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Promoting Strength and Balance in Adolescents During Physical Education: Effects of a Short-Term Resistance Training

Granacher, Urs; Muehlbauer, Thomas; Doerflinger, Britta; Strohmeier, Ralf; Gollhofer, Albert

Journal of Strength and Conditioning Research: April 2011 - Volume 25 - Issue 4 - p 940-949
doi: 10.1519/JSC.0b013e3181c7bb1e
Original Research
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Granacher, U, Muehlbauer, T, Doerflinger, B, Strohmeier, R, and Gollhofer, A. Promoting strength and balance in adolescents during physical education: effects of a short-term resistance training. J Strength Cond Res 25(4): 940-949, 2011-Secular trends in strength and postural control have been reported for children and adolescents. Therefore, the objective of this study was to investigate the impact of a short-term ballistic strength training (BST) followed by detraining on measures of strength and postural control in adolescents. Twenty-eight high-school students participated in this study and were assigned to either an intervention (n = 14, age 16.7 ± 0.6 years, body mass index [BMI] 21.1 ± 1.7 kg·m−2) or a control group (n = 14, age 16.8 ± 0.7 years, BMI 19.9 ± 1.7 kg·m−2). The intervention class participated in a short-term (8 week) lower extremity BST program 2 times a week integrated in their regular physical education lessons. Pre, post, and follow-up tests included the measurements of maximal isometric force (MIF) and rate of force development (RFD) of the leg extensors on a leg press with the feet resting on a force platform, vertical jumping height (countermovement jump [CMJ]) on a force plate and the assessment of static (1-legged stance on a balance platform), and dynamic (mediolateral perturbation impulse on a balance platform) postural control. Ballistic strength training resulted in statistically significant improvements in MIF (p = 0.001) and CMJ height (p < 0.001), which were still present after detraining for MIF (p = 0.04). Furthermore, tendencies in terms of small to medium interaction effects yet not statistically significant improvements were found for RFD (p = 0.38), and measures of static (p = 0.15) but not of dynamic postural control. In adolescents, lower extremity BST is a suitable training modality for the application in a school setting (particularly during physical education lessons) that produced transient improvements in strength variables. These results could have an impact on improving the performance level in various motor fitness skills and sports activities in physical education.

1Institute of Exercise and Health Sciences, University of Basel, Basel, Switzerland; 2Institute of Sport and Sport Science, University of Freiburg, Freiburg, Germany; and 3Staudinger Comprehensive School, Freiburg, Germany

No funding was received for this research.

Address correspondence to Urs Granacher, urs.granacher@unibas.ch.

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Introduction

The “American Heart Association” and other expert panels recommend that children and adolescents should participate in at least 60 minutes of moderate to vigorous physical activity every day (31,32). Recent data indicate that only 35.8% of high-school students meet these recommendations (4). Furthermore, a rapid decline in physical activity can be observed from childhood to adolescence (3). The decline in physical activity does not only result in an increased obesity rate among children and adolescents (28), but also in reduced performance in motor fitness tests (36). As a consequence, secular declines in sit-ups (37), bench press (37), bent arm hang (23), standing long jump (29), and in postural control (23) have been reported.

Therefore, adolescents are an important target group for fitness promotion programs to reverse individual and population wide declines before adulthood. Furthermore, it is reported that physical fitness status developed during childhood and adolescence has a positive influence on adult health and that it tracks from childhood over adolescence into adulthood (17). Thus, adequate intervention programs should be applied that have the potential to enhance fitness in adolescents. Schools provide an excellent opportunity for fitness promotion because they access a large population of children and adolescents across broad ethnic and socioeconomic strata (25). In addition, because of governmental promotion, a tendency toward all-day schools can be noticed in Germany. As a consequence, children and adolescents spend more time of their waking hours in school during the school year and have less time to attend organized sports (e.g., sports clubs) after school hours, as practiced in the past. Therefore, the implementation of effective and health-promoting intervention programs in school settings represents a major challenge for teachers, scientists, and politicians. For 2 reasons, strength or resistance training seems to be appropriate for the promotion of fitness in adolescents. First, resistance training has recently become one of the most popular sports activities among pubescents (11). Second, strength gains of roughly 30-50% were typically observed in untrained youth after short-term (8-12 weeks) resistance training programs (10). In addition, reduced sport injury rates (22) and improvements in selected motor fitness skills (18), body composition (18), bone health (27), and psychosocial well-being (12) were reported. However, the above-cited studies were conducted in out of school settings. To the authors' knowledge, there is no study available which investigated the impact of weight machine-based resistance training in a high-school physical education setting on measures of strength and postural control. As is typical in German high schools, teaching units in physical education last between 4 and 8 weeks, and physical education is taught twice a week. Given the diversified contents of the mandatory physical education curriculum (e.g., soccer, volleyball, basketball, swimming, gymnastics, track, and field) that have to be realized over the school year, resistance training cannot be conducted on a weekly basis throughout the school year. These externally imposed conditions have to be taken into account when planning resistance training programs implemented in physical education lessons. Because of the short available training period, we decided to conduct a ballistic strength training (BST) program because strength gains were reported over this short-term training period after BST (16). Because it is only possible to conduct resistance training in one teaching unit and not on a regular basis throughout the school year, we implemented a detraining phase to find out whether possible strength gains are stable over time. Therefore, the objectives of this study were to investigate the effects of standardized BST as described by Gruber et al. (16) and detraining on leg extensor strength, countermovement jump (CMJ) height, and postural control. It is expected that leg extensor strength, jumping height, and postural control improve because of BST. Thus, performance level in various motor fitness skills and sports activities could be enhanced. The intervention was performed during regular physical education classes to ensure compliance and to test the feasibility of integrating such a program in the regular school curriculum.

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Methods

Experimental Approach to the Problem

To test our hypothesis, adaptations for BST were compared in a controlled longitudinal training study. The short-term training period lasted 8 weeks to ensure neuromuscular modifications. Training was followed by a 7-week detraining period to document the stability of training-induced changes. Neuromuscular improvements were verified by an analysis of static and dynamic force and postural control measurements. Both, gains in force production and balance control developed during regular physical education lessons are of vital importance for activities of daily living and in several sports related skills.

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Subjects

Twenty-eight high-school students with no significant differences in body mass, body height, body mass index (BMI), and Tanner stages participated in the study after experimental procedures were explained (Table 1). None had any history of musculoskeletal, neurological, or orthopedic disorder that might have affected their ability to execute resistance training, strength and balance tests. Parents' and participants' written informed consent was obtained before the start of the study. None of the students had an athletic background, and none had previously participated in systematic resistance or balance training. All students performed less than 4 hours of sports activities a week (including physical education). Out of school sports activities were primarily conducted in sports clubs (e.g., soccer clubs). Local ethical permission was given, and all experiments were conducted according to the declaration of Helsinki.

Table 1

Table 1

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Procedures

Training

Participants were recruited from 2 different physical education classes of the same high school. Classes were randomly assigned to either a BST group or a control group (CON group). Before intervention, the authors of this study comprehensively instructed the regular physical education teacher about the BST methodology. The intervention program was taught by the regular physical education teacher and by an expert on BST to keep the student-to-teacher ratio small (2 teachers vs. 14 adolescents). The intervention class conducted a BST program that proved to be effective and that has been previously used (16). In addition, youth resistance training guidelines were incorporated according to Faigenbaum et al. (9). Thus, BST was organized as a circuit training with each teacher supervising 3-4 stages. Participating students always exercised in pairs so that one student trained and the other one spotted the partner's lifting technique. All sessions were documented and supervised by the authors of the study. The applied short-term BST-training protocol is described and summarized in Table 2. Participants of the CON group attended their regular physical education lessons (also 2 times a week) during the 8-week intervention period and were primarily taught in gymnastics and swimming. No specific resistance exercises were performed during their physical education lessons. For both groups, a 7-week detraining period started immediately after the last training session, where students attend their regular physical education lessons. All subjects were not allowed to decrease or increase their daily sport activities between pre, post, and follow-up tests.

Table 2

Table 2

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Testing

Before testing, all subjects underwent a 10-minute warm-up consisting of submaximal plyometrics and skipping exercises. Pre, post, and follow-up tests included (a) measurements of static and quasidynamic postural control on balance platforms, (b) the analysis of CMJ height on a force plate, and (c) the assessment of maximal isometric leg extension force (maximal isometric force [MIF]) and rate of force development (RFD) on a leg press. This sequence of measurements was applied to keep the effects of neuromuscular fatigue minimal.

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Measurement of Maximal Isometric Force and Rate of Force Development

Maximal isometric force was measured on a leg press, with the feet resting on a one-dimensional force platform (Kistler® type 9253B, Winterthur, Switzerland). Participants were horizontally positioned on the sledge of the leg press with hip and knee angle adjusted to 90° and the ankle angle to 100°. The exact position of each subject was documented, so that it was identical in the pre, post, and follow-up tests. The waist was fixed, and participants were allowed to stabilize their upper body by holding on to handles attached to the leg press. Participants were instructed to avoid forced respiration during maximal efforts. Before the testing started, participants were asked to perform 3-5 submaximal isometric contractions to get accustomed to the testing procedure. Thereafter, each participant performed 3-4 leg-press exercises with maximal voluntary effort lasting 3-5 seconds each. For each trial, participants were thoroughly instructed to extend their legs as forcefully and as fast as possible. The force signal perpendicular to the force plate was sampled at 500 Hz. Force signals were converted analog-to-digital and stored on a computer. During later offline analysis, the best trial in terms of MIF was selected and the force signal was filtered by a digital fourth order recursive Butterworth low-pass filter, using a cut-off frequency of 50 Hz. Maximal isometric force and RFD were calculated from the individual MIF development record. Maximal isometric force was defined as the maximal voluntary force value of the force-time curve, determined under isometric condition. Rate of force development was defined as the maximal slope at deflection of the force-time curve (Δforce/Δtime). Intraclass correlation coefficients (ICCs) were calculated for MIF (ICC = 0.99) and RFD (ICC = 0.92). This protocol has recently been described in detail elsewhere (14).

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Measurement of Countermovement Jump Height

Participants performed maximal vertical CMJ while standing on a 3-dimensional force platform (Kistler® type 9281A, Winterthur, Switzerland). The vertical ground reaction force was sampled at 1,000 Hz. During the CMJ, subjects stood in an upright position on the force plate and were instructed to begin the jump with a downward movement, which was immediately followed by a concentric upward movement, resulting in a maximal vertical jump. Subjects performed 3 CMJs with a resting period of 1 minute between jumps. For each of these trials, subjects were asked to jump as high as possible. The best trial in terms of maximal jumping height was taken for further data analysis. The ICC was calculated for CMJ height (ICC = 0.99).

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Measurements of Static and Dynamic Postural Control

Test circumstances (e.g., room illumination, temperature, and noise) were in accordance with recommendations for posturographic testing (20). Static postural control was assessed by a balance platform (GKS 1000®, IMM, Mittweida, Germany). The balance platform consists of 4 sensors measuring displacements of the center of pressure (COP) in the mediolateral and anterior-posterior directions. Data were acquired for 30 seconds at a sampling rate of 40 Hz (20). Total displacements of the COP were computed (summed displacements in mediolateral and anterior-posterior directions). For experimental testing, participants were asked to stand as stable as possible on the balance platform with the dominant foot placed along the anterior-posterior axis of the plate and with the knee of the supporting limb flexed at 30°. The nonsupporting limb was flexed 45° at the knee; hands were placed on hips and gaze fixated on a cross on the wall. The dominant leg was determined according to the lateral preference inventory (6). The ICC was calculated for total displacement of the COP (ICC = 0.88).

Quasidynamic postural control involved a 1-legged postural stabilization task on a 2-dimensional platform (Posturomed, Haider, Bioswing, Pullenreuth, Germany). The platform is mounted to 4 springs and is free to move in the transversal, mediolateral, and anterior-posterior directions. The maximal natural frequency of the Posturomed is below 3 Hz. The mechanical constraints and the reliability of the system were described earlier (24). If the platform is in neutral position, the maximum range of motion in the anterior-posterior and mediolateral directions amounts to 70 mm, respectively. Mediolateral perturbation impulses were applied to investigate quasidynamic postural control of the participants. Therefore, the platform was moved 2.5 cm from the neutral position in the mediolateral direction, where it was magnetically fixed. For experimental testing, participants were asked to stand on 1 leg on the fixed platform with their supported leg in 30° flexion, hands placed on hips and gaze fixated on a cross on the wall. Several trials helped participants to get accustomed to the measuring device. After investigators visually controlled the position of the subjects, the mediolateral perturbation impulse was unexpectedly applied by detaching the magnet. The platform suddenly accelerated in the medial direction. The participants' task was to damp the oscillating platform by balancing unilaterally on the Posturomed. Summed oscillations of the platform in medio-lateral (ml) and anterior-posterior (ap) were assessed by means of a joystick-like 2D potentiometer (Megatron) that was connected to the platform. The potentiometer measured the position of the platform in degree (°). The signal was differentiated, rectified, and integrated over the 10-second test interval. Three trials were performed. The best trial (least oscillations in ml direction) was used for further analysis. Intraclass correlation coefficients were calculated for summed oscillations of the platform in ap (ICC = 0.69) and ml (ICC = 0.40) directions. This protocol has recently been described in detail elsewhere (15).

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Statistical Analyses

Data are presented as group mean values ± SDs. A multivariate analysis of variance (MANOVA) was used to detect differences between study groups in all baseline variables. The effects of the short-term training and detraining on strength and balance parameters were analyzed in separate 2 (group: BST, CON) × 3 (test: pre, post, and follow-up) ANOVA with repeated measures on test. Post hoc tests with the Bonferroni-adjusted α were conducted to identify the comparisons that were statistically significant. The classification of effect sizes (f) was determined by calculating partial η2p. f values = 0.10 indicate small, f values = 0.25 medium, and f values = 0.40 large effects (5). A priori power analysis revealed power values of 0.50, 0.85, and 0.98 for small (0.10), medium (0.25), and large effect sizes (0.40), respectively, for the n size used in the study. These findings indicate that the n size used in the present study should be sufficient to detect significant differences among groups. The significance level was set at α = 5%. All analyses were performed using Statistical Package for Social Sciences (SPSS) version 16.0.

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Results

Fourteen participants completed the short-term training and none reported any training-related injury. The BST group showed high attendance at training sessions with 94%. Means and SDs for all variables are presented in Table 3. Overall, there were no statistically significant differences in baseline values between the experimental groups.

Table 3

Table 3

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Strength Performance

After 8 weeks of short-term BST, maximal isometric leg extension force was significantly enhanced in the BST group over time (Figure 1A). The analysis detected main effects of test, F(2, 108) = 13.65, p < 0.001, η2 = 0.34, f = 0.72 and group, F(1, 26) = 4.44, p = 0.04, η2 = 0.15, f = 0.42. In addition, the analysis indicated a Test × Group interaction, F(2, 108) = 8.30, p = 0.001, η2 = 0.24, f = 0.56. Post hoc analysis revealed that participants of the BST group performed with a 17% and 9% higher MIF level on the post (p = 0.007) and follow-up test (p = 0.04), respectively, compared with prevalues (Table 3). Over the duration of training/detraining, a slightly increased maximal RFD in the BST group was found (Figure 1B). Yet, the analysis did not detect main effects of test, F(2, 108) = 0.36, p = 0.70, η2 = 0.01, f = 0.10 and group, F(1, 26) = 0.33, p = 0.57, η2 = 0.01, f = 0.10. Also, the Test × Group interaction, F(2, 108) = 0.99, p = 0.38, η2 = 0.04, f = 0.20, was not significant. Figure 1C demonstrates an 8% increase in CMJ height in the BST group from pretest to posttest. The analysis revealed a Test × Group interaction, F(2, 108) = 15.67, p < 0.001, η2 = 0.38, f = 0.78, and main effect of test, F(2, 108) = 14.64, p < 0.001, η2 = 0.36, f = 0.75, but did not detect a main effect of group, F(1, 26) = 0.03, p= 0.86, η2 = 0.01, f = 0.10.

Figure 1

Figure 1

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Balance Performance

Total displacement of the COP decreased in both groups, but was slightly more reduced for the BST group (Figure 2). The analysis detected a main effect of test, F(2, 108) = 37.22, p < 0.001, η2 = 0.59, f = 1.20. However, the main effect of group, F(1, 26) = 0.001, p = 0.98, η2 = 0.001, f = 0.03 and the Test × Group interaction, F(2, 108) = 1.97, p = 0.15, η2 = 0.07, f = 0.27, failed to reach significance. There were no significant interaction effects in quasidynamic balance control irrespective of the parameter considered.

Figure 2

Figure 2

In addition, a post hoc power analysis was calculated for the parameter RFD (α = 0.05, β = 0.20, statistical power 1 −β = 0.80, effect size f = 0.20, correlation among repeated measures r = 0.57). The analysis revealed a total sample size of 38 subjects to obtain statistically significant results.

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Discussion

To the authors' knowledge, this is the first type of study that investigated the effects of a school-based short-term ballistic strength training and detraining in adolescents on MIF and RFD of the leg extensors, jumping height in CMJ, and measures of static and dynamic postural control. The main findings of this study were that (a) MIF and CMJ height were significantly improved after 8 weeks of BST; (b) RFD showed a tendency toward an improvement in terms of small to medium effect sizes; (c) measures of static and dynamic postural control were not significantly influenced by BST; (d) strength variables deteriorated after 7 weeks of detraining with MIF remaining significantly above the prevalue.

The present results are in accordance with those in the literature regarding the effects of short-term BST on MIF and RFD of the leg extensors and jumping height in CMJ. In fact, Taube et al. (33) found increases in maximal isometric leg extension force and jumping height in CMJ after 6 weeks of BST in pubertal elite athletes (age 14.5 ± 1 years). In addition, no significant training-induced changes in RFD of the leg extensors were reported. However, other authors (16) observed increases in RFD of the plantar flexors after 4 weeks of BST (16 training sessions) in young healthy adults (age 27.0 ± 6 years). It is suggested that differences in training volume in terms of the applied strength exercises for certain muscles may account for the observed discrepancy. In the study of Taube et al. (33) and in our study, several muscles of the lower extremities were trained. In contrast, Gruber et al. (16) solely focussed on the plantar and dorsal flexors. Because of the rather limited number of trained muscles in their study and given that in both studies 16 training sessions were applied, overall training volume was higher for the plantar flexors in the study of Gruber et al. (16) compared with training volume for the leg extensors in our study. In addition, maturational factors cannot account for the absent effect of BST on RFD of the leg extensors in the present study because of spinal and supraspinal motor centers that are responsible for the generation of rapid forces are already fully developed in adolescents (1,26). Therefore, a subliminal training volume seems to be responsible for the observed results. As a consequence, future studies should incorporate a higher training volume in BST for certain muscles if the goal is to induce significant improvements in RFD of the leg extensors.

The present study was characterized by a training intensity of 30-40% of the 1RM and a training period of 8 weeks. The applied training volume suggests that primarily neural adaptations on a spinal and supraspinal level rather than changes in muscle properties account for the observed transient increases in MIF of the leg extensors and jumping height in CMJ. In fact, there is evidence that BST induces enhanced H-reflex amplitudes probably because of reduced presynaptic inhibition of Ia-afferents in pubertal athletes (33). In addition, recruitment of motor-evoked potentials induced by transcranial magnetic stimulation was increased after 4 weeks of BST in young healthy adults (1). Beck et al. (1) argued that the performance of ballistic exercises during training strongly emphasizes mental preparation preceding each voluntary contraction resulting in enhanced corticospinal activation. Thus, increased spinal excitability and facilitation of supraspinal drive to the motoneuron pool may be responsible for training-induced transient improvements in force production observed in this study. In fact, it is reported that BST resulted in an increased activation of lower extremity muscles during maximal isometric contractions of the leg extensors and during the performance of CMJ (33). In addition, an improved intermuscular coordination in terms of synergistic muscle activations and decreased coactivation of antagonistic muscles may also contribute to the enhanced force production (33). However, it has to be noted that detraining effects were not investigated in the reported studies (1,16,33). Thus, it cannot be conclusively stated whether neural adaptive mechanisms are transient or relatively permanent.

In the present study, the 7-week detraining period resulted in a decrease in MIF from post to follow-up tests that amounted to 7.1%. However, the observed MIF after detraining was still significantly higher (8.7%) than the baseline value. First, this indicates that MIF deteriorated over detraining, and second, that the training effect was at least in part present after detraining. Care has to be taken though when interpreting effects of detraining in children and adolescents because loss of strength because of the withdrawal of a training stimulus may be confounded by the concomitant growth-related strength increases (2). Nevertheless, the observed results of the present study comply with those in the literature since Ingle et al. (18) conducted a 12-week complex training program including resistance training and plyometrics in early pubertal boys (age 12.3 ± 0.3 years) and observed significant decreases in dynamic strength variables after 12 weeks of detraining. According to the results of Faigenbaum et al. (13), it seems likely that the majority of performance decrements for leg extension occurred during the first 4 weeks of detraining, and thereafter, it continued at a slower rate. Mechanisms responsible for the effect of detraining on leg extensor strength have yet to be elucidated. However, there is preliminary evidence that reduced motor unit activation and losses in motor coordination may account for the reported deteriorations (2). Only a few studies have evaluated the effects of training frequency on strength maintenance in adolescents. DeRenne et al. (7) investigated the effects of training frequency on strength maintenance in pubescent baseball players. The 12-week preseason progressive resistance training revealed significant increases in upper and lower body strength. After the 12-week preseason training, one training group continued over a 12-week in-season maintenance program at a frequency of 1 d·wk−1, and the other training group exercised at a frequency of 2 d·wk−1. After the maintenance program, no significant differences were revealed between groups for strength variables assessed at the leg press. The authors concluded that for pubescent male athletes, a 1 day a week maintenance program is sufficient to retain strength during the competitive season.

In the present study, short-term BST did not have an effect on measures of static and dynamic postural control. To our knowledge, there is no study available which investigated the impact of BST on postural control variables in adolescent high-school students. Thus, other age groups have to be consulted to discuss the present results. In a systematic review on the efficacy of progressive resistance training on balance performance in older adults, Latham et al. (21) could not find a clear effect of resistance training on various measures of standing balance (effect size = 0.11). However, recent studies indicate that high-speed power training seems to have a greater impact on balance performance in old age than traditional heavy-resistance strength training (30). In terms of adolescents, this issue is not yet resolved. Therefore, future studies should focus on potential effects of resistance training in general and BST in particular on measures of postural control in adolescents.

What might be the underlying reason why we did not observe an impact of short-term BST on postural control variables? Recently, Beck et al. (1) investigated spinal and supraspinal adaptive processes after 4 weeks of balance training or ballistic ankle strength training in young healthy adults during the performance of motor tasks that were comparable to each training type. The authors reported that after ankle BST recruitment of motor-evoked potentials was enhanced during the force production task (plantar and dorsal flexors) but not during the postural control task (standing forward and backward directed perturbation impulses). These results imply that neural adaptive processes after BST are task specific. This could explain why we did not find effects of short-term BST on measures of postural control. In addition, the observed improvements in the present study in static and dynamic postural control variables in the controls indicate a potential learning effect.

Three potential limitations of this study warrant discussion: First, the sample size applied in this study is relatively small. This is at least in part reflected in the calculations of the post hoc power analysis that revealed a total sample size of 38 participants to obtain statistically significant results for the parameter RFD. However, we aimed at integrating short-term BST in the regular physical education school curriculum. For this purpose, we chose one comprehensive school and randomly assigned 2 classes of the same year to either the intervention or the control group. Because of this methodological approach, the sample size was rather small. Nevertheless, future studies should incorporate larger sample sizes to elucidate whether training-induced improvements in RFD and variables of postural control are possible in adolescent high-school students. Second, we only reported results regarding total COP displacements as a global parameter for static postural control. Our initial analysis included velocity, range, root mean square, and the coefficient of variation for total COP displacements and for COP displacements in mediolateral and anterior-posterior direction. Because the statistical analysis revealed no additional information through the integration of the above-mentioned parameters, we decided to focus on the global parameter COP displacements only. Third, we did not include a habituation session before the baseline tests. This could explain why we observed decreased COP displacements in the CON as well. However, we applied the same testing procedure in this study as in an earlier study (14), in which we did not detect this phenomenon in a group of adolescent high-school students.

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Practical Applications

The results of this study illustrate that short-term lower extremity BST is a safe training modality that produces marked improvements in MIF of the leg extensors and jumping height in CMJ of adolescent high-school students. The observed gains were transient. They began to deteriorate after the withdrawal of the training stimulus. Coaches and physical education teachers who work with adolescents should therefore continue with conditioning exercises beyond the initial training period. In fact, a one session a week maintenance program has proven to be effective in adolescents (7).

Unfortunately, short-term BST did not result in improvements of postural control variables. Given the association between increases in postural sway, deficits in strength, and the occurrence of sport injuries in children, adolescents, and adults (8,19,34,35), balance intervention programs should be considered that have the potential to improve postural control measures in these cohorts. Recently, it was reported that 4 weeks of balance training performed during regular physical education classes resulted in significantly reduced postural sway during one-legged stance in adolescent high-school students (14). Therefore, this type of intervention program could be combined with short-term BST to induce effects on both, strength and balance variables.

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Keywords:

ballistic strength training; explosive force production; leg extensors; postural control

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