Characterization of changes in muscle performance in vivo during a chronic exposure of stretch-shortening cycles: Age effects.

The objective of this research was to investigate the skeletal muscle response to a chronic administration of stretch-shortening cycles (SSCs) in young and old rats. Dorsiflexor muscles of old and young rats were exposed 3 times per week for 4.5 weeks to a protocol of 80 maximal stretch-shortening cycles per exposure in vivo. Skeletal muscle response was characterized by force and work parameters of the stretch-shortening contraction at each exposure interval. The performance of the young and old groups was not statistically different at the start of the exposure, but increased significantly by 25%-30% in the young animals and decreased 22%-39% in the old animals by the end of the 4.5 week exposure. The significance of these findings indicates that aging impairs the ability of skeletal muscle to adapt to repetitive mechanical loading. In addition to the experimental data an analytical model was constructed to help determine how mechanical properties were affected to repetitive mechanical loading. Specifically, muscle stiffness and damping were calculated for both fast and slow eccentric contractions. The model found that the force contribution from damping changed from fast to slow contraction, but it did not change for the old or young groups over the 4.5 weeks of testing. The old age groups showed no change in muscle stiffness over the experimental protocol, but the muscle stiffness of the young age group showed marked increase in muscle stiffness. This is likely attributed to the hypertrophy of the muscles in the young age group. The performance decrement in the old age group was occurring in the force generator elements of the model indicating some failure in the excitation-contraction course of muscle contraction