Exercise during growth results in biologically important increases in bone mineral content (BMC). The aim of this study was to determine whether the effects of loading were site specific and depended on the maturational stage of the region. BMC and humeral dimensions were determined using DXA and magnetic resonance imaging (MRI) of the loaded and nonloaded arms in 47 competitive female tennis players aged 8 -17 years. Periosteal (external) cross-sectional area (CSA), cortical area, medullary area, and the polar second moments of area (I P , mm 4 ) were calculated at the mid and distal sites in the loaded and nonloaded arms. BMC and I P of the humerus were 11-14% greater in the loaded arm than in the nonloaded arm in prepubertal players and did not increase further in peri-or postpubertal players despite longer duration of loading (both, p < 0.01). The higher BMC was the result of a 7-11% greater cortical area in the prepubertal players due to greater periosteal than medullary expansion at the midhumerus and a greater periosteal expansion alone at the distal humerus. Loading late in puberty resulted in medullary contraction. Growth and the effects of loading are region and surface specific, with periosteal apposition before puberty accounting for the increase in the bone's resistance to torsion and endocortical contraction contributing late in puberty conferring little increase in resistance to torsion. Increasing the bone's resistance to torsion is achieved by modifying bone shape and mass, not necessarily bone density. (J Bone Miner Res 2002;17:2274 -2280)

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)

Figure 6Serum bone specific alkaline phosphatase, osteocalcin, collagen propeptide of type I collagen (PICP), and urinary type I C-telopeptide breakdown products (CrossLaps) versus bone age. Prepubertal (filled circles), peripubertal (open circles), and postpubertal (crosses).

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)