ADAPTATIONS OF IMMATURE TRABECULAR BONE TO MODERATE EXERCISE - GEOMETRICAL, BIOCHEMICAL, AND BIOMECHANICAL CORRELATES

Strenuous endurance exercise can adversely affect the mechanical integrity of immature bone, but it is unclear whether a more moderate exercise regimen would have a positive effect. Thus, to investigate the response of immature trabecular bone to moderate exercise, we randomly assigned female Sprague-Dawley rats (8 weeks old) to either a basal-control, exercise, or age-matched control group. The basal-control rats were killed at 8 weeks of age, while the other two groups were killed at 18 weeks of age. Between 8 and 18 weeks, one group remained sedentary, while another group was trained progressively on a motor-driven treadmill at a moderate level of intensity. Rat femoral necks (FN) were tested in cantilever bending to failure, and the sixth lumbar vertebral bodies (L6) were compressed to 50% of their initial height. Both tissues were analyzed for calcium, hydroxyproline, and collagen-crosslinking concentrations, and for changes in geometry. The adrenal mass per unit body mass was significantly greater in the exercised group, compared to the age-matched controls. L6 calcium content, compressional stress, and elastic modulus were significantly less in the exercise group as compared to the age-matched control group. Nonreducible collagen crosslinks (hydroxylysylpyridinoline [HP] and lysylpyridinoline [LP]) were significantly greater in the older exercise and age-matched control L6 and FN. In the weight-bearing FN-but not L6-the LP concentration of the exercise group was significantly greater than the age-matched controls. FN hydroxyproline concentrations were less and FN fracture angles were different than age-matched controls, but FN mechanical properties were not affected significantly by the exercise regimen. These responses to moderate exercise suggest an intricate interplay between local mechanical effects and systemic factors that drive the adaptations in the bone.