There is widespread acceptance by various scientific associations (1,10,27,28,51,59,65) and review articles (14,15,27,36,37) that resistance training (RT) for children can improve muscular strength and endurance while decreasing the severity and incidence of sport injuries (29,37,55). The Canadian Society for Exercise Physiology (CSEP) position stand (10) reiterated this information but also explored more advanced training concepts such as plyometrics, instability RT, periodization, and Olympic-style weightlifting (OWL). Other organizations such as the United Kingdom Strength and Conditioning Association (51) and the NSCA (28) have also advocated that advanced multi-joint exercises such as Olympic-style lifts and plyometrics can be incorporated into a youth RT program. However, Olympic-style lifts involve a more complex neural activation pattern, youth would need a greater learning period with a relatively light load to become competent at the movements. Faigenbaum et al. (35) states that in some countries, children learn these lifts as early as 8 years, but resistance is not added to the bar until they have developed the proper coordination. The possibility of an extended learning period for OWL training might attenuate the strength or power gains compared with traditional RT over the same duration.
According to Haff et al. (40), Olympic-style lifts such as the clean and jerk and snatch lifts can generate some of the highest power outputs. They report in a 100 kg man, that the calculated power of the clean, jerk, and snatch would range from approximately 3000 to 5400 W compared with squats and deadlifts at 1100 W each, typical of power lifters (54). Similar results from 8- and 15-week adult training studies showed that OWL training provided a significant advantage over power lifting (43) and vertical jump (66) training, respectively for vertical jump improvements. In one of the few studies employing OWL in a children's RT program, Ebada (26) instituted a 3-month children (mean age of 13.2 years) for OWL training program resulting in an average 4.9% increase in 9 strength tests (e.g., snatch, clean, squats). An 8-week training program compared OWL and power (e.g., squats, deadlifts) training with high school males averaging 15.9 years of age (20). OWL training exhibited a modest advantage over conventional power training for vertical jump performance. Thus, as might be expected, with training specificity, training programs emphasizing power movements as found with OWL tend to improve power tests such as vertical jumps to a greater extent than strength measures with traditional strength type training programs. Other training programs also emphasize power such as plyometrics, albeit with only body mass as a resistance.
Meta-analyses by Saez de Villareal et al. (61,62) have indicated that plyometric training could procure substantial improvements in strength in both trained and untrained adult men and women. Similarly Johnson et al. (47) report in their meta-analysis that plyometric training with children had large positive effects on jumping and running performance with further evidence to suggest associated improvements in kicking distance balance and agility. However, there has been some reticence to apply these explosive training activities with children. Prior recommendations for high intensity adult plyometric training that stated that the individual should squat at least 1.5 times body weight before performing lower-body plyometrics (60) may have inhibited coaches from implementing plyometric training for youth. The CSEP position stand recommends that plyometric training can be safe and effective for enhancing muscle power in children (10). Studies employing plyometric training programs for youth reported improvements in vertical jump height (31), rebound jump height (56), and running speed (49). Meta-analysis by Behringer et al. (13) on the transfer of RT gains to motor performance in youth reported the highest effect sizes with a combination of plyometric and traditional training programs. However there are no studies, pediatric or adult, that have compared plyometric training to OWL training or conventional/traditional RT.
Thus, it was the objective of this study to compare the training effects of 12 weeks of OWL, plyometric, and traditional RT in 10- to 12-year-old boys. Based on the concept of training specificity (12), it was anticipated that the OWL and plyometric training would provide the greatest power improvements, whereas traditional RT would benefit force measures to a greater extent.
Experimental Approach to the Problem
The objective of this study was to evaluate the training effects of OWL, plyometrics, and traditional RT with children on functional and physiological measures. Sixty-three children (10–12 years) were randomly allocated to a 12-week control, OWL, plyometric, or traditional RT program. Pre- and post-training tests included body mass index (BMI), sum of skinfolds, countermovement jump (CMJ), horizontal jump, balance, 5- and 20-m sprint times, isokinetic force and power at 60 and 300°·s−1. Magnitude-based inferences were used to analyze the likelihood of an effect having a standardized effect size exceeding 0.20.
Sixty-three healthy boys between 10 and 12 years of age recruited from 4 youth Judo and Wrestling development centers affiliated with public primary schools in the area of Tunis, Tunisia volunteered to participate in this study. All participants were from similar socioeconomic status and had the same daily school-training schedules. None of the boys had an athletic background and were not previously involved in any organized strength and conditioning training programs before this experiment. As Judo and Wrestling involves body weight training, which possibly may affect the training outcomes, no specific training was performed by the subjects during the study period. Pubertal stage was determined by self-assessment questionnaire, in which subjects assess their genital and pubic hair development according to the criteria of Tanner (3). Subjects ranged from stage 1 to stage 2 of the Tanner score. The children were randomly allocated into 4 groups (Table 1): (a) OWL training, (b) plyometric training, (c) traditional RT, or (d) a control group without a training program. Groups were matched for age, maturation status, and physical characteristics. All 4 groups were balanced as for the sport (Judo, Wrestling) of the subjects (after minor initial adjustments), and they were shown to be equivalent on all preexperimental measures (p > 0.05), so that post-training differences could not be ascribed to unequal group composition or to preexperimental biases. Because the study period fell during the summer break, the subjects' usual training programme was in the form of games and technical learning, which did not interfere with the experimental conditions.
Parental permission and child assent was obtained after they were informed they could decide whether to be in the study or not, the objectives of the study, possible risks, and benefits. The study was conducted according to the Declaration of Helsinki, and the protocol was fully approved by the Ethics Committee of the National Centre of Medicine and Science of Sports of Tunis (CNMSS) before the commencement of the assessments.
Before the beginning of the experiments, children were examined by a physician in the CNMSS (Tunis), and were assessed as having no injury, chronic pediatric disease, orthopedic limitations, or illness that might impair their ability to execute RT or to perform power tests.
Before the commencement of the study and the initiation of testing, all the subjects completed a 2-week orientation period (3 sessions per week) to become familiar with the general environment, form, and technique on each fitness test used to evaluate force, power and balance, technique for each training exercise, equipment, and the experimental procedures. During this time, the children received consistent instructions on proper techniques for OWL movements, free weight RT, plyometric exercises, and landing from certified strength and conditioning specialists and weightlifting coaches. Technique for OWL exercises was the first priority, with the children using wooden sticks in lieu of a bar progressing to an unloaded aluminum bar. The pedagogic sequence of each session followed the guidelines proposed by Faigenbaum and Polakowski (33) and Lloyd et al. (52). Subjects worked repetitively in modified cleans (from blocks just above the knee: clean pull without explosion, clean pull + shrug, clean pull + jump and shrug) and snatch balance to a full squats, not only to learn the skill, but to work on flexibility as well. Once the overhead squat was mastered, the snatch from the upper thigh (start position of second pull) was implemented. Progression to the snatch and clean from the knee (hang clean and hang snatch) and finally from the ground completed the first sequences of drills or technique (16). Procedures on how to “miss” a lift (respond to incorrect technique) properly were reviewed, and instructions were regularly provided on how to correctly return the bar to the hang or floor position so that participants became automatic in their response to an undesirable bar position (32). Pictures and videos were used to make the instructions appropriate for children. Participants' questions were answered during this time.
Each subject's height and body mass were collected using a wall-mounted stadiometer and electronic scale, respectively. Body mass index was calculated as weight per height squared (kg·m−2). The sum of skinfolds was monitored with Harperden's skinfold calipers (Baty International, West Sussex, England). Body measurements were conducted according to Deurenberg et al. (24) who reported similar prediction errors between adults and young adolescents. Deurenberg's prediction equation used was: Body fat percentage = −22.23 + 26.56 × log (Biceps, triceps, subscapular, and suprailiac). Body mass, height, BMI, and sum of skinfold measures (biceps, triceps, supailiac, and subscapular sites) were also collected after training.
Performance testing occurred before and after the 12-week training period. The testing protocol included assessment of lower-body strength and power (isokinetic force and power, horizontal, and vertical jumps), acceleration (stationary 5-m sprint), maximal speed (flying 20-m sprint run), and static balance (Standing Stork Test). Testing was conducted pre- and post-training at the CNMSS (Tunis, Tunisia).
After the initial baseline testing session, subjects were randomly assigned to one of the four groups (OWL, plyometric, traditional RT, or control). Each experimental group participated in a 12-week group-specific training protocol using one of the three methods (Table 2). The OWL group trained using Olympic-style lift exercises. The plyometric group followed a structured plyometric training program using their body weight as resistance. Both the OWL and plyometric exercises were performed with a ballistic intent concentric contraction (11,12) (contract as forcefully and rapidly as possible). Unlike the traditional RT group, there was no enforced pacing rhythm for the concentric or eccentric segments of the movement. The traditional RT group trained with free weights using slower speed movements. The eccentric, isometric, concentric contraction pacing for the traditional RT group was 1s-1s-1s, respectively. No strength training activities were permitted outside of the supervised training sessions. The control group did not participated in any training program and was limited to their normal daily activities during the entire research project.
Experimental groups followed the 12-week training program with a frequency of 2 sessions per week performed on nonconsecutive days (Monday and Thursday afternoons). Previous investigations have clearly demonstrated that RT twice per week is sufficient for enhancing the muscle strength of children (10,30). A standardized warm-up including jogging, dynamic stretching exercises, calisthenics, and preparatory exercises (e.g., fundamental weightlifting exercises specific to their training program) was provided for all experimental groups before the beginning of each training session. Each training session ended with 5 minutes of cool-down activities including dynamic stretching. To ensure an equal volume of training, each training program session was composed of 4 different exercises and 1 to 3 sets of 8–12 repetitions. As all exercises were either compound lifts or jumps, involving multi-articular movements and multiple muscle groups (Table 2), 4 exercises per group provided sufficient stimuli. Furthermore, the time schedule of the children did not permit a more extensive training routine. All groups performed 8–12 repetitions.
The variables of volume and intensity were selected based on the previous recommendations and training guidelines for pediatric population (10,52). Training volume (i.e., number of sets × repetitions) was altered similarly for the 3 groups. During the first week, all groups trained with 1 set of 12 repetitions and progressed to 2 sets of 12 repetitions during second week. During the third, fourth, and fifth weeks, they altered the volume to 3 sets of 12, 10, and 8 repetitions, respectively. During the final week of the first mesocycle, training volume was reduced to 1 set of 12 repetitions. As it has been indicated (35,52) that when prescribing training volume for the learning stage of OWL, multiple sets and lower repetitions are most effective for young athletes to learn the snatch and clean; the total repetition number for these 2 exercises was performed in multiple sets of 4–6 repetitions using a 5–10 kg youth-sized weightlifting bar with wooden plates. Each program was periodized in 2 similar progressive mesocycles of 6 weeks. The exercises remained the same over the first and second mesocycle of training.
The amount of weight lifted in the training sessions was determined by the ability of each individual (48) established from each subject's 10 repetitions maximum in the selected resistance exercises. This procedure is similar to those prescribed in the literature (42,64). Each subject had to lift their maximum weight for the given number of repetitions while using the correct technique. The amount of weight was increased carefully, and correct technique was the focus. During each training session, instructors reviewed proper exercise technique and made appropriate adjustments in training resistance. The amount of weight was increased at the next training session whether the subject could lift the given weight with the proper technique in the respective exercises variant. If a child could not complete the required number of repetitions sequentially within a set, they were given 30 seconds–1 minute of rest before attempting to complete the set. Throughout the study, all subjects were encouraged to increase the amount of weight lifted and to achieve concentric fatigue within each designated repetition range. Because fatigue can influence the performance of explosive movements and possibly increase the risk of injury (32), and based on previous strength training studies in youth (63), rest intervals between sets within each exercise session were 3 minutes in length for the 3 groups to allow for adequate recovery. All training sessions were directly supervised by certified strength and conditioning specialists and weightlifting coaches who were knowledgeable of pediatric RT guidelines and the pedagogical aspects of teaching weightlifting to school-aged youth. All the subjects completed a minimum of 95% of scheduled training sessions.
The plyometric training group performed the following drills for their training sessions: maximum CMJs, drop jumps, ballistic-type push-ups/clapping push-ups, and medicine ball throws. Traditional strength exercises consisted of squats, lunges, alternate flat and incline chest press, and unilateral shoulder flyes. The pool of exercises for the OWL program included power cleans and snatches (progression model), in addition to shoulder push press and cross body pull with kettle bell/dumbbell. There were no injuries reported over the training program period. All training sessions took place after school at the weight fitness venue of each center that was used exclusively by the subjects in this study on designated training days. Throughout the training period, children typically exercised in groups of 6–8, and an instructor-to-participant ratio of at least 1:4 was maintained. In the course of training, there was constant concern to ensure safety and maintain sufficient hydration level, and to encourage all children to do their best to achieve the best results. Clear instructions about the importance of adequate nutrition were also provided.
Both groups were tested before and after the 12-week training period for all variables. Each testing session was conducted over 2 separate days with the same test order. Testing was completed at the same time on each testing day for both pre-and post-tests, at the same indoor venue and by the same trained investigators. The participants were asked to wear the same type of clothing and footwear and to avoid vigorous physical activity before or the day of any study procedure. Subjects were prohibited from consuming food, beverages, or any known stimuli (e.g., caffeine) that would possibly enhance or compromise alertness during the period of investigation. Each player was instructed and verbally encouraged to give a maximal effort for each performance test. Performance testing was initiated after a standardized 15-minute warm-up, including submaximal intensity running, dynamic stretching, low intensity forward, sideways, and backward running; several acceleration runs; and jumping at a progressively increased intensity. Throughout all testing procedures, instructor-to-participant ratio of 1:1 was maintained, and uniformed verbal encouragement was offered to all participants.
The maximal voluntary isokinetic concentric strength of the dominant leg was measured at 2 velocity contractions 60 and 300°·s−1 using an isokinetic dynamometer (Cybex NORM; Henley Healthcare, Cybex International, Inc., Medway, MA, USA) according to the previously described procedures (22). The parameters used for analysis were peak torque (Force60 and Force300) and mean power (Power60 and Power300) at 60°·s−1 and 300°·s−1, respectively. Excellent isokinetic reliability measurements with children in our laboratory have been reported elsewhere (intraclass correlation coefficients of 0.928–0.988) (22).
Countermovement jump height was assessed using a portable platform (Quattro-Jump; Kisler, Winterthur, Switzerland) according to the procedure described by Chaouachi et al. (23). Participants were instructed to keep their hands on their hips to minimize lateral and horizontal displacement during performance, to prevent any influence of arm movements on the vertical jumps, and to avoid coordination as a confounding variable in the assessment of the leg extensors' neuromuscular performance (23). Participants were encouraged to perform the eccentric phase of the jump as quickly as possible to maximize jump height. Three trials were performed with approximately 2 minutes recovery, and the best result was used for analysis.
Standing Horizontal Jump
The participant stood stationary with the toes aligned level with the start line and were instructed to push off vigorously and jumped forward as far as possible. Participants were allowed the use of a countermovement with arms and body swing. The distance jumped from the start line at take-off to the point where the back of the heel nearest to the take-off line landed was measured in centimeters using a metal tape measure. The test was repeated 3 times, and the maximum distance achieved was recorded in centimeters and used for analysis (19,21).
Acceleration and maximal running speed were evaluated using a stationary 5-m sprint and flying 20-m sprint. Stationary 5-m sprint involved sprinting 5-m as fast as possible from a stationary start position. Flying 20-m sprint involved sprinting 20-m as fast as possible from a maximal speed start. Time was automatically recorded using photocell gates (Brower Timing Systems, Salt Lake City, UT, USA; accuracy of 0.01 seconds) placed 0.4 m above the ground. Subjects performed 2 maximal attempts, and the best time was retained for analysis.
Static balance was assessed using the Stork stand balance protocol. To perform the Stork stand test (21,57), participants stood with their opposite foot against the inside of the supporting knee and both hands on the hips. On the command, the subject raised the heel of their foot from the floor and attempted to maintain their balance as long as possible. The trial ended if the subject either moved his hands from his hips, the ball of the dominant foot moved from its original position, or if the heel touched the floor. This test was carried out on the dominant leg acting as the standing leg. The test was timed (in seconds) using a stopwatch. The total time in seconds was recorded. The recorded score was the best of 3 attempts. Previous test-retest reliability scores for sprint, vertical and horizontal jumps, and balance measures from our laboratory with similar pediatric population have been high (Typical error of measurement range from 0.3 to 3.2%) (21).
To avoid the shortcomings of research based in null-hypothesis significance testing, magnitude-based inferences and precision of estimation were employed (46). Magnitude-based inferences were conducted to assess the clinical (practical) difference in the training effects on the independent variables between training types. Differences were calculated within each training type (before and after training) and between the changes affected by each training type.
Qualitative descriptors of standardized effects were assessed using these criteria: trivial <0.2, small 0.2–0.6, moderate 0.6–1.2, large 1.2–2.0, and very large >2.0. Effects with 95% confidence limits substantially overlapping the thresholds for small positive and negative effects (exceeding 0.2 of the SD on both sides of zero) were defined as unclear. Clear small or larger effect sizes (i.e., those with >75% likelihood of being >0.20, as calculated by a previously available spreadsheets (44,45)) were defined as substantial. Precision of estimates was indicated with 95% confidence limits, which defines the range representing the uncertainty in the true value of the (unknown) population mean.
All groups increased height to a similar extent (d = 0.24–0.29, all comparisons <75% likely). Changes in sum of skinfolds were found to be trivial (d < 0.7, all comparisons <75% likely) for all groups.
The control condition elicited likely substantial improvements in balance (87% likely, “small”) and Power300 (82% likely, “small”) (Table 1, Figures 1–10). All other effects were likely “trivial” (CMJ, BMI) or “unclear” (time to 5 m, flying 20-m time, horizontal jump, Force60, Force300, Power60, sum of skinfolds).
Plyometric training was very likely (>99%) to elicit substantial improvements in nearly all variables (time to 5 m, CMJ, horizontal jump, balance, Force60, Force300, Power300) with effect sizes ranging from “moderate” to “very large” (Table 1, Figures 1–10). The effect on flying 20-m time and Power60 was “unclear” while BMI was 80% likely to be “trivial.”
Traditional Resistance Training
Traditional RT was >85% likely to elicit substantial improvements in all variables, effects ranging from “small” to “large” (Table 1, Figures 1–10). Only traditional RT elicited substantial changes (96% likely) in BMI increasing by a “small” 1.06 kg·m−2 (95% CL 0.52–1.60).
Changes in BMI with OWL were 80% likely to be trivial (Table 1, Figures 1–10). All other variables were >85% likely to be substantially improved, effects ranging from “moderate” to “very large.”
Between-groups pretest differences were classified as trivial.
Countermovement Jump and Horizontal Jump
Although there was no clearly substantial difference in CMJ between plyometric and traditional RT, OWL was 96% likely to be better than plyometric training (mean difference, lower to upper 95% confidence limits, effect size; 3.2 cm, 0.6–5.8, 0.78) and 93% likely to be better than traditional RT (mean difference, lower to upper 95% confidence limits, effect size; 4.1 cm, 0.2–7.9, 0.71). The only likely substantial difference between intervention groups on horizontal jump was that OWL was 90% likely to be superior to plyometric training (mean difference, lower to upper 95% confidence limits, effect size; 13.5 cm, −0.9 to 27.9, 0.63).
Balance and Anthropometrics
Olympic Weightlifting was 87% likely to elicit substantially greater improvement to balance than traditional RT (mean difference, lower to upper 95% confidence limits, effect size; 5.5 seconds, −0.8 to 11.8, 0.60). Furthermore, plyometric training was 98% likely to be substantially better than traditional RT (mean difference, lower to upper 95% confidence limits, effect size; 9.1 seconds, 2.2–16, 0.86).
Traditional RT was 92% likely to elicit substantially greater increases in BMI than plyometric (mean difference, lower to upper 95% confidence limits, effect size; 0.6 kg·m−2, 0.0–1.2, 0.67) and 90% more likely than OWL (mean difference, lower to upper 95% confidence limits, effect size; 0.6 kg·m−2, 0.0–1.2, 0.64) training. There were no substantially greater between-group changes in height and sum of skinfolds (d = 0.24–0.29, all comparisons <75% likely).
Force and Power
Plyometric training was 84% likely to elicit substantially greater increases in Force60 than traditional RT (mean difference, lower to upper 95% confidence limits, effect size; 13.0 kg, −3.7 to 29.8, 0.54) and 81% likely than OWL (mean difference, lower to upper 95% confidence limits, effect size; 13.8 kg, −5.1 to 32.6, 0.50) training. Plyometric training was 79% likely to elicit substantially greater improvements in Force300 than traditional RT (mean difference, lower to upper 95% confidence limits, effect size; 6.7 kg, −3.1 to 16.4, 0.48).
Traditional RT was 96% likely to be substantially superior to plyometric training at eliciting improvements in Power60 (mean difference, lower to upper 95% confidence limits, effect size; 13.2 W, 2.3–23.8, 0.80). Regarding Power300, plyometric training elicited more power than traditional RT (78% likely; mean difference, lower to upper 95% confidence limits, effect size; 15.8 W, −7.9 to 39.6, 0.47). Olympic weightlifting also elicited more power than traditional RT (76% likely; mean difference, lower to upper 95% confidence limits, effect size; 20.7 W, 12.0–53.4, 0.44).
Time to 5 m (Acceleration) and Flying 20-m Time (Speed)
For the time to 5 m, plyometric training demonstrated the lowest improvement when compared with traditional RT (>99% likely; mean difference, lower to upper 95% confidence limits, effect size; 0.05 seconds, 0.03–0.07, 1.2), and OWL (93% likely; mean difference, lower to upper 95% confidence limits, effect size; 0.04 seconds, 0.002–0.08, 0.70) thought there were no clearly substantial difference between traditional RT and OWL. Plyometric training demonstrated the lowest improvement in flying 20-m time when compared with traditional RT (86% likely; mean difference, lower to upper 95% confidence limits, effect size; 0.12 seconds, −0.03 to 0.27, 0.57), and OWL (81% likely; mean difference, lower to upper 95% confidence limits, effect size; 0.14 seconds, −0.05 to 0.33, 0.50) thought there were no clearly substantial difference between traditional RT and OWL.
The most important finding in this study was that 12 weeks of OWL or plyometric training were generally equal to or more effective for enhancing performance than traditional RT for male youth. In summary, OWL training was likely to provide better improvements than plyometric training for CMJ, horizontal jump, and 5- and 20-m sprint times while exceeding traditional RT for balance and isokinetic Power300. Tricoli et al. (66) reported similar findings with greater relative improvements in CMJ, squat jumps, and 10-m sprint speed after OWL vs. vertical jump training in college-aged males. The many nonsignificant differences in strength and sprint measures between OWL and traditional RT in this study are complemented by similar findings by Hoffman et al. (43) in a 15-week training program comparing OWL and powerlifting in college-aged males. However in their study, OWL provided significantly greater improvements in vertical jump performance than powerlifting training. Conversely, an 8-week training program comparing OWL and powerlifting training in high school boys did not find statistically significant differences in vertical jump height (20), similar to the findings in this study. From our results, we conclude that while all training systems can improve performance of youths, OWL is generally equal to or superior to the other 2 forms of training.
Plyometric training was more likely to elicit better training adaptations compared with traditional RT for balance, isokinetic Force60 and 300, Power300. Adult strength gains were similar with 6-(53) and 12-week (67) plyometric vs. conventional RT training programs (average age of subjects: 22 and 25 years, respectively). However, muscle power increased almost exclusively with plyometric training in the 12-week training program (67). Studies employing plyometric training programs for youth reported improvements in vertical jump height (31), rebound jump height (56), and running speed (49). However, these studies did not attempt to compare plyometric training with other forms of strength or power training. The present findings support the CSEP (10), NSCA (28), and UKSCA (51) position stands which recommended that OWL and plyometrics can be included in children's RT programs to enhance strength and power gains. This is the first study to directly compare plyometric, OWL, and traditional RT in children.
Given that balance and coordination are not fully developed in children (58), the implementation of more complex, coordinated activities such as OWL and plyometrics into youth RT programs has been controversial (10). Because children rely less on glycolytic metabolism (68) have blunted muscle hypertrophic responses (37) and lower type 2 fibers composition areas (34) than adults, it might be considered that high intensity strength and power training programs may not benefit children. As OWL involves more complex coordination (10), a prolonged duration motor learning curve might be expected delaying potential beneficial training adaptations compared with the slower velocity, less complex movements involved with traditional RT. Furthermore, the recommendation for initiating adult plyometric training indicated that the individual should squat at least 1.5 times body weight before performing lower-body plyometrics (60) may have inhibited the implementation of plyometric training for youth. However, these potential impediments were not realized in this study.
It is well documented that traditional RT can provide significant strength training adaptations in children (1,10,27,36,37,59,65). OWL training in this study was equally or more effective than traditional RT. OWL has been shown to produce higher power outputs than traditional RT (40). OWL involves an explosive intent contraction that will involve either low or higher velocity movement dependent on the resistance employed. Behm and Sale (11,12) demonstrated that it was the intent to contract explosively that determined the high velocity-specific training response and not the movement velocity. An important advantage with OWL vs. traditional RT is the emphasis on rate of force development (RFD) and its importance for activities such as jumping (39). Harries et al. (41) in a meta-analysis reported a positive effect for RT programs on vertical jump performance (mean difference, 3.08 cm; 95% CI, 1.65–4.51; Z = 4.23; P < 0.0001) providing sufficient evidence that RT can improve muscular power in adolescent athletes. Olympic weightlifting exercises are explosive, multi-joint movements against a resistance, which exemplify many athletic actions (66) accentuating the task or action specificity of the training.
The coordinated control and stability to efficiently move a resistance through an extended range of motion with a high RFD necessitate strong balance capabilities. Many studies have demonstrated that instability or balance perturbations can impair force and power (5,7,6,25). Balance in this study was improved to a greater extent with OWL and plyometrics than with traditional RT. Behm and Colado (6) in a review of balance and instability RT studies reported that balance training alone in adults with no strength, power, or functional training improved measures of functional performance such as strength, power, running, and other activities by 31% with an effect size of 0.58 indicating a moderate magnitude of change. Thus, the improvement of balance or stability without concomitant RT can improve functional performance. Because the balance capabilities of children are not fully mature (58), greater improvements in balance with OWL and plyometrics could translate into greater power production.
Plyometric training in this study was superior to traditional RT for balance, Force60, Force300, Power300, and provided similar results for CMJ and horizontal jump. Traditional RT only exceeded plyometric training for BMI and Power60. Some of the advantages of plyometrics are similar to OWL. Similar to OWL, plyometrics involve a high RFD and explosive intent contractions that would be beneficial for power adaptations (12). Although plyometrics do not typically involve an external resistance, the momentum (mass × velocity) of the body moving at high velocities generates high ground reaction forces that must be absorbed and redirected with the stretch-shortening cycle actions. Cappa and Behm (17) reported ground reaction forces ranging from 2000 to 4000 N for CMJ and drop jumps, respectively. Auro (2) reported ground reaction forces ranging from 2 to 7 times body weight for agility and jump actions. Thus, even without the added stress of an external resistance, plyometrics using body mass as the resistance provides considerable tension training stress to the individual.
Plyometrics differ from OWL, in that the movement is more rapid providing the proprioceptive system with high velocity movement information for both eccentric and concentric contractions (17,18). Attempting to control the movement of the body with high velocity descending, ascending, and change of direction movements place substantial stress on the body to maintain balance and joint stability. Johnson et al. (47) in their meta-analysis of plyometric training for children indicated that plyometric training had a large effect on improving the ability to jump and run with a smaller effect on improving strength. In accordance with the training specificity principle (12), plyometrics in the Johnson's meta-analysis had the strongest effect on jumping and running, which involve substantial high velocity eccentric and concentric actions. Hence again similar to OWL, the subjects in this study responded to these balance stressors by improving balance to a greater extent than traditional RT. Improved balance and stability would contribute to greater force and power output (5,9,8,6).
Although most pediatric RT articles emphasize the neural component in children's strength gains (10), increases in muscle mass in children have been reported (38). Traditional RT in this study resulted in a substantially greater BMI and body mass after training than with OWL or plyometric training. All groups increased height (d = 0.24–0.29) to a similar amount, so we cannot account for the difference in BMI with greater skeletal mass of 1 group over another. Furthermore, all groups had “trivial” changes in skinfolds (d < 0.07). Based on a 3-compartment model of body mass (i.e., skeletal mass, muscle mass, and fat mass), we eliminate skeletal and fat mass differences, and therefore account for likely substantial difference in BMI to be because of greater muscle mass. Further exploration of the capacity of children to increase muscle mass with RT to confirm this deduction may be warranted using more involved techniques of body composition analysis (e.g., hydrostatic weighing, DEXA). Although, all 3 training programs can provide high overload tensions for the muscles, traditional RT with its slower movements provides a greater time under tension especially for the eccentric component (4,50) of the movements compared with plyometrics and OWL.
The limitations of this study would include the limited age group and sex of the subjects (10- to 12-year-old males). Although the use of Tanner stages can be considered a strength of the protocol, the self assessment may not be as valid as an assessment by a physician. Furthermore, there were no physiological measures such as magnetic resonance imaging, electromyography, or evoked contractile properties to aid in describing the mechanisms underlying the training adaptations.
This study demonstrates that more advanced RT techniques such as OWL and plyometrics can be included in a training program for adolescent children. The training gains from OWL and plyometrics for jump height, balance, strength and power measures, and sprint time were generally comparable or superior to traditional RT. The advantages associated with OWL and plyometrics may derive from the explosive contractions and high contraction speeds that tend to produce higher power outputs, and the increased demand placed on balance.
The present results do not suggest that traditional RT should be precluded from RT programs for children. In light of the common misperceptions that high intensity, high velocity, more complex coordinated activities like OWL and plyometrics may be ineffective and lead to injury in children, this study demonstrates the effectiveness of these training modalities. As the competitiveness of sport is reaching into younger ages, coaches and young athletes are seeking training advantages. Because coordination, balance, and power are underdeveloped in youth, training programs implementing OWL and plyometrics can accelerate positive training adaptations leading to competitive advantages. The results of this study and others (10,62) recommend that a combination of traditional RT, OWL, and plyometrics be introduced to children who wish to resistance train to provide a variety of overload stimuli and enhance neuromuscular training adaptations. As suggested by a number of professional organizations, the implementation of any RT program should be under professional supervision and involve an orderly training progression.
This study was financially supported by the Tunisian Ministary of Scientific Research, Technology and Development of Competences, Tunisia. The authors would like to thank the staff of the National Center of Medicine and Science in Sports, as well as the young athletes and coaches for their participation in this study. We especially thank Dr Mohamed Nassef for medical screening; Dr Moktar Chtara, Dr Olfa Turki for, Dr Lamia-Belkhiria Turki, Mme Narjess Touati, Mr Sofiene Kassmi and Mme Khawla Hidri for their assistance in testing.
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