and TG, respectively (Table 5). A relationship was observed both between training frequency (sessions per weeks) and COD ability ES gains (r = 0.436) and TG (r = −0.624) and between intensity of plyometric exercise (r = 0.493) and COD ability ES gains.
The purpose of this systematic review and meta-analysis was to describe the effects of PT on COD ability and to compare the effects of PT on COD ability in PRE, MID, and POST PHV youths. The main results of this study were that PT enhances COD ability in youths and 2 training sessions per week applied for 7 weeks with moderate intensity seems to be affective dose (ES = 0.86). Moreover, performing a total-program volume of 1,400 jumps (i.e., 200 per week), with 75 seconds and 48 hours of rest between plyometric exercises and training sessions, respectively, could be meaningful programing to enhance COD ability in youths. When age groups were compared, no significant differences were found across the PRE, MID, and POST PHV groups in COD ability gains. However, older youths (MID and POST) showed a meaningful tendency toward greater adaptive responses to PT compared with younger youths (PRE).
Regarding the effect of PT on COD ability in youths, PT improved COD ability in PRE, MID, and POST youths compared with controls (Table 3), which is consistent with the findings of previous researchers (3,10,23,26,35). In youth team-sport athletes, improvements in COD ability might be transferred to key explosive competitive actions (24,25,36). Potential mechanisms underpinning the improvements in COD ability after PT might be related to neuromuscular adaptations (1,2), such as enhanced motor unit recruitment and firing frequencies (2). These physiological changes may lead to a greater rate of force development and power output and, consequently, COD ability improvement after PT (34,36). In addition, PT may reduce ground contact times through an increase in muscular force output and movement efficiency, which positively affects COD ability (38). Moreover, PT may improve the eccentric strength of the thigh muscles, a prevalent component in COD during the deceleration phase of impulsive movements (34), which may involve a rapid switch from eccentric to concentric muscle action in the leg extensor muscles.
Regarding the effects of age on PT-induced changes in COD ability, older youths (MID and POST) showed a meaningful tendency toward greater COD ability changes in responses to PT compared with younger youths (PRE), although it was not statistically significant. It might be that older youths express greater plasticity after PT in muscle size, transition from type I to type II muscle fibers, muscle contractile ability, fascicle angle, motor unit recruitment, intermuscular coordination, stretch-reflex excitability, utilization of the SSC properties, and neural drive to agonist muscles (15). However, further studies are needed to clarify which of these potential underlying physiological mechanisms may help to explain the results observed in this systematic review and meta-analysis more effectively.
In the PRE (i.e., 10–12.9 years of age) group, greater improvement in COD ability was observed after PT in the experimental group compared with the control group (ES = 0.68 vs. 0.12, TG = −0.4 vs. −0.01, respectively). This finding is aligned with those of previous research on PRE athletes (9,23–25). This consistency is probably due to the inclusion of similar training program variables across studies (3).
In the 2 older groups in this meta-analysis, the experimental groups showed greater improvement in COD ability compared with the control groups (Table 3). This result is consistent with those of previous research on MID (6,17,26,27,32) and POST athletes (2,22,36). The improvement in COD ability in the MID (ES = 0.95) and POST (ES = 0.99) groups was not statistically significantly greater than in the PRE (ES = 0.68) group. However, a difference in improvement of −0.31 to −0.34 seconds (Figure 3) could be meaningful in a competitive athletic context (2,6,7,9,10,17,21–27,32,35,36). Therefore, coaches working with MID and POST athletes should aim for PT interventions to take advantage of their increased effect on COD ability improvement in this age group. It may be possible that the fastest period of growth that typically occurs at MID allows for greater increases in total, trunk, and leg length; bodyweight; muscle mass, and muscle length, which permits increased tolerance to greater plyometric drills intensity (8) and therefore, PT-induced changes. These growth-related adaptations may be retained in older age (POST). Overall, the results of this study showed minimal differences between the MID and POST groups in COD ability improvement (ES = 0.95 vs. 0.99) after PT. It seems that the marked differences between age groups in COD ability improvements after PT occur between the PRE and MID groups, and the elevation of anabolic hormone concentrations that occur with increased age may attenuate adaptation differences (14,29). However, it seems that adaptive responses to PT are dependent on maturation status. Additional studies are necessary to clarify the maturation-related PT effects on COD ability gains.
Moreover, other potential mechanisms that lead to further enhancements in COD after PT for the MID and POST groups could be due to maturation-related development of the central nervous system and increases in fascicle length (18). Another possible explanation involves the elevation of anabolic hormone concentrations during maturation and their effects during the MID and POST maturation phases. Hormone-related hypertrophy of type II muscle fibers as well as the growth spurt-related increases in muscle coordination and motor unit activation greatly influence COD ability (14,18,29). Moreover, COD ability gains after PT might relate to improvements in muscle strength and power (34).
Some potential limitations are perceived for this review: (a) PT interventions differed across studies (i.e., type of plyometric exercise used, number of exercises performed, number of jumps during training sessions, type of testing, and training duration); (b) as research in the area is lacking regarding measures of subjects' maturity status, the categories used were based on chronological age; (c) an small number of studies and ESs were available. Therefore, caution should be used when generalize the results of this study.
In conclusion, PT significantly improve COD ability in youth subjects (ES = 0.86, TG = −0.61 seconds). In comparison of age groups, older youths showed more adaptive responses to PT in COD ability gains. Therefore, adaptations in COD ability after PT are related to age. To achieve COD ability gains, it seems that 2 training sessions per week with 1,400 jumps (i.e., 100 per session) for 7 weeks at moderate intensity could be a meaningful dose.
Plyometric training can be recommended as an effective training modality for improving agility or COD performance; yet, the positive effects of PT is in relation to several factors including training program design, training level, the specific sport activity, familiarity with PT, program duration, and training volume, or intensity (3). One of the important variables is age or maturation status of subjects. Regarding the results of this meta-analysis improvements in COD ability, performance may be more pronounced in the MID and POST stage of maturation, in line with periods of acceleration in physiological adaptation. Current findings should be taken into account by strength and conditioning professionals working with youth athletes. Two PT sessions per week, with 1,400 moderate-intensity jumps for 7 weeks, seem to be an adequate dose.
The authors disclose funding received for this work from any of the following organizations: National Institutes of Health (NIH); Welcome Trust; Howard Hughes Medical Institute (HHMI); and other(s).
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