Exercise during growth may contribute to the prevention of osteoporosis by increasing peak bone mineral density (BMD). However, exercise during puberty may be associated with primary amenorrhea and low peak BMD, while exercise after puberty may be associated with secondary amenorrhea and bone loss. As growth before puberty is relatively sex hormone independent, are the prepubertal years the time during which exercise results in higher BMD? Are any benefits retained in adulthood? We measured areal BMD (g/cm 2 ) by dual-energy X-ray absorptiometry in 45 active prepubertal female gymnasts aged 10.4 ؎ 0.3 years (mean ؎ SEM), 36 retired female gymnasts aged 25.0 ؎ 0.9 years, and 50 controls. The results were expressed as a standardized deviation (SD) or Z score adjusted for bone age in prepubertal gymnasts and chronological age in retired gymnasts. In the cross-sectional analyses, areal BMD in the active prepubertal gymnasts was 0.7-1.9 SD higher at the weightbearing sites than the predicted mean in controls (p < 0.01). The Z scores increased as the duration of training increased (r ‫؍‬ 0.32-0.48, p ranging between <0.04 and <0.002). During 12 months, the increase in areal BMD (g/cm 2 /year) of the total body, spine, and legs in the active prepubertal gymnasts was 30 -85% greater than in prepubertal controls (all p < 0.05). In the retired gymnasts, the areal BMD was 0.5-1.5 SD higher than the predicted mean in controls at all sites, except the skull (p ranging between <0.06 and <0.0001). There was no diminution across the 20 years since retirement (mean 8 ؎ 1 years), despite the lower frequency and intensity of exercise. The prepubertal years are likely to be an opportune time for exercise to increase bone density. As residual benefits are maintained into adulthood, exercise before puberty may reduce fracture risk after menopause. (J Bone Miner Res 1998;13:500-507)

Cross-sectional studies of elite athletes suggest that growth is an opportune time for exercise to increase areal bone mineral density (BMD). However, as the exercise undertaken by athletes is beyond the reach of most individuals, these studies provide little basis for making recommendations regarding the role of exercise in musculoskeletal health in the community. To determine whether moderate exercise increases bone mass, size, areal, and volumetric BMD, two socioeconomically equivalent schools were randomly allocated to be the source of an exercise group or controls. Twenty boys (mean age 10.4 years, range 8.4 -11.8) allocated to 8 months of 30-minute sessions of weight-bearing physical education lessons three times weekly were compared with 20 controls matched for age, standing and sitting height, weight, and baseline areal BMD. Areal BMD, measured using dual-energy X-ray absorptiometry, increased in both groups at all sites, except at the head and arms. The increase in areal BMD in the exercise group was twice that in controls; lumbar spine (0.61 ؎ 0.11 vs. 0.26 ؎ 0.09%/month), legs (0.76 ؎ 0.07 vs. 0.34 ؎ 0.08%/month), and total body (0.32 ؎ 0.04 vs. 0.17 ؎ 0.06%/month) (all p < 0.05). In the exercise group, femoral midshaft cortical thickness increased by 0.97 ؎ 0.32%/month due to a 0.93 ؎ 0.33%/month decrease in endocortical (medullary) diameter (both p < 0.05). There was no periosteal expansion so that volumetric BMD increased by 1. 14 ؎ 0.33%/month, ( p < 0.05). Cortical thickness and volumetric BMD did not change in controls. Femoral midshaft section modulus increased by 2.34 ؎ 2.35 cm 3 in the exercise group, and 3.04 ؎ 1.14 cm 3 in controls ( p < 0.05). The growing skeleton is sensitive to exercise. Moderate and readily accessible weight-bearing exercise undertaken before puberty may increase femoral volumetric BMD by increasing cortical thickness. Although endocortical apposition may be a less effective means of increasing bone strength than periosteal apposition, both mechanisms will result in higher cortical thickness that is likely to offset bone fragility conferred by menopause-related and age-related endocortical bone resorption. (J Bone Miner Res 1998;13:1814-1821)

More than ever, many young athletes are being encouraged to train intensely for sporting competitions from an early age. Compared with studies in adults, less is known about the physiological trainability of adolescents. The velocity of physical growth during the adolescent years makes research with a group of young athletes particularly difficult. The purpose of this review is to discuss a number of physiological issues that surround the performances of the adolescent athlete. Research has highlighted the role of growth hormone (GH) in the abrupt acceleration of linear growth that occurs during adolescence. In addition, GH has been shown to be sensitive to exercise following short term intervention studies. The reduced anaerobic power of the adolescent athlete compared with that of an adult athlete has been attributed to the intrinsic properties of the muscle that are yet to be fully understood. Resistance training studies in male adolescents, and to a lesser extent female adolescents, highlight the substantial relative strength gains that can be obtained. Aerobic trainability in young boys appears to improve markedly during the adolescent years. One of the most plausible explanations for this observation is the 'trigger hypothesis' which links increased aerobic improvements in adolescence with hormonal changes and substantial growth of the cardiorespiratory and musculoskeletal systems. Studies of aerobic trainability in adolescent girls are too scarce to be conclusive. An understanding of the impact of long term intensive training on adolescent athletes is difficult to ascertain because physical stresses vary both between and within sports. There is, however, limited evidence to suggest that 'intense' training does not impair normal growth, development or maturation. Adolescent athletes who experience rapid growth as well as large increases in training volumes may be vulnerable to overuse injuries.