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Strength Training for Children and Adolescents Jeffrey A. Guy, MD, and Lyle J. Micheli, MD 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 Because of the increasing demands ticipate in some form of strength for performance and the decreasing training to enhance performance ages of participation and peak per- and endurance and to reduce the formance, young athletes are con- risk of injury. While the effective-tinually being asked to perform at ness, risks, and methods of training higher levels and to improve at a for the adult population have been quicker pace than ever before. As extensively studied,1,2 the role of the demands increase, the athletic strength training for children and community has been asked to sup- adolescents remains a topic of con-ply the means to increase athletic troversy and often heated debate.3-5 performance, and the medical com- A number of important questions munity has been asked to validate have been asked. Can strength the safety of these methods. training increase the muscular Strength training has become one strength in young athletes? Is of the most popular and rapidly strength training safe? Can strength evolving modes of enhancing ath- training result in increased athletic letic performance. Although initial- performance? ly limited to those sports thought to The initial controversy surround-require strength for optimal perfor- ing strength training for the young mance, such as football and rugby, athlete evolved from unfounded some form of strength training has statements and three misconcep-now been adopted in virtually every tions regarding the risks and poten-sports activity. tial benefits to the athlete. The first It is commonplace for adult ath- misconception is that the prepubes-letes, both male and female, to par- cent athlete cannot benefit from Vol 9, No 1, January/February 2001 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. 29 Strength Training for Children and Adolescents 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 Bradley4 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, Vrijens16 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 al17 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- 30 Journal of the American Academy of Orthopaedic Surgeons Jeffrey A. Guy, MD, and Lyle J. Micheli, MD 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 al7 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 al8 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-baum5 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 al18 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 al9 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 al22 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 Vol 9, No 1, January/February 2001 31 Strength Training for Children and Adolescents 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 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 Micheli10 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 al12 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). 75 65 55 In a study by Faigenbaum et al8 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 T MT DT C Persistence of Training-Induced Gains 45 Pretraining Posttraining Detraining 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 32 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.) Journal of the American Academy of Orthopaedic Surgeons Jeffrey A. Guy, MD, and Lyle J. Micheli, MD 35 * 30 25 * 20 15 10 Pretraining Posttraining Mid-detraining Post-detraining Figure 2 The effects of strength training and detraining on children demonstrated in the study by Faigenbaum et al8 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.) least slow down this regression has based sports programs.3 To ad-yet to be determined. While these dress the question of whether findings may bring into question strength training by the prepubes-the need for maintenance programs cent child is associated with an un-for children, more information is acceptable risk of injury, we must required before specific recommen- first revisit the relevant definitions. dations can be made. The terms “strength training” and “resistance training” are used to refer to progressive resistance to Risks of Resistance enhance performance or ability by Training for Young using submaximal amounts of Athletes weight. The terms “weight lifting” and “power lifting” usually refer to The past 20 years have seen a the use of maximal amounts of marked increase in the participation weight at high intensities during of children in competitive sports, competition. and the popularity continues to It has been estimated that more grow. Approximately 30 million than 17,000 weight-lifting or power-children (50% of boys and 25% of lifting injuries in adolescents re-girls) are involved in either competi- quiring emergency room visits oc-tive organized sports or community- cur annually.25 However, most of Vol 9, No 1, January/February 2001 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 only25 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 al21 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- 33 ... - tailieumienphi.vn