Differential effectiveness of low-intensity exercise in young and old rats.

Low-intensity exercise increases strength and function in old adults, but it is unclear if change occurs secondary to "neural adaptation" or to intrinsic muscle adaptation. Whether function and strength change concomitantly is also unclear. We examined effects of a modest intensity, 10-session exercise program on muscle mass, contractile force, and function (gait) in 6-month-old and 30-month-old rats. Animals underwent 45 minutes of activity (e.g., ramp walking, balancing) 5 days/week. In old animals, a significant increase in muscle mass and peak contractile force occurred with exercise in soleus, plantaris, extensor digitorum longus, and peroneus longus compared with controls, but did not restore values to those for young controls. The increase in muscle force in old rats was accompanied by a significant lengthening of stride (90 +/- 9 to 103 +/- 15 mm), which was still 23% less than stride values for young rats. Changes in muscle function and gait with exercise were not apparent in young rats. Results suggest that (a). rapid and significant changes in muscle mass and strength in an aged organism can occur with a modest activity program, (b). the threshold for muscle adaptation may differ in young versus old rats, and (c). changes in strength and function in old rats may occur concomitantly.