Complexity of age-related change in skeletal muscle.

Age-related changes in skeletal muscle mass, fiber area, and contractile function were examined in pathogen-free rats at 6, 12, 28 and 36 mos of age. The intent of this study was to clarify age-related decline, particularly in contractile force, and to determine if the decline in contractile tension with age is due to alterations at the neuromuscular junction. A variable amount of age-associated reduction in muscle mass was noted for the soleus (18%), extensor digitorum longus (EDL-16%), plantaris (37%), and gastrocnemius (38%) muscles. The decline in fiber area for these four muscles was between 5 and 16% greater than the loss in muscle wet weight. A variable amount of change in peak contractile force between 6 and 36 mos was observed for the soleus (62%), EDL (48%), and plantaris (34%). For soleus and EDL, the decline in peak tetanic tension exceeded the decline in muscle mass and fiber area. Most of the declines for the animals used in this study did not become significant until after the age of 28 mo. The marked reduction in peak tetanic tension, fiber area, and muscle mass between 28 and 36 mos indicates an accelerated age-related decline in this time period. The reduced peak twitch and peak tetanic tension in the oldest animals was not due to likely age-related changes at the neuromuscular junction. Peak values for tetanic tension were similar, whether tension was elicited via direct muscle stimulation or through stimulation of the nerve. Results underscore the complexity of age-related change and suggest that multiple mechanisms contribute to the decline of skeletal muscle.