by Cameron Joe Blimkie & Alain Marion
Resistance Training during Preadolescence: Issues, Controversies, and Recommendations
This article is a synopsis of scientific data whose content has been adapted for coaches. The main source of references for this text is a review of the literature by Cameron J. R. Blimkie: Resistance Training during Preadolescence: Issues and Controversies, Sports Medicine 15 (6): 389–407, 1993.
Cameron Blimkie, PhD, is an associate professor of kinesiology at McMaster University in Hamilton, Ont. His main research interest is in the area of pediatric exercise physiology, with a particular interest in the effects of physical activity and sport training on growth and development of the neuromuscular and skeletal systems in children.
Alain Marion is a coaching consultant with the Coaching Association of Canada, where he is responsible for advanced coach education programs including NCCP Level 4/5. He has a master’s degree in exercise physiology from the Université de Montréal.
Introduction
A highly controversial issue for coaches is whether or not activities specifically aimed at improving strength should be used and encouraged during preadolescence. Traditionally, resistance training during childhood has been considered both ineffective and potentially harmful. On the other hand, for years coaches from a variety of sports have had to manage programs whose very nature imposed significant mechanical stress on the musculoskeletal system of preadolescents. These coaches have observed that controlled and progressive loading could be useful in improving performance. The latter is perhaps best exemplified by the sport of gymnastics, where young children repeatedly generate high levels of power outputs and impose heavy loads to their muscles, bones, and connective tissues in the process of learning and performing specific skills.
In recent years, the area of strength training prior to puberty has been closely examined by sport scientists. This has brought forward new perspectives on the issue which, to a certain extent, also challenge some of the traditionally established positions. The purpose of this article is threefold:
• to present an overview of some of the arguments traditionally invoked in the scientific or sport literature against strength training during preadolescence
• to outline how some of the conventional advice on the subject may need to be reconsidered in light of recent data
• to provide coaches with basic guidelines regarding the development of strength prior to puberty.
The authors define strength or resistance training as a program of regular exercises which use one or a combination of training methods and devices (free weights, pneumatic and hydraulic machines, body weight) in an attempt to increase strength. It is considered to be a completely separate activity from the sports of power lifting or competitive weightlifting. Preadolescence, or childhood, is defined as the period encompassing pre- and early puberty as determined by Tanner (1962), with an upper age of approximately 11 years for females and 13 years for males. An increase in strength is indicated either by improved performance in some strength-related fitness test, by an increased weightlifting capacity (for example, lifting more weight or the same weight a greater number of times) or by the ability to exert increased force as measured by special devices such as a dynamometer.
Traditional arguments against resistance training during preadolescence
Although the effectiveness, benefits, and risks of strength training in adults are relatively well documented in the scientific literature, little information was available in North America regarding children until a few years ago. Nonetheless, based on a limited number of studies, it has traditionally been recommended in the coaching literature that resistance training should be avoided during preadolescence on the basis that significant strength gains could not be made before puberty and that it could be harmful to children.
It is well-established that growth is a key factor responsible for increases in strength during childhood, and that strength tends to improve naturally and in a similar fashion in both boys and girls during this period (Shephard 1982). Based on this, strength gains as a result of training were often not considered a likely possibility until fundamental hormonal changes occur with puberty. In other words, gains in strength were associated with increases in muscle size (hypertrophy), and because the muscles of children show only a limited capacity to hypertrophy, the benefits of resistance training during preadolescence appeared questionable. Early studies conducted in the 1960s and 1970s (Kirsten 1963; Vrijens 1978) as well as a more recent investigation (Docherty et al. 1987) did not report strength gains as a result of training in preadolescents. Despite the fact that only low to moderate loads were used in these studies, these data suggested that resistance training was ineffective in children.
Based on early measurements of cardiac dimensions in adults (Morganroth et al. 1975; Chignon et al. 1975), it has also been inferred that high training loads, including resistance training, could be potentially harmful to the normal development of the cardiorespiratory system in children. More specifically, it was believed that intense short-duration training could lead to a thickening of the left ventricular wall and a reduced cardiac cavity. Although scientific reviews have provided strong evidence that these apprehensions were unfounded (Perreault et al. 1994), this notion was commonly held in the coaching community until recently. Other arguments against strength training before puberty included potential loss of flexibility and reduced muscle contraction speed.
The risks for injury and potential growth-related problems that may result from undue stress imposed on the musculoskeletal system of preadolescents have also been extensively emphasized in the literature, especially the sensitivity of the joint structures and the growth plates of the long bones (Shephard 1982; Israel 1993). Appropriately, a position of extreme caution was advocated throughout the coaching community in order to guard against possible misuses of strength training during childhood. This, in addition to the lack of specific guidelines in the literature, including the National Coaching Certification Program, and the fact that resistance training is often associated with heavy weightlifting could also explain why strength training during preadolescence has traditionally been largely discouraged.
When planning sport programs for children, many coaches therefore face the following ethical dilemma:
• Are there potential short and long term benefits on performance that may result from controlled resistance training during preadolescence?
• Are the benefits, if any, worthwhile in light of potential risks?
Strength gains are possible during preadolescence
As mentioned previously, it was generally accepted until recently that resistance training prior to adolescence would be ineffective in increasing strength. However, most of the studies conducted since the middle of the 1980s and which have (i) incorporated moderate to high training loads and (ii) controlled for the effects of growth and motor skill acquisition on strength gains, provide convincing evidence that significant and substantial strength gains can be made during preadolescence. Based on the information provided by these studies, it appears that children are equally, if not more, trainable in a relative sense (e.g. percentage improvements) than adolescents and young adults. It seems, however, that preadolescents are less trainable in terms of absolute strength.
Strength training and performance-related parameters
A comparative analysis of studies reveals that similar strength gains may be achieved through a variety of methods and programs. It also appears that of all training parameters (i.e. intensity, duration, mode, and type of contraction) intensity is the key determinant of effectiveness. However, and although guidelines have been proposed in the literature, coaches should realize that the optimal training mode, number of sets, repetitions per set, or training sessions per week in preadolescents has yet to be determined.
Regarding the magnitude of the load, Kraemer and Fleck (1993) report that children should be capable of executing at least six to eight repetitions and that heavier loads should be avoided. Basic recommendations regarding resistance training in preadolescents have also been proposed by a variety of organizations, including the American College of Sports Medicine, the National Strength and Conditioning Association, the American Orthopedic Society for Sport Medicine, the United States Olympic Committee, and the New Zealand Federation of Sports Medicine. These recommendations can be summarized as follows (from Wilmore and Costill 1994):
• The frequency of training should be two or three times a week.
• The duration of the main part of a resistance training session should not exceed 30 minutes.
• Each set should consist of 6–15 repetitions. One to three sets of a given exercise should be performed.
• Resistance should be increased very progressively (for example, 1 to 1.5 kg increments) once the child is capable of executing 15 repetitions of a given exercise while maintaining good technical execution.
It may be tempting for coaches to speculate that strength gains resulting from appropriate resistance training could lead to improved performances and/or facilitate skill acquisition in preadolescents. For instance, a greater relative strength could represent an advantage when the young athlete must exert force to displace his or her own body, as is the case in most sports. Even in activities where the mass of a standard implement must be overcome, as in ball games, a greater absolute force could be transferred to the object which could represent a significant factor for performance improvement. Unfortunately, scientific data on the effects of strength training on athletic performance during preadolescence are scarce. Two studies involving young swimmers have been published, but the results are inconclusive (Ainsworth 1970; Blanksby and Gregor 1981). Due to the lack of specific data, it is therefore currently not possible to draw any firm conclusion as to whether or not there is a direct relationship between strength gains and improved sport performance in children. Although better results have been reported in vertical jumping as a result of strength training (Nielsen et al. 1980; Weltman et al. 1986), suggesting improvements in motor fitness, it remains to be determined whether or not these types of improvements in isolated tests translate into enhanced performances in more complex sport movements.
The effects of resistance training on cardiorespiratory fitness during childhood appear to be dependent on the nature of the training program. Based on the limited information available, conventional dynamic resistance training appears to have little effect on either absolute (l/min) or relative (ml/kg/min) VO2max in preadolescents (McGovern 1984; Blimkie 1993). Short-term (<20 weeks) resistance training therefore does not seem to have any negative effect on the normal development of cardiorespiratory fitness during preadolescence, provided there is an opportunity for participation in a variety of sports or recreational activities. On the contrary, there is some evidence that resistance training programs using hydraulic or isokinetic devices, and featuring alternative concentric contractions between agonistic and antagonistic muscles, may provide concurrent improvements in both strength and VO2max in preadolescents (Weltman et al. 1986).
Based on the information available from the studies listed in Table 1, it appears that strength training has no significant effect on the body composition (body fat and lean body mass) of preadolescents.
Mechanisms underlying strength gains in preadolescents
A review of the studies listed in Table 1 provides convincing evidence that resistance training during preadolescence can improve strength per cross-sectional muscle area (e.g. relative strength), since no significant increase in muscle size appears to have taken place in the subjects studied. That skeletal muscle may be incapable of hypertrophy in the absence of high level circulating androgens has in fact been one of the key arguments put forth in the past to support the non-relevance of resistance training during preadolescence; because hypertrophy could not be observed in children it was concluded that no strength gains were possible. However, this view implied that muscle size alone would be responsible for an individual’s strength. We now know this is not the case, and that increases in strength following training can be explained both by increased muscle size and neuro-motor adaptations. Neural adaptations could result in a greater number of motor units activated simultaneously, a higher frequency of firing of motor units and/or reduced inhibitory signals on motor neurons from the central nervous system. Improved motor unit activation has been demonstrated in adults as well as in children following resistance training, particularly during the early stages of the program (Blimkie et al. 1989; Ramsay et al. 1990; Sale 1993; Ozmun et al. 1994). Improved movement coordination is probably another important factor contributing to strength gains in preadolescents, especially for complex multi-joint exercises such as arm curl or leg press.
Very little information is available regarding the maintenance of strength gains with detraining in children. Current data suggest that training-induced strength gains in preadolescents are impermanent, as is also the case with older age groups. However, available information suggests that, contrary to adults, a single high-intensity training session per week is not sufficient for maintenance. The reasons for this are unknown, but could perhaps be explained by the lack of hypertrophy observed in children and by the decreased motor unit activation resulting from the reduced training stimulus.
Resistance training and the risk of injury
Perhaps the single most important argument against strength training during preadolescence has been the risk for injury and/or potential growth-related problems, due to the sensitivity of the joint structures and the growth zones of the bones during childhood. However, with regard to the latter argument, it is clear that heavy resistance training is not the only activity which can potentially impose undue stress on the musculoskeletal system of preadolescents. Injuries to growth plates have been reported in baseball (Adams 1965), while tennis and swimming share an increased risk for this type of injury when young athletes are subjected to excessive training loads (Wilmore and Costill 1994).
Clearly, it must be emphasized that strength or any high-intensity training during preadolescence must be approached with a high degree of caution. Injury data indicate that weightlifting in unsupervised conditions may indeed be a risky activity for preadolescents (U.S. Consumer Product Safety Commission 1987). However, the risks for injuries such as fracture or damage to growth plates appear to be less dramatic than previously thought. To the authors’ knowledge, only one case of epiphyseal fracture resulting from weightlifting during preadolescence has been reported (Gumbs et al. 1982), and there is no mention of such injuries in any of the prospective strength training studies involving preadolescents listed in Table 1. In a recent review of the literature, Mazur et al. (1993) noted that most cases of injuries caused by weightlifting exercises in children and adolescents (10 to 19 years of age) were the result of accidents in the home, not the result of supervised weight-training exercises or competitive weightlifting. They also concluded that there was no increased risk of injury in prepubescent athletes participating in carefully supervised strength-training programs. One incident of soft-tissue injury has been reported by Rians et al. (1987). Weltman et al. (1986) also report one case of injury related to strength training in a prepubescent child, but the injured subject missed only three training sessions. By contrast, however, competitive weightlifting, power lifting, and body building may prove riskier activities for children and, according to some professional bodies, should not be recommended for preadolescents (American Academy of Pediatrics 1990; Proceedings of the Conference on Strength Training and the Prepubescent 1988). This opinion is not shared by expert weightlifting coaches who contend that, under careful supervision, their sport is not dangerous.
Therefore, it appears that general strength training is not a particularly risky activity for preadolescents in terms of musculoskeletal injury when it is closely supervised, properly instructed, and appropriately prescribed through proper progression. In fact, controlled resistance training might offer some protection against injury by strengthening the muscles that cross a joint (Kraemer and Fleck 1993). Whether or not connective tissues such as tendons and ligaments can adapt as a result of strength training remains to be determined, but there is evidence to this effect in the literature (Zernicke and Loitz 1993).
Conclusion and practical recommendations
Recent studies provide new perspectives in the area of strength training during preadolescence, and suggest that some of the traditional views on the subject may need to be reconsidered. However, it is also clear that additional research is needed to provide coaches with more precise and practical information, such as loading parameters, long-term effects, and specific impact of resistance training on performance. In light of the information currently available, the following conclusions can be formulated :
• Strength gains are possible during preadolescence and, in terms of relative changes, are comparable to those made by adolescents and adults.
• Intensity appears to be the critical loading parameter that determines strength gains during preadolescence.
• The optimal loading parameters in terms of number of repetitions, sets, and training sessions per week remain to be determined.
• Strength gains during preadolescence can be attributed primarily to improvements in neuromuscular activation and motor coordination, not hypertrophy.
• Maintenance of strength gains during preadolescence cannot be achieved on the basis of one high intensity training session per week.
• Short-term resistance training does not interfere with the normal development of cardiorespiratory fitness during preadolescence, and may even provide a positive stimulus under specific conditions.
• A direct relationship between improved sport performance and strength gains made as a result of training during preadolescence remains to be demonstrated. Whether or not this is more a shortcoming of the limited literature currently available than proof of non-correlation between the two variables is unclear. However, motor fitness appears to be increased.
• Body composition of preadolescents (body fat and lean body mass) is unaltered by resistance training.
• The risk of musculoskeletal injury resulting from resistance training during preadolescence cannot be excluded, but the risk is low in competently supervised training conditions where competition among subjects is prohibited.
Based on the above, it appears that the answer to the two questions posed earlier should be: “Yes, but …” The following are therefore offered as general guidelines to coaches and parents concerning resistance training during preadolescence:
• Before initiating a resistance training program, preadolescents should be examined by a physician and declared fit.
• Resistance training should be encouraged as only one of a variety of normal recreational and sport activities.
• Children involved in the program must be mature enough to accept coaching and instruction.
• Resistance training using body weight should be encouraged.
• Resistance training with weights, machines, or other devices should always take place under the supervision of a qualified adult.
• Thorough warm-up and cool-down periods should be included in any resistance training session involving preadolescents.
• Loading should be based on each child’s own capacity and follow a progression throughout the program.
• As a general rule, intensity should not exceed what has been reported in Table 1. Extremely high intensity efforts, such as maximal or near-maximal lifts with free weights or machines, should be avoided. Children must be capable of performing six to eight repetitions of an exercise.
• Proper technique should be emphasized at all times over the amount of weight lifted or the number of repetitions achieved, and exercises should be stopped when the quality of technical execution starts to break down. Particular attention should be paid to proper alignment of body segments during exercises.
• A variety of training modalities should be used, such as body weight, free weights, machines, and springs.
• Competition between children should be discouraged, and the emphasis should be on personal improvement.
• Eccentric training involving isolated muscles should be avoided, and the emphasis should be on dynamic concentric contractions.
• Circuit training with low to moderate resistance should be considered as a means of introducing children to correct technique, and to capitalize on possible cardiorespiratory benefits.
• Balance should be achieved between upper- and lower-body development and between agonistic and antagonistic muscles when performing resistance training.
• If weight training machines are used, only those specifically designed for children, or those for which the loads and levers can be easily adjusted to accommodate the reduced strength capacity and size of children, should be used.
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