The aging of bone and muscle.

The purpose of this article has been to summarize the existing body of literature on the changes in the function of muscle and bone that occur as a consequence of aging. Throughout this article, the importance of distinguishing between aging and disuse has been a recurring theme. Much of what had previously been considered aging is now regarded as functional disuse. With aging, muscle undergoes a reduction in size, and consequently strength, which is related to a loss of muscle fibers and a reduction in the size of existing fibers. There may be a selective loss of the Type II, or fast twitch fibers, but this is still under debate. This loss of muscle fibers is more than likely related to the loss of functional motor units. Muscle strength is reduced as a consequence of these changes, but there appears to be little loss in the metal olic potential of the muscle with aging. Exercise training can increase the size and strength of the trained muscles through hypertrophy of both Type I and II fibers and neural alterations but appears to have little effect on decreasing fiber number. Losses of bone start in the third and fourth decade of life, but the pattern of bone loss differs between cortical and trabecular bone, the former being less influenced by menstrual status. Exercise training appears to attenuate the normal decreases in bone with aging and can even lead to small increases in bone density and mass.