Meeting Abstract

50.7  Sunday, Jan. 5 11:45  The effect of muscle relaxation rate on tendon recoil during energy dissipating tasks ABBOTT, EM*; AZIZI, E; Univ. of California, Irvine emily.m.abbott@gmail.com

Series elastic elements, such as tendons and aponeuroses, can amplify muscle power, minimize energy consumption and limit injury susceptibility. Specifically, during energy dissipating tasks, muscles initially store energy in tendons before recoil of the tendon stretches the fascicles as muscle force declines. This mechanism is thought to reduce the rate of stretch directly applied to a muscle fascicle and may function to protect the muscle from stretch-induced damage. We hypothesized that the rate of muscle relaxation determines the rate of tendon recoil. Previous studies have shown that the rate of muscle relaxation can vary as a function of temperature as well as muscle fiber type composition. Here, we used a temperature manipulation protocol in an in vitro muscle-tendon preparation to examine how relaxation rate alters the rate at which tendons recoil and subsequently stretch the muscle fascicles. We measured muscle fascicle lengths by instrumenting the plantaris muscles of Rana catesbeiana (bullfrog) with sonomicrometry crystals. MTU force and length were measured by a servomotor. To determine the twitch kinetics, we characterized the rate of force development and relaxation during twitches at 10, 20 and 30ºC. The MTU was also actively lengthened at the various temperatures with a 50ms tetanic stimulation and a simultaneous 100ms stretch. As expected, the rate of force development and relaxation increased with increasing temperature. We also observed that the rate of tendon recoil correlated positively with rising temperature during eccentric contractions. These data imply that factors that alter relaxation rate can influence the rate of fascicle stretch in a muscle tendon unit and thus alter the likelihood of muscle damage. Supported by NSF grant 1051691.