The effect of exercise on bone mass and structural geometry during growth.

Regular weight-bearing exercise is widely reported to have beneficial effects on bone mineral content and areal bone mineral density during growth, but the structural basis underlying these changes remains uncertain. In young athletic children, participation in high-impact sports has been shown to enhance bone formation on the periosteal and/or endosteal surfaces of long bones at loaded skeletal sites. Participation in moderate physical activity, recreational play or school-based exercise interventions designed to specifically load bone have also been shown to enhance bone mineral accrual. However, few data are available on the surface-specific effects of exercise training or general physical activity on bone. Based on the limited data available, it would appear that the structural response of bone to exercise during growth is maturity dependent and sex specific; prior to puberty exercise appears to increase periosteal apposition in both sexes, whereas during or late in puberty exercise appears to result in periosteal expansion in boys but endocortical contraction in girls. In most cases, these geometric changes lead to an increase in bone bending strength. However, there are contrasting results as to whether the pre- or peripubertal years are an optimal time to intervene for the greatest osteogenic response; it is likely that both periods represent an important time for incorporating physical activity to optimize bone health. There are also many unresolved questions as to the optimal dose of exercise (intensity, frequency, duration and rate of progression) needed to enhance bone strength in children and adolescents. We know that weight-bearing exercise is important, and that activities should be dynamic, variable in nature, applied rapidly and intermittently, and that relatively few loading cycles are required. Although several effective interventions have been designed for improving bone mass, further research is needed to define the specific exercise programs or activities that will optimize bone structure and strength during growth. Perhaps most importantly, further work is also needed to determine whether any exercise-induced alterations in bone mass and structure during growth are maintained into old age when fractures occur.