Meeting Abstract
66.2 Thursday, Jan. 6 Energy absorption by muscle during steady and non-steady movement KONOW, N*; ROBERTS, TJ; Brown University; Brown University nkonow@brown.edu
An important function of skeletal muscle is to absorb energy via active lengthening. This occurs in leg extensors during leg flexion and with changes in energetic demand—when running terrain slopes downhill or landing height increases—the priority on energy absorption is altered. Footfall during steady and unsteady movement leads to a period of rapid force-rise and extensive leg flexion. Direct energy absorption by muscle during this force-rise period, where the rate of force-rise and stretch of the muscle due to leg flexion is unpredictable, can result in damage to active fascicles. The risk may be mitigated by a delay in muscle energy-absorption until force decays, which is achieved if energy is stored transiently in series elastic tendon. We measured muscle force, length and activity dynamics in turkey lateral gastrocnemius at several levels of energetic demand. Pitching the running surface steepness from level to 28º downhill resulted in no change in rate of force rise or peak forces, and did induce a shift in fascicle behavior during force rise from near-isometry to lengthening. This fascicle lengthening then continued into the period of force decay. Muscle fascicle behavior during landings from low height was similar to that of downhill running: fascicles lengthened throughout the force-rise and force-decay periods. During landings from greater heights, however, fascicle lengthening occurred almost exclusively late in the contraction during force-decay, indicating that in these high-intensity contractions the tendon buffered muscle energy absorption by transiently storing energy. Our results suggest that temporary energy storage in tendon during the period of force-rise could be a facultative mechanism, which is force-limited and recruited via changes in muscle contraction behavior and intensity. Supported by NIH #AR055295.
