Three experiments were conducted to examine immature trabecular bone's response to intense and moderate exercise. In the first study, Biomechanical and Biochemical Changes in Lumbar Vertebrae of Rapidly Growing Rats (Salem et al., 1989;​ Appendix A) the sixth lumbar vertebral bodies of 44-day old and 54-day old, male, Sprague Dawley rats were compressed at a fast strain rate. Growth-related increases in vertebral-body geometry, calcium concentrations, and calcium-to-collagen ratios were consistent with the biomechanical increases in loads, energy absorption, stiffness, stress, strain-energy density, and elastic modulus.

In the second study, Structural and Mechanical Adaptations of Immature Trabecular Bone to Strenuous Exercise (Hou et al., 1990;​ Appendix B), female Sprague-Dawley rats (8 wk) were progressively exercised for 10 wk at 80-90% of their maximum oxygen consumption on a motor-driven treadmill, 5 days/wk for approximately 1 hr/day. Sedentary, age-matched rats served as controls. The femoral neck of the exercise group had a significant decrease in cortical-to-trabecular bone ratio and in structural and material properties. The lumbar vertebrae were not significantly affected by strenuous running.

In the third study, Response of Immature Femoral Neck and Lumbar Vertebrae to Moderate Exercise, female, Sprague-Dawley rats (8 wk) were assigned randomly to a basal-control (n = 15), exercise (n = 15), or age-matched control group (n = 15). The basal-control rats were killed at 8 wk of age, while the exercised and age-matched controls were killed at 18 wk of age. The exercise group trained progressively on a motor-driven treadmill (3 days/wk, 1 hr/day), so that by the tenth week they were exercising at a workload of approximately 71-77% of the maximum oxygen capacity for untrained rats.

Results indicated that the moderate exercise protocol significantly increased adrenal mass, vertebral body bone spicule-to-marrow ratio and femoral-neck lysylpyridinoline crosslinks. Vertebral body compressional stresses, elastic modulus, and calcium content, and femoral neck hydroxyproline content were significantly less, while femoral neck fracture angles were significantly altered in the exercise group, compared to the control group.