Role of exercise in prevention of involutional bone loss.

Physical inactivity has been cited as a possible cause of osteoporosis. Because involutional bone loss in the female can begin as early as age 40, the purpose of this investigation was to compare the skeletal status of two groups of premenopausal middle-aged (30-49 yr) women of diverse physical activity levels. Bone mineralization was determined by x-ray densitometry (middle phalanx of fifth finger and os calcis) and photon absorptiometry (distal and midshaft radius) in 42 marathon runners and 38 sedentary females. Mean values for bone mineral content (BMC) and bone density were greater in the marathon runners at the midshaft radius (P less than 0.05) and at the middle phalanx of the fifth digit (P less than 0.001). Mean density of the os calcis was higher in the physically inactive women (P less than 0.001). Following normalization of the data for differences in age and body size, regression analysis suggests that the runners maintain their bone mass longer at the distal radius, a site frequently fractured in women after midlife.