The Effect of Long-Term Resistance Training on Anthropometric Measures, Muscle Strength, and Self Concept in Pae-Pubertal Boys

Transcription

1 Pediatric Exercise Science, 2001, 13, O 2001 Human Kinetics Publishers, Inc. The Effect of Long-Term Resistance Training on Anthropometric Measures, Muscle Strength, and Self Concept in Pae-Pubertal Boys Eliahu Sadres, Alon Eliakim, Naama Constantini, Ronnie Lidor, and Bareket Falk The purpose of this study was to examine the effect of 2 school years (21 months) of a twice-weekly resistance training program on stature, muscle strength, and self-concept among prepubertal boys. The experimental group (E, n = 27) aged 9.2 f 0.3 yrs, participated in progressive resistance training, while the control group (C, n = 22) aged 9.4 f 0.3 yrs, participated in standard physical education classes (as advised by the Ministry of Education). Training sessions included 1-4 sets of 3-6 exercises, with 5-30 repetitionsfset. The load ranged between 30% and 70% 1RM. No differences were observed in the gain in body height between groups. Muscle strength increased significantly more in E (e.g., knee extensors: to 0.77 f 0.16 kglkg body mass), compared with C ( to kgkg body mass). One minor injury was reported throughout the study. Initial scores of self-concept were high in both groups, with no trainjng effect. The results demonstrate that among prepubertal boys, a twice-weekly low-to-moderate-intensity resistance training program over a period of 2 school years (21 months) can result in enhancement in muscle strength with no detrimental effect on growth. Introduction The effectiveness of resistance training among children has been addressed recently by several comprehensive reviews, which all came to the conclusion that resistance training can be very effective for developing muscle strength among pre-pubertal children (7, 1 1,33). These reviews included prospective intervention studies, usually up to 4 months in duration. Despite this relatively short duration, enhanced muscle strength was consistently demonstrated. In fact, Falk and Tenenbaum (1 I), using a meta-analysis, concluded that following progressive resistance training, a child with an average score who was involved in resistance E. Sadres was with the Nat Holman School of Coaches and Instructors and is presently with the Zinman College of Physical Education and Sport Sciences, Wingate Institute, Netanya, Israel; A. Eliakim is with the Department of Pedatrics, Meir Hospital, Kfar Sava, Israel; N. Constantini and B. Falk are with the Ribstein Center for Sport Medicine Sciences and Research, Wingate Institute, Netanya, Israel; R. Lidor is with The Motor Behavior Laboratory, The Zinman College of Physical Education and Sport Sciences, Wingate Institute, Netanya, and Faculty of Education, Haifa University, Israel.

2 358 - Sadres et al. training was stronger than 72% of children who did not engage in resistance training (effect size = 0.57). This figure is a composite of different exercises and different muscle groups and may therefore not be representative of a specific muscle group. However, it does reflect that short-term resistance training is effective in increasing muscle strength among pre-pubertal boys. The main mechanism responsible for the strength gain described above is believed to involve neuromuscular adaptations rather than muscle hypertrophy (7, 33). This belief is based on several studies that demonstrated increased neuromuscular activity (IEMG, 28) or increased motor unit activation (30), following resistance training among children. Additionally, most previous studies failed to demonstrate any effect on muscle mass or calculated fat-free mass (see 6, 7 for review). However, it should be noted that few studies did report changes in muscle mass or girth width (14, 21, 24). Two of these studies (14, 24) employed more precise methods of measurements (MRI, US) than the commonly used anthropometric methods. It is possible that the resistance training programs reviewed above did result in greater muscle mass but that the change was too small to be detected using anthropometric methods, especially over the commonly examined duration of 2-4 months. Thus, an intervention program of a longer duration may result in measurable changes in muscle mass, even when using anthropometric methods. The American College of Sports Medicine (ACSM; I), the International Federation of Sports Medicine (FIMS; 13) as well as the National Strength and Conditioning Association (NSCA; 26) recommend progressive resistance training for children and adolescents. Most studies have employed a training schedule of three sessions per week (e.g., 4, 28-30, 32, 34, 35, 40), although few have employed a twice-weekly program (8-10,12). The ACSM (1) recommends that children engage in resistance training two times per week. The issue of safety of resistance training among children has been of concern to physicians, coaches, teachers, and parents. Most of the prospective studies report no injuries while a few report one injury. In no case did the injury cause the child to miss more than 1 week of training (21,31,34). Hamill (17), in a survey of English schools, found that weight training (progressive resistance exercises for body conditioning), as well as weight lifting (competition in snatch and the clean and jerk), had the lowest injury rate among several popular sports, including soccer, tennis, ball games, and more. It should be noted that the low occurrence of injuries in these studies may be due to the fact that the trainers were well qualified and they focused on proper exercise technique, but it may also be associated with the relatively short duration of the studies. Several studies have found a positive relationship between aspects of resistance training and self-concept variables such as self-esteem (e.g., 20, 39). According to Berger and McInman (3), resistance training appears to be an effective method to enhance body concept and self-concept in adolescents and in adults. However, further research is needed to clarify the effect of such training on the self-concept in children, especially over an extended period of time. The purpose of this study was to examine the effect of 2 school years of progressive resistance training (9 months of training, 3 months of detraining, and 9 months of training) on muscle strength, linear growth, and self-concept, as well as to monitor the injury rate during this program, among pre-pubertal boys.

3 Long-Term Resistance Training Subjects Methods Sixty boys, aged 9-10 years, were invited to participate in the study. All subjects and their parents gave their initial consent to participate. The experimental (E) group (n = 30) included all boys from two grade 4 classes in one school while the control group (C; n = 30) included all boys from two grade 4 classes from another school. Each school had three grade 4 glasses, two of which were randomly selected to participate in the study. The schools included all school-aged children in a town in central Israel. This town was selected in view of its relative vicinity to our institute and the enthusiasm of the local authorities to participate. The mean age (+ standard deviation) was 9.23 f 0.30 and 9.36 f 0.27 years for E and C, respectively. The students were of a similar ethnic and socioeconomic background, as determined by the community educational department. The two schools had similar academic and extracurricular activity programs. The experimental and control schools were allocated based on school self-selection for practical reasons. The two groups were similar in their chronological age, as well as their maturational stage, as determined by secondary sexual characteristics (i.e., pubic hair, 38). All subjects were pre-pubertal (stage 1) with a few exceptions: two subjects in each group were in stage 2, and one subject in E was in stage 3. These three subjects were of similar anthropometric characteristics and initial strength as the rest of their group. Therefore, they were included in the study. C participated in the regular physical education program within the school framework in line with the recommendations of the Ministry of Education. For E, a progressive resistance training program substituted the standard physical education program. Classes were held twice per week for both groups. In both groups, children underwent a medical examination in order to exclude those with any chronic disease that could affect the musculoskeletal system or children who were under medication (e.g., corticosteroids) that could affect bone growth or strength development. None of the subjects had any chronic disease or orthopedic disability. The local education system, parents, and subjects were informed in writing and orally about the study program and objectives. Parents gave their written consent, and children gave their verbal consent to participate. Training Program Both groups trained twice a week on non-consecutive days for 9 months during the first and second school years. No detailed information is available about the subjects' activities during the summer months, but it is assumed that it was similar in the two groups. The resistance-training program was designed and instructed by a weightlifting coach, well-experienced in working with children and adolescents ~. s - w e 1 ~ w i t h - Q L y m p i c ~ v e L a t h lstruc- e t e - ~ - - tured succeeding stages. Most of the work was performed in pairs, although the workload was designed individually for each subject's initial strength and technique level and his rate of progression. The initial load was minimal (broom stick for one month and standard 8 kg bar with no weights for another month), in order to learn the proper technique and safety procedures. This light load was used in view of the advanced multijoint lifts that were part of the program. Following this

4 360 - Sadres et al. period and at 3-month intervals, 1 RM tests were performed in order to determine the load and progression. The IRM of each exercise was evaluated by the maximal number of repetitions for a particular load, according to Baechle (2). In a few cases, when it was deemed necessary by the instructor (for example, if the prescribed training task was apparently light), more frequent 1 RM tests were performed. During each exercise, one subject within the pair performed the exercise and the other rested. Each exercise was demonstrated by the instructor, following which subjects performed the required sets. All pairs performed the same exercises, although occasionally, circuit training was also introduced. The rest period between exercises was 2-3 min. The instructors-to-subjects ratio was 1 to 15. Appendix 1 includes a detailed description of 1 month of the program. Each lesson comprised of an introduction (3-5 min), general and specific warm-up (8-10 min), weightlifting and resistance training exercises (20-25 min), general activity and games (5-10 min), and a cool-down (5 min). In general, each session included 3-6 exercises and a total of 150 repetitions. The training session included specific resistance and weightlifting exercises (clean pulls, jerk, clean, squats, dead lift, snatch and snatch pulls; see Appendix) using free weights. In addition, as part of the general activity and games, the children participated in activities such as running, jumping, and throwing, using different means such as medicine ball (1-3 kg), skipping ropes, and others. During the first school year, the load for each subject ranged between 30% and 70% of 1RM (mean for each subject was 50% of 1 RM). During the second school year the load for each individual ranged between 50% and 70% of 1RM (mean of 60% lrm, Table 1). This broad range of training loads was used as a precautionary measure. For each exercise, subjects initially performed the exercise at a low percent 1 RM and high number of repetitions. Progressively, the percentage of 1 RM was increased while the number of repetitions was decreased. The training intensity and its progression were determined according to the principle of periodization in strength training (2). Regular physical education classes for the C group included a section on track and field (sprints, long jump, high jump), basic gymnastics (e.g., rolls, head Table 1 Comparison of Resistance Training Program During the 21 Months* of the Study Period Year 1 Year 2 Training characteristic Duration Frequency Total repetitionslsession Mean load Range of load Exercises per session Repetitionslset Setslexercise 9 months 2 sessionslweek % of 1 RM 30-70% of 1 RM months 2 sessionslweek % of 1 RM 50-70% of 1 RM *including 3 months of summer vacation.

5 Long-Term Resistance Training stand, hand stand), and ball games (soccer, team-handball, and basketball). Classes also included flexibility exercises and games in motion. While we could not fully neutralize for weight-bearing activities, it should be noted that activities were performed mainly as games, or as skill acquisition, and did not focus on strength improvement. None of the control subjects were involved in resistance training during the study. Subjects in both groups maintained their normal daily activities, including participation in various after-school sports (e.g., soccer, basketball, karate). Tests and Measurements All measurements were performed at the start, middle, and end of each school year. These measurements included pubertal stage according to secondary sexual characteristics, as described by Tanner (38), and habitual physical activity as determined by a questionnaire (16). The latter was filled out in order to see whether subjects participated in strengthening activities after school. Each type of activity is scored according to an estimated MET score, and a final weighted activity score is calculated. Additionally, anthropometric measures included height and body mass. These measurements were performed by the same experienced investigator (BF) throughout the 2 years using a standard wall stadiometer (to the nearest 0.1 cm) and a scale (Shekel Scale, Bet Keshet, Israel, accuracy = 420g). Muscle strength in multijoint lifts was measured using one-repetition maximum (1RM). Concentric strength of the knee flexors and extensors was measured using the Shnell M3 Multi Muscle Machine. The training program was designed to strengthen these muscle groups, although the specific testing exercises were not included in the training sessions. The 1RM was found for all strength measurements by 3-6 attempts after a warm-up of 3-5 sub-maximal repetitions. The 1RM was measured using increments of 2.5 kilograms. An exercise was considered complete only if the proper technique was performed. It should be noted that the first 3 weeks of the program were devoted to handling of free weights and learning techniques of weight lifting. Therefore, the initial strength measurements were performed 3 weeks after the start of the program. The Perceived Physical Competence Scale (PPCS; 22,23,27) was adrninistered to the subjects at the beginning, midpoint, and end of the first year of training. The scale was administered to half of each group at a time (15 children in their classroom), by one of the researchers. The purpose of the PPCS is to assess cognitive appraisals of physical self among children, including perceptions of both physical performance capacity (physical fitness) and physical appearance. Each subject was asked to compare himself to boys of the same age in terms of physical fitness, motor ability, anthropometric characteristics, and other physical attributes. The subjects responded to each of the 10 items on the PPCS using a 5-point scale in which a discrimination must be made between bipolar adjective descriptors. ~~~lishtaazabi.(th~~a~e-ofinstruc_tionin the schools was Arabic). The Cronbach alpha internal consistency coefficient was This value was similar to those reported by Lintunen (22) and Lintunen et al. (23). In addition, the translated version of the PPCS was validated contextually by three experts in exercise physiology, physical fitness, and motor learning. The PPCS was given to the subjects on a separate occasion from the training session. The subjects were instructed how to use the scale each time it was distributed to them. It took 25 min for the subjects to complete the scale.

6 362 - Sadres et at. Data Analysis An unpaired t-test was used to assess differences between groups in baseline measures. Variables measured repeatedly were analyzed using a two-way ANOVA for repeated measures, with time serving as the within-subjects factor and physical education program (resistance training vs. regular) serving as the between-subjects factor. Post hoc analyses were performed using the Bonferoni procedure (18). Pearson product-moment correlation was used to assess relationships between selected variables. Group data are presented as means and standard deviations (SD). Statistical significance was set at CY = 0.05 for all statistical analyses. Data analysis was performed on all measurements. However, for clarity, data are presented for beginning, end of first year ("midpoint"), and end of the study. Compliance Results Each group initially included 30 subjects. All subjects in the experimental group completed the study. In the control group, one child withdrew from the study and one child's family relocated. Subjects who were absent on one of the testing days had incomplete data and were dropped from the analysis. Thus, anthropometric data is complete for 27 and 22 subjects in E and C, respectively. Strength tests are complete for 24 and 17 subjects in E and C, respectively. Injury Rate An injury was defined as an incident during the training session which caused the subject to discontinue his training. Only the E group was under injury surveillance. No injuries were reported throughout the study with one exception: on one occasion the bar slid and fell on the thighs of one of the subjects following a lift (clean). The child complained of transient non-specific pain in the anterior thigh and sat out for about 5 min. He returned to train within the same session when the pain was resolved and had no further complaints. Therefore, it was felt that no additional medical evaluation was required. The calculated injury rate was participant-hours (calculated according to the number of subjects and number of training hours during the 2 school years). Maturation By the end of the two school years, 11 boys in E and 7 boys in C had advanced from pubertal stage 1 to stage 2. There were no significant differences between the groups and no changes over time in the subjects' reported habitual activity (activity score for E: 67.6 f 17.7 and 79.1 f 29.4 at start and end of study, respectively; for C: 60.3 f 22.5 and 63.8 f 30.9 at start and end of study, respectively). Anthropometric Measures The physical characteristics of the two groups throughout the study appear in Table 2. A significant time effect was evident in all anthropometric measures, reflecting

7 Long-Term Resistance Training Table 2 Physical Characteristics of the Experimental (E) and the Control (C) Groups at the Start, Midpoint, and End of the Resistance-Training Program Statistical Variable group Start Midpoint End analysis Height (cm) E Mean SD C Mean SD Mass (kg) E Mean SD C Mean SD Body mass index E Mean SD C Mean SD Note: Midpoint is the end of the first year; a = time effect - significant change between start to end in both groups; b = interaction between time and group effects. Levels of significance: *p c.05, ***p c.001 in all variables due to a generally greater increase in body dimensions over time in E compared with C. The increase in height over the study period was similar in the two groups (9.7 vs 9.3 cm in E and C, respectively). The time-group interaction found in the height analysis is likely due to the fact that the increase in height in E was slightly smaller in the fust year (3.6 vs. 4.1 cm in E and C, respectively) but slightly greater during the second year (6.1 vs. 5.2 cm in E and C, respectively). Strength Measures A comparison of all strength measures between the two groups appears in Figures 1 and 2 and in Table 3. All strength measures increased significantly over time in both groups. The experimental group was significantly stronger in all strength measures throughout the study period. These initial differences may be partly due to the fact that initial tests were carried out 3 weeks after the start of the program -- (see- Discussion). The significant time-group interaction found in all strength measures reflects the greater strengtfnceaseokerv~aiiiii compared with C oveitlii study period. The differences between groups over time (interactions) were observed whether strength was expressed in absolute units (kg) or corrected for body mass. Of particular interest is the fact that absolute strength increased similarly in both groups during the summer vacation (between the 2 school years of exercise). No increase was observed during the summer vacation when strength was normalized to body mass.

8 364 - Sadres et al. - Experimental Control 40 Absolute T I=- X W Relative to Body Mass _--- START MIDPOINT END Figure 1 - Knee extensors strength in the experimental and control group at the beginning, midpoint, and end of the study. Strength is expressed in kg (top panel), and relative to body mass (bottom panel). *A significant group-time interaction was observed, reflecting a greater increase in strength over time in the experimental, compared with the control group. - Experimental ---- Control 30 ) Absolute 1 Figure 2 - Knee flexors strength in the experimental and control group at the beginning, midpoint, and end of the study. Strength is expressed in kg (top panel), and relative to body mass (bottom panel). *A significant group-time interaction was observed, reflecting a greater increase in strength over time in the experimental, compared with the control group. I Relative to Body Mass I START MIDPOINT END I Self-Concept The data analyses revealed neither main effects nor interaction. The means of the PPCS scores were similar for both E and C across the training program. In fact, the means were very high from the first time the PPCS was administered, after 3 weeks of training ( and 4.17 f 0.43 for E and C, respectively), at the midpoint ( and 4.30 f 0.43 for E and C, respectively), and at the end of the first year of training (426 f 0.47 and 4.11 f 0.45 for E and C, respectively).

9 Long-Term Resistance Training Table 3 Differences of Muscle Strength (1RM) in the Experimental (E) and Control (C) Groups From Start to hlidpoint and From Midpoint to End of the Resistance-Training Program Variable Group Start to midpoint Midpoint to end Knee extension Absolute/(kg) Relative to body mass (kgflrg) Knee flexion Absolute (kg) Relative to body mass (kgflrg) Note: Midpoint is the end of the first year. *Significantly greater change in E compared with C. Discussion The main finding of this study is that 21 months (9 months of training, 3 months vacation, and 9 months of training) of progressive resistance training among prepubertal boys result in a significant increase in muscle strength with no detrimental effect on linear growth. Additionally, only one minor injury occurred. This study is unique in view of the extended duration of training. Previous prospective intervention studies on the effect of resistance training among children were usually 24 months in duration (for review see 10) with few exceptions (Ramsay et al. [30]: 20 weeks; Stahle et al. [37]: 9 months). This duration is too short for the determination of the effect of resistance training on growth. The present study is more than twice as long as that reported by Stahle et al. (37). While the effect of resistance training on growth needs to be studied over a much longer period, the results of the present study indicate no detrimental effect over a period of almost 2 years. The gain in height over the study period was similar in E and C, reflecting that resistance training over the 2 years had no detrimental effect on skeletal growth. There is a commonly held belief that intensive weight training is associated with and may be he cause or snort s t n s belief. However, these studies were of relatively short duration, and the long-term effects were unknown. While this study involved low-to-moderate-intensity training, it clearly demonstrates that over a 21-month period, resistance training certainly does not hinder growth. The intensity of training (frequency and load) in the present study was relatively low in comparison with most previous studies among children. Subjects trained twice per week, whereas in most previous intervention studies subjects

10 366 - Sadres et al. trained three times per week (see 11 for review). Notable exceptions are the studies published by Faigenbaum et al. (8-lo), Stahle et al. (37), and Falk and Mor (12), who found strength improvements even while training only twice per week. This frequency of training is in line with recent ACSM guidelines for exercise testing and prescription (1). The results reported in the present study are in agreement with the above-mentioned studies. The training load used in the present study was low-moderate: 50% of 1RM during the first year (range: 30-70%) and 60% of 1RM during the second year (range: 5670%). In previous studies where load was specified, a greater load was used (50-100% of 1RM; 29, 33, 36, 40). Despite the low-moderate load in the present study, strength gains were significantly greater in E compared with C in all strength measurements (knee extensions: 83% vs. 79%; knee flexors: 63% vs. 57%). Calculated as weekly strength gains, this comes out to 0.82 and 1.01% gain per week during the first and second year, respectively, for the knee extensor and 0.50 and 1.14% gain per week during first and second year, respectively, for the knee flexors. In comparison, the weekly strength gains in studies reviewed for a recent meta-analysis (1 I), ranged between 1.5 to 10% per week. Explanations for the lower rate of strength gain in the present study include (a) the relatively low load of training; (b) the long duration of our study along with the known curvilinear relationship between strength gain and duration of training (33); (c) the fact that training included exercises other than testing exercises; and (d) the age range of participants in the present study was very narrow, while in previous studies, a wider age range (or greater SD) is reported. This can provide for a larger variation and, possibly, larger strength gains. Interestingly, the strength gains in E appear to be greater during the second compared with the first year although these differences did not reach significance (Table 3, Figures 1 and 2). This is in contrast with the usual greater strength gains during the initial portion for a training period. This can be explained by the higher load used in the second, compared with the first, year (60% vs. 50% of IRM), although other factors, such as the subjects' older age andlor maturational status, cannot be ruled out. Additionally, the larger strength gained in the second year may be due to adaptations that occurred during the fxst year and the summer vacation. Further study is required to better understand the effect of detraining, followed by resumed training. It is difficult to compare increases in strength observed in this study with those of previous studies in view of the different duration, frequency, and load of training. One way to standardize the effect of training is to calculate the effect size (ES). ES is the magnitude of an effect in terms of standard deviation units and it allows for quantitative comparison between studies (15). It is defined as the difference between the gain score means of the experimental and control groups, divided by the standard deviation of the pooled variance or of the control group's variance. The calculated ES's in the present study were 1.53 and 0.76 for the knee extensors and knee flexors performance, respectively. These ES's are higher than the mean ES of 0.57 reported by Falk and Tenenbaum (1 1) in their meta-analysis of the effectiveness of resistance training among children. Therefore, it is clear that despite the low frequency and low-moderate intensity, the resistance training program in the present study was effective in improving muscle strength and possibly more effective than previous studies of shorter duration, greater frequency, and higher load. These findings are in line with Faigenbaum et al. (8), who recently

11 Long-Term Resistance Training demonstrated that resistance training performed twice weekly with high repetitions and moderate load may be better than low repetitions and heavy load. Further research is needed in order to optimize resistance training for children. The higher initial muscle strength displayed by E compared with C, even when corrected for body mass, is somewhat difficult to explain. Part of the explanation may lie in the fact that initial testing was performed 3 weeks after training had begun. During these 3 weeks only techniques of weights handling and weight lifting were learned, and safety measures were emphasized. However, it is possible that this "pre-training" period also resulted & strength gains. ~es~it; the greater initial strength of E, the strength increments during the study were greater in E compared with C even when expressed as a percentage of initial strength. The summer vacation months provided a unique "detraining" period to this study. While absolute strength increased for both groups, there was no change in strength relative to body mass. This indicates that despite the relatively long break, no detraining took place. This is in contrast to a previous report of strength reduction within 8 weeks of detraining among pre-pubertal boys (5). The difference in the detraining effect between the two studies may be partly explained by the shorter training period prior to detraining (20 weeks vs. 9 months). In the present study, it is assumed that subjects kept active during the summer months at a level that prevented a detraining effect. Nevertheless, activity during the summer vacation was not monitored. More research is required to clarify the processes that take place during detraining and the mechanisms responsible for these processes in children, as well as in adults. The injury rate in the present study was very low ( participanthours), despite the relatively long duration of training. An even lower rate (0.004 injuries1100 participant-hours) was reported in a previous survey of year old boys who participated in weight training or weight lifting (17). It should be noted that in both studies, qualified personnel, specially trained to work with children, were involved. In the present study the intensity and frequency of training were also relatively low. These findings support the claim that properly performed resistance training can be safe, as well as effective, among pre-pubertal children (1, 13,26). The PPCS scores did not improve during the training program. Thus, the notion that participation in a progressive resistance training program can result in positive changes in self-concept (e.g., 20, 39) was not supported in this study. Typically, the experimental subjects (i.e., participants in a resistance training program) are compared with control subjects who are not exposed to any type of physical activity (3, 19). In this study, the control subjects participated in regular physical education classes in which they were exposed to sports such as track and field, basketball, and team-handball. The number of hours allocated to ball-games -- and-track-anlfield activities are almost 60% of the tot! hours allocated to physi- --- cal education activities (25). The boys that participated inie s5tyeitgs als5iz themselves from the beginning of the study. Indeed, the average of the PPCS scores of each group across the training program was about 4.2 on a scale of "1" to "5." This high value was maintained throughout the program. In conclusion, this study demonstrates that two school years of progressive resistance training (9 months of training, 3 months of detraining, and 9 months of training) at a low-moderate intensity among pre-pubertal boys can result in an

12 368 - Sadres et al. enhancement in strength, which is not accompanied by any detrimental effect on growth. Further study is required to examine the effect of a longer and more intense training program. References 1. American College of Sports Medicine. Guidelines for Exercise Testing and Prescription for Children, the Elderly, and Pregnancy. Philadelphia: Williams & Wilkins, Baechle, T. Essentials of Strength Training and Conditioning. Champaign, IL: Human Kinetics, Berger, B.G., and A. McInman. Exercise and the quality of life. In: Handbook of Research on Sport Psychology. R.N. Singer, M. Murphey, and L.K. Tennant (Eds.). New York: Macrnillan, 1993, pp Blanksby, B., and J. Gregor. Anthropometric, strength and physiological changes in male and female swimmers with progressive resistance training. Aust. J. Sport Sci. 1:3-6, Blimkie, C.J.R., J. Martin, J. Ramsay, D. Sale, and D. MacDougall. The effects of detraining and maintenance weight training in boys on strength development in prepubertal boys. Abstract. Can. J. Sport Sci. 14:102P, Blimkie, C.J.R. Resistance training during pre and early puberty: Efficacy, trainability, mechanisms, and persistence. Can. J. Sport Sci. 17: , Blimkie, C.J.R. Resistance training during preadolescence: issues and controversies. Sports Med. 15: , Faigenbaum, A.D., W.L. Wescott, R.L. Loud, and C. Long. The effects of different resistance training protocols on muscular strength and endurance development in children. Pediatrics 104: l/e Faigenbaum, A.D., W.L. Westcott, L. Micheli, A. Outerbridge, C. Long, R. La-Rosa Loud, and L. Zaichkowsky. The effects of strength training and detraining on children. Med. Sci. Sports Exerc. Suppl. 27:s 1 15, Faigenbaum, A.D., L.D. Zaichkowsky, W.L. Westcott, L.J. Micheli, anda.e Fehlandt. The effect of a twice-a-week strength training program on children. Pediatr Exerc. Sci. 5:339-46, Falk, B., and G. Tenenbaum. The effectiveness of resistance training in children. Sports Med. 3: , Falk, B., and G. Mor. The effects of resistance and martial arts training in 6- to 8-yearold boys. Pediatr Exerc Sci. 8:48-56, FTMS (The International Federation of Sports Medicine). Resistance training for children and adolescents. In: Sports and Children, K-N. Chan, and L.J. Micheli (Eds.). Hong Kong: Williams & Wilkins Asia-Pacific Ltd., 1998, pp Fukunaga, T., K. Funato, and S. Ikegawa. The effects of resistance training on muscle area and strength in prepubescent age. Ann. Physiol. Anthrop. 11: , Glass, G.V., B. McGaw, and M.L. Smith. Meta-Analysis in Social Research. Beverly Hills, CA. Sage, Godin, G., and R.J. Shephard. A simple method to assess exercise behavior in the community. Can JAppl Sports Sci 10: , Hamill, B.P. Relative safety of weightlifting and weight training. J. Strength and Cond. Res. 8:53-57, 1994.

13 Long-Term Resistance Training Hochberg, Y. A sharper Bon ferroni procedure for multiple tests of significance. Biometrika 75: , Holloway, J.B., and T.R. Baechle. Strength training for female athletes: A review of selected aspects. Sports Med , Holloway, J.B., A. Benter, and J.L. Duda. Self-efficacy and training for strength in adolescent girls. J. Appl. Social Psychol. 18: , Lillegard, W.A., E.W. Brown, D.J. Wilson, R. Henderson, and E. Lewis. Efficacy of strength training in prepubescent to early postpubescent males and females: Effects of gender and maturity. Pediatr: Rehab , Lintunen, T. Perceived physical competence scales for children. Scand. J. Sports Sci., , Lintunen, T., E. Leskinen, M. Oinonen, M. Salinto, and P. Rahkila. Change, reliability, and stability in self-perceptions in early adolescence: A four-year follow-up study. Int. J. Behav. Devel. 18: , Mersch, E, and H. Stoboy. Strength training and muscle hypertrophy in children. In: International Series on Sports Sciences. Children and Exercise XIII, S. Oseid, and K.H. Carlsen (Ed.). Champaign, IL: Human Kinetics, 1989, pp Ministry of Education and Culture. Physical Education Cum.culum for Kindergarten, Elementary and High Schools. Jerusalem: Author, (Hebrew) 26. NSCA (National Strength and Conditioning Association). A Position Paper and Literature Review of Youth Resistance Training. Colorado Springs, CO: Author, Ostrow, A.C. (Ed.). Directory of Psychological Tests in the Sports and Exercises Sciences. Morgantown, WV Fitness Information Technology, Ozmun, J.C., A.E. Mikesky, and P.R. Surburg. Neuromuscular adaptations following prepubescent strength training. Med. Sci Sports Exerc. 26: , Pfieffer, RD., and R.S. Francis. Effects of strength training on muscle development in prepubescent, pubescent, and postpubescent males. Phys. Sports Med. 14: , Ramsay, J.A., C.J.R. Blimkie, and K. Smith, S. Garner, J.D. MacDougall, and D.G. Sale. Strength training effects in prepubescent boys. Med. Sci. Sports Exerc. 22: , Rians, C.B., A. Weltman, B.R. Cahill, C.A. Janney, S.R. Tippett, and EI. Katch. Strength training for prepubescent males: Is it safe? Am. J. Sports Med. 15: , Sailors, M, and K. Berk. Comparison of responses to weight training in pubescent boys and men. J. Sports Med. 27:30-36, Sale, D.G. (1989). Strength training in children. In: Perspectives in Exercise Science and Sports Medicine, Vol. 2. Youth Exercise and Sports, C.V. Gisolfi, and D.R. Lamb (Eds.). Indianapolis, JN:Benchmark Press, 1989, pp Sewall, L., and L.J. Micheli. Strength training for children. J. Pediatr: Orthop. 6: , Siegel, J.A, D.N. Camaione, and T.G. Manfredi. The effects of upper body resistance -- -tr~onprepubescent_~ldren~e&at~ Exerc Sci. 1 : , Slaughter, M.H., T.G. Lohman, B.A. Boileau, C.A. Horswill, R.J. Stillman, M.D. Van Load, and D.A. Bemben. Skinfold equations for estimation of body fatness in children and youth. Hum. Biol., 60: , Stahle, S.D., S.O. Roberts, B. Davis, and L.A. Rybicki. Effect of a 2 versus 3 times per week weight training program in boys aged 7 to 16. Med Sci Sports Exec 27 Suppl.:S 114, 1995.

14 370 - Sadres et al. 38. Tanner, J.M. Growth at Adolescence. Oxford, England: Blackwell Scientific Publishers, Tucker, L.A. Effect of weight training on self-concept: A profile of those influenced most. Res. Quart. Exerc. Sport, 54: , Weltrnan, A., C. Janny, C.B. Rians, K. Strand, B. Berg, S. Tippit, J. Wise, B. Cahill, and F. Katch. The effects of hydraulic resistance strength training in pre-pubertal males. Med. Sci. Sports Exerc. 18: , Westcott, W.L. Female response to weight training. J. Phys. Educ. 77:31-33, Acknowledgments The authors thank the boys who participated in the study and their parents. Special thanks to the teachers and Auny Gara and Chatem Charkia, the principals of A1 Zahara Elementary School and Ibn-Rushed Elementary Schools, respectively, for their cooperation and enthusiastic support. Additionally, we would like to thankylftach Gordoni for his assistance in the statistical analysis. This study was supported by the Israeli Sports Authority and the Terner Fund.

15 ~ppehdix 1 A Detailed Description of the Resistance Training Program for 1 Month I Week 1 Week 2 j Monday Thursday Monday Exercise Sets Reps Load Sets Reps Load Sets Reps Load Thursday Dead lift Clean pulls* Snatch pulls* Snatch (power) Clean* Jerk % % Circuit training: 3 rounds of exercises for: % One. Abdominal muscles Two. Leg extension and flexion Three. Arm extension and flexion Four. Back extension using: bars medicine balls s dumbbells a % CD i $ Front squat Back squat 25 50% % i (P * from the thighs I Y -4 3, g.

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