Aging does not affect contractile properties of type IIb FDL muscle in Fischer 344 rats

The purpose of this study was to examine the influence of aging on the contractile, metabolic, and histochemical properties of the predominantly fast-twitch flexor digitorum longus muscle (FDL) in the Fischer 344 (F344) rat strain. Contractile measurements were made in situ by stimulating the motoneuron while the animals were under pentobarbital sodium anesthesia. Fiber-type composition, capillarity, aerobic capacity, and myosin light chain composition were also examined. The results of this investigation were striking in that there was no alteration in any parameter measured. Collectively, these results suggest that aging does not induce a functional alteration in the neuromuscular relationship in the predominantly fast-twitch FDL of F344 rats, and consequently, there is no loss of functional capacity of the muscle. This study demonstrates that alteration in predominantly fast-twitch muscle is not a universal phenomenon and that care must be taken when generalizing about the influence of aging on skeletal muscle. Some of the conflicting results regarding the influence of aging on predominantly fast-twitch muscle in rodents may be either strain or muscle specific.