EFFECTS OF VOLUNTARY EXERCISE ON BONE-MINERAL CONTENT IN RATS

We used a voluntary running model to explore the relationship between average daily running distance and bone mineral status of rats. A total of 60 male Sprague-Dawley rats were randomly assigned at 6 weeks of age to a sedentary control group (n = 22) or to a group with unlimited access to a running wheel (n = 38). The running distance of exercising rats was monitored daily, and steady-state running levels ranged from 3.2 to 18.1 km/day. At the end of the experimental period, femora and tibiae were dissected and bone mineral content (BMC, g/cm) and bone mineral density (BMD, g/cm2) were measured by single-photon absorptiometry. Cross-sectional morphometry was examined by taking a transverse section of the femoral middiaphysis. Hindlimb percentage fat was significantly higher in controls than in runners (20.0 +/- 1.2 versus 11.1 +/-0.6, p < 0.001), and soleus mass was greater in runners than in controls (371 +/- 8.1 versus 320 +/- 0.8 mg, p < 0.001). Femoral and tibial lengths, weights, and volumes were significantly higher in runners than in controls (p < 0.005). BMC and BMD were higher in runners than in controls at all sites apart from the distal femur. Cross-sectional areas at the femoral midshaft were greater in running rats than in sedentary controls (6.26 +/- 0.1 versus 5.45 +/- 0.3 mm2, p < 0.02), as was the polar moment of inertia (15.6 +/- 0.6 versus 12.7 +/- 0.2 mm4, p < 0.05). No positive correlation was found between distance run and BMC, BMD, cross-sectional area, or polar moment of inertia. Conversely, there was a highly significant positive correlation between body weight and most bone measurements and a negative correlation between distance run and body weight and femoral and tibial wet weight and volume. Partial correlations correcting for differences in body weight suggested that exercise level was not an independent predictor of BMC. In summary, voluntary running activity markedly increased femoral and tibial BMC in young male rats compared to weight-matched sedentary controls. However, within the exercising group substantially higher running distances produced no further increase in BMC.