Vol 9, No 1, January/February 2001 29 Because of the increasing demands for performance and the decreasing ages of participation and peak per- formance, young athletes are con- tinually being asked to perform at higher levels and to improve at a quicker pace than ever before. As the demands increase, the athletic community has been asked to sup- ply the means to increase athletic performance, and the medical com- munity has been asked to validate the safety of these methods. Strength training has become one of the most popular and rapidly evolving modes of enhancing ath- letic performance. Although initial- ly limited to those sports thought to require strength for optimal perfor- mance, such as football and rugby, some form of strength training has now been adopted in virtually every sports activity. It is commonplace for adult ath- letes, both male and female, to par- ticipate in some form of strength training to enhance performance and endurance and to reduce the risk of injury. While the effective- ness, risks, and methods of training for the adult population have been extensively studied, 1,2 the role of strength training for children and adolescents remains a topic of con- troversy and often heated debate. 3-5 A number of important questions have been asked. Can strength training increase the muscular strength in young athletes? Is strength training safe? Can strength training result in increased athletic performance? The initial controversy surround- ing strength training for the young athlete evolved from unfounded statements and three misconcep- tions regarding the risks and poten- tial benefits to the athlete. The first misconception is that the prepubes- cent athlete cannot benefit from strength training because of insuffi- cient circulating levels of andro- gens. 6 However, this has been dis- proved over the past decade, as research has documented that young athletes do in fact gain strength with a properly planned and super- vised training regimen. 4,7-13 The second misconception is that athletes participating in strength training lose both the flexibility and the range of motion necessary for optimal performance in their chosen sport. This has also been refuted by recent studies, with some research- ers reporting increased flexibility when flexibility training was incor- porated into a training regimen. 10 The third misconception is that strength training is dangerous and exposes the young athlete to unnec- essary risk of injury. This particu- lar question remains a cause for concern for parents and general physicians. The persistence of this concern is largely due to the inap- propriate comparison of injury rates with different modes of train- Dr. Guy is Fellow in Sportsmedicine, Boston Children’s Hospital, Boston, Mass. Dr. Micheli is Director, Division of Sports Medicine, Boston Children’s Hospital; and Associate Clinical Professor of Orthopaedic Surgery, Harvard Medical School, Boston. Reprint requests: Dr. Micheli, Boston Children’s Hospital, 319 Longwood Avenue, Boston, MA 02115. Copyright 2001 by the American Academy of Orthopaedic Surgeons. Abstract Strength, or resistance, training for young athletes has become one of the most popular and rapidly evolving modes of enhancing athletic performance. Early studies questioned both the safety and the effectiveness of strength training for young athletes, but current evidence indicates that both children and adoles- cents can increase muscular strength as a consequence of strength training. This increase in strength is largely related to the intensity and volume of load- ing and appears to be the result of increased neuromuscular activation and coor- dination, rather than muscle hypertrophy. Training-induced strength gains are largely reversible when the training is discontinued. There is no current evi- dence to support the misconceptions that children need androgens for strength gain or lose flexibility with training. Given proper supervision and appropriate program design, young athletes participating in resistance training can increase muscular strength and do not appear to be at any greater risk of injury than young athletes who have not undergone such training. J Am Acad Orthop Surg 2001;9:29-36 Strength Training for Children and Adolescents Jeffrey A. Guy, MD, and Lyle J. Micheli, MD Strength Training for Children and Adolescents Journal of the American Academy of Orthopaedic Surgeons 30 ing, such as weight training, resis- tance training, and power lifting. Injury rates with these modes of training can vary greatly, and ex- trapolation from one to another can be misleading. The literature in recent years has helped dispel some misconceptions about strength training for children and adolescents. Unfortunately, however, information from the med- ical community on these topics may appear to be inconsistent, depending on the experience of the practitioner and his or her knowledge of recent studies on strength training. Not surprisingly, parents, coaches, and trainers remain confused and uncer- tain about strength training and often refrain from its use. Definitions The term “strength training” is defined as the use of progressive resistive methods to increase one’s ability to exert or resist force. 4 The term “resistance training” may also be used in the same context and is often considered synonymous. This type of training is both con- trolled and progressive, often utiliz- ing various modalities, such as free weights, individual body weight, hydraulics, and elastic bands, to name a few. To be successful, a par- ticular training regimen must be individualized and must involve a timely progression in intensity, thereby stimulating strength gains that are greater than those associ- ated with normal growth and de- velopment. One particular area of confusion is in the use of the terms “strength training” and “resistance training” in relation to the terms “weight lift- ing” and “power lifting.” The latter terms should be used only to de- scribe techniques of training at high intensities with the goal being to lift maximal amounts of weights, often in competition. When reviewing the literature, the age group involved in discus- sion can be particularly confusing. For the purposes of this review, the definitions by Faigenbaum and Bradley 4 will be utilized. The terms “prepubescent” and “child” refer to girls and boys prior to the develop- ment of secondary sex characteris- tics, roughly defined as up to the age of 11 years for girls and up to age 13 for boys. The terms “pubes- cent” and “adolescent” are applied to girls aged 12 to 18 and boys aged 14 to 18. The term “young athlete” is a more comprehensive term and will be used when discussion in- cludes both the prepubescent and the pubescent athlete. Effectiveness of Strength Training for Young Athletes The topic of strength training by adult athletes has been the subject of extensive research. 14 However, the role of strength training for the young athlete remains controversial despite recent studies at a number of centers. During the 1970s, there were few studies available. As a result, many clinicians discouraged strength training for children. It was felt that prepubescent children were incapable of developing much strength and that physical weak- ness after puberty is merely the result of insufficient physical exer- tion. 15 This stance was reflected in a 1983 position paper of the Ameri- can Academy of Pediatrics in which it was stated that “prepubertal boys do not significantly improve strength or increase muscle mass in a weight training program because of insuffi- cient circulating androgens.” 6 Furthermore, several early stud- ies failed to demonstrate increased strength in children engaged in strength-training programs. 16,17 In 1978, Vrijens 16 reported no strength gains in a study of 10- to 17-year- old boys undergoing training ses- sions three times a week for a total of 8 weeks’ duration. Of interest, the training program involved low resistance and employed only one set of exercises per session. In a similar study, Docherty et al 17 found that 12-year-old boys did not benefit from strength training fol- lowing their competitive season. The frequency of training was three times weekly for a total of 4 to 6 weeks. However, both the low intensity of two sets per session and the short duration of the study may have compromised the results of the study. These studies have been cited in the literature as proof that strength training is ineffective for young ath- letes; however, careful evaluation suggests that these results may have been flawed by methodologic shortcomings. The nature of con- trol groups is important because as children continue to grow, a prepu- bescent athlete may in fact develop an increase in strength from normal growth alone, thus confounding any benefit from a training pro- gram. In addition, the training pro- gram itself may not provide the intensity, frequency, or length of training necessary to allow the prepubescent athlete to develop enough muscular strength to over- come differences observed with normal growth alone. The past 15 years has seen a pro- gressive and increased interest in the topic of strength training, and a number of controlled studies have examined the benefits and risks of youth strength training. One of the earliest clinical studies supporting strength training for prepubescent children was by Sewall and Mich- eli. 10 Eighteen prepubescent boys and girls participated in pneumatic resistance training for three 30- minute sessions per week for a total of 9 weeks. The children involved in training had a statistically signif- Jeffrey A. Guy, MD, and Lyle J. Micheli, MD Vol 9, No 1, January/February 2001 31 icant (P<0.05) mean increase in strength of 42%, compared with a 9% increase for control subjects. The study also showed that, even over a 9-week period, prepubescent children have a baseline increase in strength due to normal growth and maturation. Similar findings were demon- strated by Weltman et al, 18 who examined the effects of hydraulic strength training on prepubertal boys. Twenty-six boys participated in a strength training program three times a week for 14 weeks, and dif- ferences in isokinetic strength for flexion and extension at the knee and elbow joints were evaluated. Compared with an untrained con- trol group, subjects involved in training had an increase in strength of up to 36% for concentric work and an increase in torque of up to 45% for all eight motions tested (P<0.05). The findings in this study suggest that short-term, supervised concentric strength training with use of hydraulic resistance is both effective and safe for prepubertal boys, with no injuries sustained while training. As further evidence in support of strength training for prepubescent children accumulated, researchers began to manipulate training regi- men variables (e.g., frequency, in- tensity of exercise, and duration of training) in search of an optimal pro- gram. Because overuse injuries are not uncommon in the pediatric pop- ulation, 19,20 Faigenbaum et al 7 investi- gated the effects of a shortened fre- quency of training (twice a week) while maintaining a high level of in- tensity. In an 8-week study, prepu- bescent subjects underwent a twice- weekly training schedule based on an individual’s 10-repetition-maximum (10-RM) strength (i.e., the maximum weight that could be lifted ten times with good form). The prepubescent children were found to have a mean increase of 74% in 10-RM strength values compared with nontrained control subjects. Faigenbaum et al 8 found similar results in prepubescent subjects in a 1996 study: a mean increase of 53% in leg extension and a 41% mean increase in chest-press val- ues after 8 weeks of strength training. Thus, at a given intensity, twice- weekly training programs appear to increase strength in children to a level equivalent to that found with schedules requiring participation three times per week. Taking into consideration the number of variables involved in determining the effectiveness of resistance training, Falk and Tenen- baum 5 conducted a meta-analysis of nine studies demonstrating in- creased strength. All children in the studies were under the age of 13 years. In the combined studies, the resistance training group had a 71.6% increase in strength over the control group. There was no ad- vantage at any particular age, and there were no differences between the sexes. Thus, current evidence indicates that resistance training can result in marked strength gains in the pre- pubescent child. While the ultimate duration and intensity continue to be debated, children develop strength gains with workouts as infrequent as twice weekly. At this time, there do not appear to be any sex- or age- related differences. Physiologic Mechanisms for Strength Development Although the literature supports the contention that children may demonstrate strength gains with a proper training regimen, it is more difficult to define how and why this occurs and what the underlying mechanisms are. Numerous fac- tors, including muscle hypertrophy, increase in muscle cross-sectional area, motor-unit coordination, cen- tral nervous system activation, and psychological drive, may all con- tribute to increases in strength. These factors have been extensively studied in adults, but few studies have evaluated the underlying mechanism of strength gains in children. In an attempt to determine the contribution of muscle hypertrophy to increased strength, several re- searchers have included morpho- logic variables in their evaluation of strength changes. 7,9,18,21-23 Weltman et al 18 found little or no change in anthropometric and body composi- tion measures in prepubescent boys over a 14-week training period. No statistically significant differences were found in body circumference or skin-fold measurements. Body density as measured by hydrostatic weighing was also unchanged. Ramsay et al 9 found no statistically significant changes in anthropomet- ric indicators in prepubescent boys over a 20-week resistance training period. No changes were seen in the cross-sectional area of either the midportion of the upper arm or the midthigh as measured with com- puted tomography. Because prepubescent children lack circulating androgens, it is not surprising that strength gains seen in resistance training are not associ- ated with the muscle hypertrophy seen in the adult population (at least not in short-term studies). Neural adaptations have been implicated by some as primarily responsible for strength gains. 9,22 Ozmun et al 22 addressed this issue in a study of the effects of thrice-weekly biceps curls on prepubescent children over the course of 8 weeks. Significant isotonic and isokinetic strength in- creases were found in the trained group (22.6% and 27.8%, respective- ly), with no changes in either skin- fold or arm-circumference measure- ments. While these findings confirm that strength gains are not the result of muscle hypertrophy, the increased electromyographic measurements (17% greater amplitude in the trained Strength Training for Children and Adolescents Journal of the American Academy of Orthopaedic Surgeons 32 group) suggest that the early gains in strength seen in prepubescent children are due in part to increased muscle activation. Only one other study has ad- dressed the neural adaptations in strength training in children. Blimkie et al, 12 looking at isotonic strength changes in prepubescent children, found a significant (P< 0.05) increase in strength over a 10- week training period. Although there were no differences in muscle cross-sectional area, an increasing trend in motor unit activation was noted, as determined by interpolar twitch. It has also been suggested that intrinsic muscle adaptations, increased motor activation, im- proved motor skill performance, and coordination of the involved muscle groups may all play a role in the muscle strength seen with resistance training. 9 Although at this time it may be difficult to separate out the contri- butions and relative importance of each variable, it appears that neu- romuscular activation, motor coor- dination, and intrinsic muscular adaptations all contribute to the increased strength seen in prepu- bescent athletes undergoing resis- tance training. Similar mechanisms are found in adolescents and young adults, 14 but strength gains seen in prepubescent children ap- pear to be largely independent of muscle size. Not surprisingly, the training-induced gains in strength seen in postpubertal boys are accompanied by increased cross- sectional area of muscle. 16 Persistence of Training- Induced Gains The removal of stimulus, or “de- training,” is defined as the tempo- rary or permanent reduction or with- drawal of a training stimulus, which may result in the loss of physiologic and anatomic adaptations, as well as a decrease in athletic performance. 8 There are few studies of detraining in adults and even fewer in the pre- pubescent population. Furthermore, attempts to evaluate the persistence of resistance-induced strength gains in prepubescent subjects after with- drawal of a training stimulus may be confounded by the concomitant growth-related strength increases. 24 In a study of detraining in pre- pubescent children, Sewall and Micheli 10 suggested that the loss of strength due to withdrawal from training was greater than, and not offset by, the anticipated growth- related increases in strength over the same time period. In 1989, Blimkie et al 12 proposed a model of the effects of growth, resistance training, maintenance training, and detraining on strength devel- opment in children. In a study using that model, 13 the strength gains seen in the training group regressed over time in both the maintenance and detraining groups to levels close to, but still above, those of the untrained control sub- jects (Fig. 1). In a study by Faigenbaum et al 8 evaluating the effects of strength training and detraining on children, the results were consistent with those of Blimkie. 13 Despite a 53% increase in training-induced leg-extension strength over 8 weeks, a subsequent 8 weeks of detraining led to rapid and significant (P<0.05) decreases in both leg extension (−28%) (Fig. 2) and chest press performance (−19.3%). In the same period, the performance of the untrained control subjects in- creased slightly. The magnitude of loss for the trained group was ap- proximately 3% per week. A com- parison of groups at completion of detraining found no statistically sig- nificant difference in leg extension. Although the available data are limited, it appears that strength gains secondary to resistance train- ing during prepubescence are tran- sient and regress toward untrained control levels. The degree of regres- sion appears to depend on the mag- nitude of strength gains, level of inactivity, and duration of detrain- ing. Unfortunately, the amount of training required to maintain or at Pretraining 75 T MT DT C 65 55 45 Posttraining Detraining Leg strength, N • m Figure 1 Graphic illustration of Blimkie’s model demonstrating the effects of resistance training (T), maintenance training (MT), and detraining (DT) on strength development during normal growth (C) during childhood. The values for both the maintenance and detraining groups regressed with time to levels close to, but above, those of the untrained control subjects. (Adapted with permission from Blimkie CJR: Resistance training during pre- and early puberty: Efficacy, trainability, mechanisms, and persistence. Can J Sport Sci 17;4:264-279.) Jeffrey A. Guy, MD, and Lyle J. Micheli, MD Vol 9, No 1, January/February 2001 33 least slow down this regression has yet to be determined. While these findings may bring into question the need for maintenance programs for children, more information is required before specific recommen- dations can be made. Risks of Resistance Training for Young Athletes The past 20 years have seen a marked increase in the participation of children in competitive sports, and the popularity continues to grow. Approximately 30 million children (50% of boys and 25% of girls) are involved in either competi- tive organized sports or community- based sports programs. 3 To ad- dress the question of whether strength training by the prepubes- cent child is associated with an un- acceptable risk of injury, we must first revisit the relevant definitions. The terms “strength training” and “resistance training” are used to refer to progressive resistance to enhance performance or ability by using submaximal amounts of weight. The terms “weight lifting” and “power lifting” usually refer to the use of maximal amounts of weight at high intensities during competition. It has been estimated that more than 17,000 weight-lifting or power- lifting injuries in adolescents re- quiring emergency room visits oc- cur annually. 25 However, most of these injuries happen at home or school and are not the result of su- pervised activity. In several stud- ies of adolescents, the incidence of injury ranged between 7% and 40%. 26,27 Almost 75% of the inju- ries were strains, with the most common site being the lower spine. There are also numerous case re- ports or small series of serious weight-lifting and power-lifting injuries, such as cardiac rupture due to impact by a dropped bar- bell, 28 spondylolysis and spondy- lolisthesis, 29 and growth-plate injuries in the wrist. 30 Most of these injuries were attributed to improper lifting techniques, exces- sive loading, or inadequate teaching or supervision. Not surprisingly, recommendations about the partici- pation of young athletes in these activities vary from supervised par- ticipation only 25 to proscription of weight lifting, power lifting, and body building, as well as the use of maximal amounts of weight in training programs, for both chil- dren and adolescents. 31 Strength training for young ath- letes has received widespread sup- port. 3,4,10,11,18,24,32,33 Rians et al, 33 looking at subclinical musculo- skeletal injury (as evaluated on bone scan) or muscle damage (as estimated on the basis of serum creatine phosphokinase determina- tion), found no evidence of injury in prepubescent boys after 14 weeks of resistance training. Similar find- ings by Blimkie et al 21 found only mildly elevated creatine phospho- kinase values and concluded that short-term (duration of 20 weeks) resistance training by prepubertal boys did not pose any particular risk in terms of subclinical or clini- cal musculoskeletal injury. Perhaps a better assessment of the risk of injury associated with resistance training would come from prospective studies of closely monitored and supervised training programs with appropriately pre- * * 35 30 25 20 15 10 Pretraining Posttraining Mid-detraining Post-detraining Leg-Extension Strength (6 repetition maximum), kg Figure 2 The effects of strength training and detraining on children demonstrated in the study by Faigenbaum et al 8 were consistent with Blimkie’s model. 13 The trained group (solid circles) had a 53% increase in training-induced leg-extension strength over 8 weeks, but a subsequent 8 weeks of detraining led to a rapid and significant decrease (−28%) in leg-extension performance, while the performance of the untrained control subjects (open circles) increased slightly (asterisk indicates statistically significant [P<0.05] difference between control value and previous value for trained group). A comparison of groups at the completion of the 16-week detraining period revealed no significant difference from the control value for leg extension. (Adapted with permission from Faigenbaum AD, Westcott WL, Micheli LJ, et al: The effects of strength training and detraining on children. J Strength Cond Res 1996;10:109-114.) Strength Training for Children and Adolescents Journal of the American Academy of Orthopaedic Surgeons 34 scribed training loads. There have been no reported cases of serious injuries in these studies. 9,10,18 There- fore, it appears that the risks and concerns associated with youth strength training are no greater than those associated with other sports and recreational activities common to this age group. 4 However, this is based on the understanding that a given strength training program is competently supervised and the young athlete is properly instructed and underscores the need for pre- participant history, blood pressure measurements, flexibility screening, and a preparticipation physical examination. As with adult ath- letes, while no studies have demon- strated enhanced performance with strength training, experience strong- ly supports its use. Anabolic Steroid Use For years, athletes have taken exog- enous substances to manipulate their athletic performance. It is not surprising that modern athletes often turn to ergonomic aids like anabolic androgenic steroids to enhance muscle growth, increase strength, and improve physical performance. It has been esti- mated that over 1 million persons in the United States are currently using anabolic steroids, with a total expenditure of more than $100 million a year. 34 Although there is a potential for enhancing performance, anabolic androgenic steroids can have severe physio- logic and emotional side effects, such as a heightened risk for coro- nary disease, cholestatic jaundice, abnormal liver function, hepatic tumors, stunted growth, gyneco- mastia, and many psychotic disor- ders. In addition, there is the risk of transmission of diseases such as acquired immunodeficiency syn- drome and viral hepatitis through needle sharing. Early use of anabolic steroids in the United States was primarily by individuals involved in weight training. However, gains in size and strength prompted their use by other athletes. Today, anabolic steroids are consumed by both male and female power athletes, endurance athletes, and nonath- letes. Given the increasing pres- sure for athletes to perform better and earlier, it is no surprise that the use of anabolic steroids has breached the boundary of age. The use of steroids in the adoles- cent population brings with it an additional level of concern com- pared to its use by older athletes. Estimates of steroid use in the ado- lescent population have placed the prevalence at approximately 5% to 7% for boys and 1% to 3% for girls. 35-37 In a recent study of pre- adolescent middle-school students ranging in age from 9 to 13 years, approximately 2.7% of the students admitted using steroids. 38 The majority of the students felt that steroids would make their muscles bigger and stronger. While usage is not exclusive to any segment of the population, the literature suggests that the highest level is among ado- lescents from more affluent neigh- borhoods, presumably because of easier access to this relatively ex- pensive drug. 39 Most of the steroids used by young athletes appear to have been obtained illegally, in- creasing the risk of purchasing mis- labeled or impure agents. The physical side effects in ado- lescent boys can range from acne and gynecomastia to more serious conditions, such as priapism, sodium retention edema, and liver dysfunc- tion after prolonged use. In girls, clitoromegaly, hirsutism, and amen- orrhea are common, as well as per- manent deepening of the voice after prolonged use. Use by children of both sexes may also result in dimin- ished adult height, as premature closure of the physis is possible. Perhaps the most serious side effects of steroid use occur in the behavioral sphere; in the transition to adulthood, adolescents may be particularly vulnerable to the conse- quences of heightened aggression. 40 As the relatively high consump- tion of steroids by young athletes continues, the need for early educa- tional intervention concerning their effects is becoming more apparent. One such intervention is the ATLAS (Adolescents Training and Learn- ing to Avoid Steroids) program. 34 The goal of that program is to edu- cate adolescent athletes, enhance healthy behaviors, and minimize the factors that encourage steroid use. Although such programs ap- pear to be quite successful, they are limited in both number and avail- ability. Therefore, one cannot over- emphasize the role of health pro- fessionals, educators, and parents in providing a healthy and informed atmosphere for young athletes. Initiation of Training The proper initiation of strength training for children and adoles- cents is critical. Those supervising young athletes—coaches, trainers, and parents—should address several issues before initiating a program of training. First is whether the ath- lete is prepared psychologically and physically to participate in the pro- gram. This includes making sure that the athlete has had a prepartici- pation physical at school or at a physician’s office. In addition, supervising adults should strive to minimize pressure and stress placed on the athlete to perform. The second issue is whether the athlete understands what strength training is and what the goals of the program are. This point cannot be overemphasized, as misinformed athletes are at increased risk for injury. The athlete should under- stand the fundamental differences Jeffrey A. Guy, MD, and Lyle J. Micheli, MD Vol 9, No 1, January/February 2001 35 between strength training and weight lifting and the goals of each. Athletes should understand that while increasing one’s performance is a reasonable and attainable goal, increasing muscle size prior to the onset of puberty is not. Safety while training should also be emphasized. The third issue is which strength training program the athlete should follow. While the specifics of indi- vidual training programs are be- yond the scope of this article, the program chosen should be tailored to the athlete in question on the basis of age, size, experience, and sport. 41 Access to certain facilities and specific types of supervision are important considerations, as not everyone has a gym membership or the finances to hire a personal trainer. Parents interested in being involved in the training process can also con- sult the wealth of information in the literature on strength training for adolescents. 41-43 The objective is to have a well-informed, carefully supervised athlete participating in a balanced strength-training program with the goal of increasing strength and improving mental attitude and performance in sport. Summary The past decade has seen growing support from both the medical and the scientific communities regarding the participation of young athletes in strength training programs. Current evidence indicates that both prepu- bescent and pubescent children can, in fact, increase muscle strength, but not necessarily athletic performance, as a consequence of resistance train- ing. This increase in strength is largely related to the intensity and volume of loading and appears to be the result of increased neuromuscu- lar activation and coordination. These increases in strength do not ap- pear to be a consequence of muscle hypertrophy, as they are in adults. The training-induced strength gains are largely reversible when the train- ing is discontinued. There is no current evidence to support the misconceptions that chil- dren need androgens for strength gain, lose flexibility with training, or are at increased risk of injury. Given the proper supervision and appropri- ate instruction and program design, children involved in resistance train- ing do not appear to be at greater risk of injury than other young ath- letes who have not undergone such training. However, parents, coaches, and trainers should be aware that participation in unsupervised train- ing or in activities involving rapid and maximal loading places prepu- bescent children at increased risk of injury and is not recommended. References 1. Kraemer WJ, Duncan ND, Volek JS: Resistance training and elite athletes: Adaptations and program considera- tions. 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Faigenbaum AD, Westcott WL, Micheli LJ, et al: The effects of strength train- ing and detraining on children. J Strength Cond Res 1996;10:109-114. 9. Ramsay JA, Blimkie CJR, Smith K, Garner S, MacDougall JD, Sale DG: Strength training effects in prepubes- cent boys. Med Sci Sports Exerc 1990; 22:605-614. 10. Sewall L, Micheli LJ: Strength training for children. J Pediatr Orthop 1986;6: 143-146. 11. Webb DR: Strength training in chil- dren and adolescents. Pediatr Clin North Am 1990;37:1187-1210. 12. Blimkie CJR, Ramsay J, Sale D, MacDougall D, Smith K, Garner S: Effects of 10 weeks of resistance train- ing on strength development in prepu- bertal boys, in Oseid S, Carlsen K (eds): Children and Exercise XIII. Champaign, Ill: Human Kinetics, 1989, pp 183-197. 13. Blimkie CJR: Resistance training dur- ing pre- and early puberty: Efficacy, trainability, mechanisms, and persis- tence. Can J Sport Sci 1992;17:264-279. 14. 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