Exercise and skeletal health.

Involutional osteopenia contributes to older persons' susceptibility to fractures of the hip, spine and wrist. Experimental evidence is reviewed that supports the hypothesis that involutional bone loss can be prevented by physical exercise. Weightlessness and immobilization result in accelerated bone loss. In cross-sectional studies measuring total body potassium (TBK), both muscle mass and bone mass in normal humans have remained strikingly constant. The usual decrease in total body calcium (TBCa) and TBK with increasing age was not observed in a cross-sectional study of marathon runners. Localized hypertrophy of bone and muscle has been reported for a variety of occupations and sports. Two prospective studies have demonstrated an increment in bone mass after a program of physical exercise. It would seem that certain involutional changes in body composition (loss of TBCa and TBK) can be prevented by increased physical activity, but the effectiveness of specific types and durations of exercise should be investigated.