Chronic exposure to stretch-shortening contractions results in skeletal muscle adaptation in young rats and maladaptation in old rats.

The objective of this research was to investigate skeletal muscle response to a chronic administration of stretch-shortening cycles (SSCs) in young and old rats. Dorsiflexor muscles of old (30 months, n = 5) and young (12 weeks, n = 6) rats were exposed 3 times/week for 4.5 weeks to a protocol of 80 maximal SSCs per exposure in vivo. Skeletal muscle response was characterized by isometric and dynamic performance, as well as by muscle wet mass and quantitative morphological analyses following the exposure period. The performance of the young and old groups was not statistically different at the start of the exposure. By the end of the exposure, however, a statistical difference was noted, as performance increased significantly in the young animals and decreased significantly in the old animals. Muscle wet mass of the left tibialis anterior (TA) in the treated limb was significantly greater in the youngthan in the old animals (p < 0.001), whereas there was no difference in the contra-lateral TA. No degenerative myofibers or changes in non-cellular interstitium were noted in either age group, but a significant increase was observed in the volume of the cellular interstitium in the exposed limb of the old animals (p = 0.01), which is indicative of an inflammatory response. Thus, a chronic exposure of SSCs results in significant performance increase and muscle hypertrophy in young animals, and a significant performance decrease and an increased cellular interstitial response in old animals. These findings suggest that age may impair the ability of skeletal muscle to adapt to repetitive mechanical loading, even in the absence of degeneration.