Evaluating the dose-response relationship between drop-jump height and bone adaptation: A randomized controlled trial

Exercise interventions frequently prescribe impact activities assuming that identical tasks produce similar mechanical loads, yet few studies quantify the external loads experienced during such interventions. This study examined drop-jump height and skeletal adaptation and whether quantified external load is associated with changes in bone in low-active, healthy young adults. Forty-eight participants (22 ± 2 years) were randomized to perform diagonal drop jumps (DDJ) from 0 cm, 40 cm, or 60 cm, or to a non-exercise group for 16 weeks (40 jumps, 4 days·week^-1). Force plates, inertial measurement units (IMU) and motion capture were taken at baseline, 6-, 12- and 16 weeks. Whole-body dual-energy X-ray absorptiometry (DXA) and tibial peripheral quantitative computed tomography (pQCT) were performed at baseline and week 16, with additional pQCT at week 12. Cortical density (Ct.D) increased at the dominant distal tibia from the 40 cm jumps (0.4%, p = 0.006) and control group (0.7%, p = 0.027), whereas the 60 cm jumps decreased (-1.7%) at week 12. Similarly, Ct.D at the tibial diaphysis increased in the 40 cm group (1.6%) but decreased in the control group (-1.6%, p = 0.033). No significant effects were detected for DXA-derived outcomes. Negative associations were found between the change in cortical and trabecular outcomes and loading variables, such as impact peak, load rate and ankle moment. Findings demonstrate that increased prescribed jump height does not necessarily reflect loading dose, highlighting the importance of quantifying mechanical load when designing exercise interventions. Furthermore, as the changes in cortical density did not exceed the pQCT least significant change, the results should be interpreted cautiously.