Meeting Abstract

34.2  Tuesday, Jan. 5  Geared up to stretch: pinnate muscle behavior during active lengthening AZIZI, E*; ROBERTS, TJ; Brown University; Brown University manny_azizi@brown.edu

Many locomotor activities require muscles to actively lengthen and absorb energy in order to decelerate the body. These eccentric contractions can disrupt cytoskeletal structures within myofibrils and reduce force output. Here we examine how architectural features of pinnate muscles can provide a protective mechanism against eccentric muscle damage by limiting fiber lengthening. It has been previously shown that the angled fibers of pinnate muscles change orientation when shortening. This change in fiber orientation can amplify fiber velocity, resulting in a velocity advantage at the level of the muscle-tendon unit (MTU) that is characterized by a gear ratio (MTU velocity/fiber velocity). Muscle gearing has been shown to be variable during shortening, while gearing during lengthening remains unknown. We used sonomicrometry to measure fiber length and a servomotor to measure the MTU length and force in vitro in the bullfrog plantaris. We characterized the muscle’s force-velocity curve and gear ratio during both shortening and lengthening across a broad range of forces (10%-190% peak isometric force). Gearing was measured during the isotonic portion of each contraction, to eliminate possible contributions of series elasticity to MTU length changes. During shortening the muscle operated with a variable gear ratio that decreased with increasing force. A similar pattern was observed during lengthening, however, the gear ratio was significantly higher than during shortening, indicating that lengthening of the MTU results in a relatively small and slow stretch at the level of the fiber. Since the magnitude of fiber strain is considered an important determinant of muscle damage, a high gear ratio likely affords pinnate muscles significant protection against the damaging effects of active lengthening. Supported by NIH AR054246 to EA AR055295 to TJR.