Age-related differences in the adaptive potential of type I skeletal muscle fibers.

This study was designed to investigate whether the relationship of fiber size and force is maintained with aging and inactivity. We hypothesized that fiber size and fiber force-generating capacity would decrease in parallel, thus resulting in no change in specific force with either age or inactivity. Thirty male Fischer 344/Brown Norway F1 hybrid rats, 6-23-month old (young adult), 24-33-month old (middle-aged) and 34-40 month old (old) were hindlimb unweighted for 14 days. Single permeabilized type I fibers from the soleus and gastrocnemius muscles were evaluated for size and contractile function. The diameter of the fibers from the soleus muscle declined with age and unweighting. In contrast, the fibers from the gastrocnemius showed no age-related atrophy. In both the soleus and gastrocnemius, there was a significant decrease in the force generation with age and unweighting. When comparing size and peak absolute force in the type I fibers from control young and middle-aged animals a positive relationship was observed whereas no significant relationship between size and peak absolute force was observed in the old animals. Following unweighting, fibers from young and middle aged rats showed a significant relationship between force and size. Fibers from old animals did not exhibit a relationship between size and force following unweighting. These results suggest aged skeletal muscle has an attenuated ability to adapt to inactivity by altering its size in response to inactivity.