Does Bone Exhibit a Dose-Respone to Jumps of Increasing Height and Repetitions?

Abstract

Jump exercise has been shown to be an effective mode of training for increasing bone density in adult women, but little is known about the optimal combination of jump height (magnitude) and jump repetitions to improve bone health. PURPOSE: We conducted a 6-month pilot study to evaluate the effect of jump height and jump repetitions on hip and lumbar spine bone mineral density (BMD) in premenopausal women. METHODS: Ninety premenopausal women participated in this study (mean age: 31.3 ± 9.4 years; mean BMI: 23.3±2.9; mean hip t-score: +0.7± 0.6). Women were randomly assigned to one of nine jump groups (n=10 per group) comprised of the 9 combinations of jump height (4″, 8″ or 12″ box height) and number of jump repetitions (30, 60, or 90 jumps per session). Boxes were constructed and provided to women to be used for jump training at home. Women were instructed to wear athletic shoes and to step up onto their box and then jump off onto a hard, stable surface, for their specified repetitions three times per week for six months. BMD at the proximal hip and lumbar spine was measured by dual energy x-ray absorptiometry (Hologic QDR-1000Wi) at baseline and 6 months. Baseline BMI, energy and calcium intake, minutes of physical activity and BMD were similar between groups. The effect of jump height and repetitions on BMD was evaluated using a 3 × 3 × 2 repeated measures ANOVA, with time as the repeated factor. Of interest were both 3-way (height × repetitions × time) and 2-way (height × time and repetitions × time) interactions. RESULTS: Seventy-three women (81%) returned for post-testing. Compliance to the 6-month, home-based jump training program averaged 60% among the 55 subjects who returned training logs. There were no significant 3-way interactions at any bone site. However, the interaction term for repetitions × time approached significance at the greater trochanter (p = .056). Graphical inspection of this 2-way interaction revealed a dose-response effect of jump repetitions such that the greater trochanter bone response improved with increasing jump repetitions. This effect was not apparent for different jump heights, nor was there any effect at the lumbar spine. CONCLUSIONS: Data from this pilot study suggest that the number of jump repetitions may influence the magnitude of the bone response to jump training, in a dose-dependent fashion, independent of jump height. However, these data are limited by low sample sizes per group, short study duration and modest compliance to training. A larger, longer and perhaps supervised program is warranted in order to better examine the dose-response of bone to jump exercise.