Bone involution poses serious health risks for aging women. Bone mass is subject to both local (mechanical) and systemic (hormonal) homeostatic control mechanisms. The local forces acting on bone are due to gravity and muscular contraction. There are several theories concerning the mechanisms of local control. When bent, bone functions as a piezoelectric crystal with calcium accumulation on the negatively charged concave surface. Microfractures that occur in response to stress greater than normal levels stimulate osteoclastic activity to remove the damaged structure. Studies of astronauts and immobilized subjects have consistently found bone atrophy. The degree of bone loss is related to the difference in levels of stress normally applied and those at bedrest in the site studied. Correspondingly, athletes have greater bone mass than the sedentary population, with the greatest hypertrophy found in the areas most stressed. Exercise intervention also promotes bone hypertrophy. Both middle-aged and elderly women increase bone mass or reduce the rate of loss in response to physical activity intervention programs.