Cortical bone adaptation to weight-bearing exercise: A mechanical and histomorphometric analysis

To quantify bone response to training, bone size, mechanical strength, and formation were examined in swine femurs. Adult crossbred sows (3 yr, 229 kg) were randomly assigned to either Basal (B, n = 6), Control (C, n = 7), or Trained (T, n = 7) groups. The B and C groups did not exercise and were killed initially (B) or after 20 wks (C). The T group walked on a treadmill 5 d/wk for 20 wk at 5 km/hr and 5% grade. There was no significant difference with time (B vs C) or with training (C vs T) in bone size (length, area, or fat free wt) or mechanical properties (bending moment, ultimate stress, or modulus of elasticity). In the femur midshaft, fluorescent labeling of formation indicated greater periosteal active surface and mineral apposition rate (MAR) in T vs C femurs. There were no differences among groups in the number of labeled osteons but intracortical MAR was greater in T than C femurs. There were no differences among groups in endosteal active surface or MAR. Pooled across groups the quadrants of the femur cross section showed different activity patterns, however, there were no interactions between groups and quadrants. The increased formation on the periosteal and osteonal surfaces did not alter mechanical properties between sows, but may have increased mass and strength within a femur.