Bone mass increases in less than 4 wk of voluntary exercising in growing rats

Introduction: Physical exercise is known to augment bone mass, but the spec ific mechanisms by which physical activity influences skeletal metabolism i s still not thoroughly understood. So far, time related events of bone tiss ue adaptation to physical training have not been investigated. We, therefor e, studied the time-course effects of daily spontaneous exercise training o n bone metabolism in the rat by biochemical and tissue analyses. Methods: F orty-five 4-wk-old female Dark Agouti rats were used, randomly assigned to voluntary exercising groups of five animals for 1, 2, 3, and 4 wk, or to ag e-matched sedentary controls. At sacrifice. blood was sampled for determina tion of parameters of bone and calcium metabolism (osteocalcin, alkaline ph osphatases, total and ionized calcium, phosphorus, parathyroid hormone). Ri ght and left tibiae were removed for bone mass (dry and ash mass, mineral c ontent) and histomorphometric analyses. Results: We found that the animals performed large amounts of exercise (reaching 50 +/- 8 km.wk(-1)), and that this voluntary physical training induced significant changes in bone metab olism An increase (similar to 32%) in serum alkaline phosphatase activity w as already measurable at the end of the first week of exercising and persis ted throughout the experiment, accompanied by a concomitant increase in ost eoblastic bone formation (similar to 27%), as evaluated in bone tissue. The se changes were associated with consecutive increases in tibial bone mass ( similar to 7.6%), mineral content (similar to 7.4%) and metaphyseal bone vo lume (similar to 17%) measurable 4 wk after the onset of exercise, with int eresting positive correlations between tibial bone mass parameters and sole us muscle mass. Conversely, no change was evidenced in biochemical paramete rs of calcium metabolism except for an increase in phosphorus serum levels in trained animals. Conclusion: Daily spontaneous exercise training induced an increase in bone mass and bone volume in less than 4 wk. Further studie s are needed to fully investigate these changes during the very first days of exercising.