Age-related deficit in load-induced skeletal muscle growth.

The growth response of ankle flexor and extensor muscles to two models of increased loading, functional overload (FO) and hind-limb reloading following hind-limb suspension, was measured by wet weight in Fisher 344-Brown Norway rats at ages ranging from 6 to 30 months. In response to FO, there was a 40% decrease in absolute growth of the plantaris beginning in middle age. Interestingly, the growth response to FO of 30-month old rats maintained on a 40% calorie-restricted diet improved by more than twofold relative to 30-month old rats on a normal chow diet. Recovery of muscle mass upon reloading following disuse was significantly impaired (reduced 7-16%) in predominantly fast, but not slow, muscles of 30-month relative to 9-month old rats. Initial investigation of the Akt signaling pathway following FO suggests a reduction or delay in activation of Akt and its downstream targets in response to increased loading in old rats.