Introduction
It is well documented that developmentally appropriate, well-supervised resistance training interventions are safe and effective in stimulating positive adaptations on a range of physical performance measures in children and adolescents (21 , 22 21 28 19 7 26 , 36 11 , 37
Previous meta-analyses have attempted to examine the effects of resistance training on muscular strength (3 2 2 34 , 35 10 20 , 28 , 31
Sprinting and jumping ability are fundamental locomotive skills that form part of the athletic motor skill spectrum (23 33 12 32 32 2 , 3
Therefore, the purpose of this study was to determine the effectiveness of different resistance training modes (traditional strength training, plyometric training, or combined training) on measures of neuromuscular performance (squat jump height, reactive strength index, acceleration, and maximal running velocity) in boys of different maturational status. We hypothesized that boys who were pre-PHV would respond more favorably to plyometric training, whereas boys who were post-PHV would show a greater training response to traditional strength training or combined training.
Methods
Experimental Approach to the Problem
A between-group repeated measures design was used to examine the effects of different resistance training programs on measures of sprinting and jumping. Pre- and post-PHV male youth were placed within 1 of 3 experimental groups (traditional strength training, plyometric training, or combined training) or a control group. The experimental groups participated in their respective training programs twice weekly for 6 weeks, whereas the control group completed their regular physical education lessons. The training period of 6 weeks was selected to match the typical duration of a school term while also reflecting a recognized mesocycle duration (13
Subjects
Eighty young male school children (n = 40 pre-PHV and n = 40 post-PHV) from a local secondary school in the United Kingdom volunteered to participate in the study and were subsequently divided into 1 of 3 training groups (traditional strength training, plyometric training, or combined strength and plyometric training) or a control group (n = 10 × pre-PHV and 10 × post-PHV per group). Participant characteristics per maturation group and training intervention are presented in Table 1 . All participants had previously been engaged in physical education–based activities; however, they were not involved in any formalized strength and conditioning program. The training groups completed 2 training sessions per week for 6 weeks instead of their regular physical education classes. Conversely, the control group continued with their physical education curricula. Parental informed consent and participant assent were obtained in advance of the study, and ethical approval for the research was granted by the University Research Ethics Committee in accordance with the Declaration of Helsinki.
Table 1: Descriptive statistics for anthropometrics per group.*
Testing Procedures
Before the start of the intervention period, all participants took part in a familiarization session, which provided opportunities to practice both jumping and sprinting test protocols. Participants were allowed to complete as many practice trials as required to ensure they fully understood the protocols and could demonstrate consistent technical execution as determined by the principal investigator. For the purposes of the actual test sessions, participants completed a standardized 10-minute dynamic warm-up inclusive of 3 minutes of submaximal multidirectional running and 7 minutes of light dynamic mobilization and activation exercises targeting the main muscle groups of the upper and lower extremities. After the warm-up and practice attempts of the test protocols, participants completed the battery of tests in the following order: anthropometrics, squat jump test, 5-maximal rebound test, and 10 m and flying 20 m sprint tests. For each test, participants completed 3 trials, with the best of 3 trials being used for further analyses. Two- and 5-minute rest periods were given between each trial and test, respectively, to limit the effects of fatigue on consecutive efforts.
Anthropometrics
Standing height (centimeters) and seated height (centimeters) were measured using a stadiometer (SC126; Holtan, Wales), whereas body mass (kilograms) was measured using a balance beam scale (Seca 700; seca gmbh, Hamburg, Germany). These data were then incorporated into a sex-specific regression equation (equation 1) to predict whether participants were either pre- or post-PHV (29
Jump Protocols
Squat jump height (centimeters) and reactive strength index (millimeter per millisecond) were calculated from a squat jump and a 5-maximal rebound test, respectively, both of which were performed on a mobile contact mat (Smart Jump; Fusion Sport, Queensland, Australia). Both protocols have been shown to be a valid and reliable means of assessing neuromuscular performance in youth (squat jump, intraclass correlation coefficient = 0.93; reactive strength index, intraclass correlation coefficient = 0.90; (24 24 8 24
Sprinting Protocols
Sprint times were recorded using wireless timing gates (Smart Speed; Fusion Sport, Brisbane, Australia) in an indoor sports hall. Data were instantaneously collected via a handheld PDA (iPAQ; Hewlett Packard, Palo Alto, CA). Acceleration was measured over 0–10 m with a stationary start from a line 30 cm behind the first timing gate. Maximal running velocity was measured over a 20-m distance with a flying start.
Training Programs
Training took place twice per week for 6 weeks, and training sessions were designed and implemented by a fully accredited strength and conditioning coach. Training sessions were separated by at least 48 hours to enable full recovery. Within each session, a fully qualified physical education teacher was present, which enabled a staff-to-pupil ratio of 1:10. To be included in the final analyses, participants were required to complete at least 80% of the total training sessions within their respective program. Correct technical execution was stressed at all times throughout the program with relevant feedback provided on an individual basis; whereas intensity was never increased at the expense of technical competency. In the event of participants being unable to competently perform any given exercise, relevant exercise regressions were prescribed on an individual basis. Within all training programs, training sessions lasted no longer than 60 minutes and prescribed inter-set rest periods ranged between 1 and 2 minutes dependent on the relative intensity of the exercise, an approach that is commensurate with recommended guidelines for youth resistance training (21
Traditional Strength Training Group
Within traditional strength training sessions, participants completed 3 sets of 10 repetitions of a barbell back squat, barbell lunge, dumbbell step up, and leg press. To enable the prescription of individualized training intensities, 10 repetition maximum (10RM) loads were calculated for participants in the traditional strength training group before the start of the training period using a protocol previously identified in the literature (16
Plyometric Training Group
Plyometric training prescription included a combination of exercises that were geared toward developing both safe jumping and landing mechanics (e.g., drop landings, vertical jumps in place, single-leg forward hop and stick) and also to stress stretch-shortening cycle activity (e.g., pogo hopping, drop jumps, multiple horizontal rebounds). Within each session, participants were exposed to multiple sets of 4 exercises to enable sufficient repetition to develop motor control programs. The plyometric training program (Table 2 ) was progressed conservatively according to number of foot contacts completed within each session (week 1 foot contacts = 74 per session, week 6 foot contacts = 88 per session).
Table 2: Overview of the plyometric training program.
Combined Training Group
The combined training program involved exposure to 2 traditional strength training exercises (barbell back squat and barbell lunge) and 2 varied plyometric exercises, each session taken from the plyometric training program (Table 3 ). As per the traditional strength training group, individualized training intensities were prescribed based on baseline 10RM loads. Similarly, a 5% increment in external load was selected to progressively overload the traditional strength training exercises, whereas plyometric exercises were progressed according to total foot contacts per exercise per session.
Table 3: Overview of the combined training program.
Statistical Analyses
Descriptive statistics (means ± SD ) were calculated for all performance variables for both pre- and posttraining intervention data. Differences in all performance variables were analyzed using separate 2 × 4 × 2 (time × training group × maturity) repeated measures ANOVA, where “time” denotes pre- to posttraining data, “training group” represents plyometric training, traditional resistance training, combined training, or control groups, and “maturity” refers to pre- vs. post-PHV. Sphericity of data was tested by Mauchly's statistic, and where violated, Greenhouse-Geisser adjustment was used. Bonferroni and Games-Howell post hoc tests were used to determine the origin of any between-group differences when equal variance was or was not assumed, respectively. Effect sizes were calculated for all performance variables in each training group and assessed using the magnitude of effect sizes according to Cohen's d statistic. Descriptive statistics and repeated measures ANOVA were computed using SPSS V.20 (SPSS Inc., Chicago, IL, USA), with statistical significance for all tests set at an alpha level of p ≤0.05.
Inferential statistics were used to examine the meaning of differences in the training response between pre- and post-PHV groups for each training modality. The smallest worthwhile effect was used to determine whether the observed changes were considered negative, trivial, or positive. The smallest worthwhile effect was calculated as 0.20 of the pooled between-group SD before training (1 14 15
Results
Mean changes in sprint and jump performances, including effect sizes, are displayed in Table 4 for pre- and post-PHV groups, respectively, and where significant, changes in sprint and jump performances in both pre-PHV and post-PHV groups ranged from small (<0.20) to large (>0.90). Irrespective of maturation, none of the control groups made any significant changes in performance over the 6-week training period. Attendance rates for pre-PHV (plyometric training = 91%, traditional strength training = 89%, and combined training = 90%) and post-PHV (plyometric training = 83%, traditional strength training = 88%, and combined training = 87%) experimental groups and both control groups (>82%) were above the predetermined attendance threshold across the intervention period.
Table 4: Changes in running speed and jump performances for pre-PHV subjects after 6-week training intervention (mean ± SD ).*
Significant main effects in acceleration and maximal running velocity were reported for time and maturity. For both indices of sprinting, post-PHV boys demonstrated faster performances after the training intervention period. For acceleration, significant interactions were found for time × maturity, time × training group, and time × maturity × training group. Acceleration significantly improved in all 3 training groups from within the pre-PHV cohort and in the resistance and combined training groups within the post-PHV cohort. Analysis of maximal running velocity data revealed a significant interaction for time × training group. Maximal running velocity significantly increased in the plyometric and combined training groups of both pre- and post-PHV cohorts.
Analysis of squat jump and reactive strength index data showed main effects for both time and maturity. For both jumping variables, post-PHV youth demonstrated greater performances after the training intervention period. Significant interactions were found for time × maturity and time × training groups for both squat jump and reactive strength index. Additionally, there was a significant time × maturity × training group interaction for squat jump performance. Significant improvements in squat jump height and reactive strength index were seen in all pre-PHV training groups. In the post-PHV cohort, squat jump height increased significantly in the resistance and combined training groups, whereas reactive strength index improved significantly in the plyometric and combined training groups.
Although within-group analysis showed that all training groups improved some aspect of neuromuscular performance in response to their specific training interventions, significance testing failed to determine any significant differences in training response between pre- and post-PHV groups. Nearly all the differences in training responses between pre- and post-PHV cohorts were not significant and “trivial” for traditional strength training, combined training, and control groups. However, pre-PHV children who completed the plyometric training intervention showed changes in acceleration and squat jump height that were “very likely greater” than post-PHV youth who also followed the plyometric training intervention (Figure 1 ).
Figure 1: Mean difference (90% confidence interval) between pre- and post-peak height velocity groups in 10 m sprint time, 20 m sprint time, squat jump (SJ) height, and reactive strength index (RSI) across different training interventions. Training interventions are presented as PLY = plyometric training; TST = traditional strength training; COM = combined training; and CON = control group. The gray shaded area represents the smallest worthwhile effect. Magnitude-based inferences are represented by U = unclear; T = trivial; VL-N = very likely negative; and VL-P = very likely positive.
Discussion
From the results of this study, it was observed that boys, both pre- and post-PHV, were able to make significant improvements in jumping, sprinting, or both qualities after a range of 6-week resistance-based training programs. Irrespective of maturation, none of the control groups showed any significant changes in performance across the intervention period. Plyometric training seemed to stimulate the greatest gains in pre-PHV children, with significant improvements (range of percentage or effect size) reported for all sprinting and jumping variables. Across all pre-PHV training groups, effect sizes were greater for changes in jumping versus sprinting performances. For the post-PHV cohort, combined training seemed to be the most effective in eliciting change across all performance variables, with moderate to large effects reported in acceleration, maximal running velocity, and squat jump height. Inferential statistics showed that for almost all variables, comparable performance changes between pre- and post-PHV boys were trivial after both resistance and combined training interventions. However, after plyometric training, changes in squat jump and acceleration performances were significant and very likely greater in pre-PHV children versus their post-PHV peers.
It should also be noted that in addition to the current study showing beneficial effects for all resistance training modes, performance gains were achieved without any occurrence of musculoskeletal injury. This finding provides further support to recent recommendations from international consensus statements that children should participate in a varied, technical competency driven, and age-appropriate strength and conditioning program to facilitate athletic development (4 , 21
Developmentally appropriate strength training (28 7 26 9
Of greater interest to the youth practitioner, however, was the significant time × training group × maturity interaction for both squat jump and acceleration performances. Furthermore, magnitude-based inferences confirmed that in response to the plyometric training intervention, pre-PHV children had improvements in acceleration and squat jump height that were very likely greater than post-PHV children. Almost all other between-maturity group differences were nonsignificant and trivial, irrespective of training intervention. Intuitively, this suggests that the pre-PHV boys responded more favorably to the plyometric training program. These findings may reflect the process of “synergistic adaptation,” which refers to the symbiotic relationship between specific adaptations of an imposed training demand with concomitant growth and maturity-related adaptations. It is acknowledged that appropriately prescribed plyometric training enhances stretch-shortening cycle function in youth (26 18 5 , 30 , 34
Squat jump and acceleration performances are indicative of slower stretch-shortening cycle activity (6 25 17 , 25
Although magnitude-based inferences indicated that the post-PHV boys failed to show an enhanced training response compared with the pre-PHV children for any of the performance variables, it is worth noting that statistical significance testing showed that the training mode that led to the largest and highest number of significant performance changes was combined training. Combined training incorporates both plyometric and traditional strength training exercises, and although speculative, it could be suggested that the stimulus of this training mode more closely reflected the internal milieu of the post-PHV participants. Performance of plyometrics and traditional strength training differs in the time available in which to produce force. Plyometrics involve rapid movement speeds and high rates-of-force development, whereas traditional strength training allows for much longer contraction times to attain higher peak force outputs. Developmentally, post-PHV boys will experience morphological changes that facilitate force generation (e.g., increased motor unit size and pennation angles) in addition to continued neural adaptations as a consequence of maturation (27 32
In conclusion, to acutely improve vertical jump and acceleration capacities, pre-PHV children seem to benefit more than post-PHV children from plyometric training that requires high rates-of-force development and high-velocity movement speeds. Post-PHV children seem to make greater adaptations from combined training, which uses a greater range of exercises that stress different regions of the force-velocity continuum. Although speculative, these specific training responses seem to be age and maturity related, respectively, reflecting the natural adaptive processes experienced by both pre- and post-PHV children. We suggest these concomitant adaptations may reflect the process of synergistic adaptation.
Practical Applications
The novel findings of the study suggest that when seeking to induce specific acute adaptations (6 weeks) in vertical jump and acceleration capacities in boys who are pre-PHV, practitioners may benefit from devoting increased training time to plyometrics. Alternatively, post-PHV children may benefit more from exposure to a combination of plyometric and traditional strength training methods. Given the short-term nature of the current study, it should be stressed that the resistance training stimulus should be changed periodically to facilitate continued progressive neuromuscular adaptation. Thus, although a focus on plyometrics may initially provide a preferential training response for pre-PHV boys, practitioners should routinely change the primary training mode to facilitate long-term adaptation. Practitioners must ensure that youth of all ages are prescribed varied, periodized, and developmentally appropriate training programs. Furthermore, rather than an independent entity, resistance training should be a component of an integrated approach to youth physical development, which targets multiple physical fitness qualities and aligns with the goals of long-term physical development strategies (22
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