43.5 Saturday, Jan. 5 The timing of muscle recruitment alters series elastic function during lengthening contractions ABBOTT, EM*; AZIZI, E; Univ. of California, Irvine; Univ. of California, Irvine email@example.com
During eccentric contractions muscles are actively lengthened to dissipate energy. While eccentric contractions are physiologically common, muscle injury and soreness are often associated with actively lengthened muscle fibers. It has been suggested that series elastic elements (tendons and aponeuroses) may act as mechanical buffers to mitigate these potentially damaging outcomes. During eccentric contractions, strain energy is initially loaded into series elastic elements before tendon recoil stretches the muscle fascicles. By temporarily storing elastic energy, tendons can slow the rate of stretch applied directly to fascicles and can therefore function in attenuating power. Here we examine how the timing of muscle activation relative to muscle lengthening may alter the effective utilization of tendons in attenuating power. Using an in vitro muscle-tendon preparation, we test the prediction that muscle recruitment in anticipation of lengthening can reduce the rate and magnitude of stretch applied directly to muscle fascicles. Plantaris muscles of Rana catesbeiana (bullfrog) were instrumented with sonomicrometry crystals to measure muscle fascicle length. MTU length, force and power were measured by a servomotor. The MTU was lengthened at a constant speed while being stimulated at either 50ms before (early-start), concurrent with, or 50ms after (late-start) MTU lengthening. We find that the magnitude and rate of stretch applied directly to the fascicles decreased when the muscle was stimulated in advance of lengthening (early-start). We conclude that motor control strategies used during energy dissipating tasks can determine the degree of power attenuation by series elastic elements. Supported by NSF grant 1051691.