Bone response to jumping is site-specific in children: a randomized trial.

Skeletal loading during growth may be one way of increasing bone mass early in life. We hypothesized that children randomized to a jumping program (25 jumps/day from a 45-cm box, 5 days/week for 12 weeks) would have greater increases in hip and spine bone mineral content (BMC) and 4% distal tibia volumetric bone density than children randomized to the control group. Our secondary hypothesis was that jumping would not be as beneficial among peripubertal children as compared to prepubertal or pubertal children due to the relatively high growth rate that occurs during the peripubertal period. Fifty-four children (31 girls) ages 3-5, 7-8, 11-12, and 15-18 years were enrolled. We performed bone, anthropometric, and force plate measurements at baseline and 12 weeks. Twenty-four-hour diet recall and Tanner's self-report of pubertal development were completed at baseline. Jumpers had a lower calcium intake than nonjumpers at baseline (965 +/- 403 vs 1295 +/- 465 mg/day, P < 0.01), but the groups were otherwise similar. Overall, jumpers had greater increases in total body BMC (45.0 +/- 4.9 vs 29.4 +/- 5.3 g, P = 0.03) and regional dual energy x-ray absorptiometry leg BMC (19.8 +/- 2.6 vs 11.5 +/- 2.8 g, P = 0.03) than nonjumpers at all pubertal stages. However, the 4% distal tibia bone response to jumping appeared to be modified by pubertal stage, with the greatest bone benefit from jumping observed in pubertal children (interaction of jumping group by pubertal stage, P < 0.05, for both BMC and volumetric BMD). A similar pattern was observed for spine BMC (interaction, P = 0.10). We conclude that skeletal loading increases total body and leg BMC in children, but may not have a positive effect at sites that are predominantly trabecular bone during periods of rapid growth (i.e., peripubertal period).