Inactivity, age, and exercise: single-muscle fiber power generation.

We examined the effects of mild therapeutic exercise during a period of inactivity on size and contractile functions of myosin heavy chain (MHC) type I (n = 204) and type II (n = 419) single fibers from the medial gastrocnemius in three age groups. Young adult (5-12 mo), middle-aged (24-31 mo), and old (32-37 mo) F344BNF1 rats were assigned to one of three groups: weight-bearing control, non-weight bearing (NWB), and NWB plus exercise (NWBX). Fourteen days of hindlimb suspension were applied in NWB rats. The NWBX rats exercised on the treadmill for 15 min, four times a day, during the period of NWB. The NWBX did not improve peak power, but increased normalized power of MHC type I fibers in young adult rats. In MHC type II fibers, NWBX did not change peak power, isometric maximal force, V(max), and fiber size from young adult and middle-aged rats. NWBX did not improve peak power and isometric maximal force and showed a dramatic decline in V(max) and normalized power in the old rats. Collectively, mild treadmill exercise during a period of inactivity does not improve peak power of MHC type I or type II fiber from the gastrocnemius in young, middle-aged, and old rats. However, NWBX is beneficial in enhancing normalized power of MHC type I fibers in young adult rats, most likely due to the stimulus intensity and the ability of the individual fibers to adapt to the stimulus. In contrast, several factors, such as impaired adaptation potential, inappropriate exercise intensity, or increased susceptibility to muscle damage, may contribute to the lack of improvement in the older rats.