Morphologic and functional alterations in aging rat muscle

To understand better the causes of reduced contractile force in aging skeletal muscle, we performed a physiologic and morphologic analysis of plantaris muscle in old rats. The peak twitch tension (Fmax) and rates of force development and relaxation were significantly lower in old (24 months old) rats than in young (six month old) rats. In teased muscle fiber preparations, there was a 5% reduction in the mean number of fibers in the aging plantaris muscle. Histologically, a net loss of fibers occurred only in the muscle belly. Histochemically, fewer Type I fibers were seen in the belly and proximal regions, whereas distally fewer Type IIa fibers were seen. The loss of Types I and IIa oxidative fibers suggested a conservation of fast-twitch Type IIb fibers in a fast-twitch muscle. The relatively small loss of muscle fibers does not explain the large decline in muscle contractile performance which, despite established doctrine, was independent of muscle mass, fiber number or size, or number of fast-twitch fibers. The reduced force production in aging rat muscle appears to be due to a defect in excitation, contraction performance or metabolic activity, rather than a purely anatomical abnormality of muscle.