Meeting Abstract

43.1  Jan. 6  Variable Gearing in Pinnate Muscles AZIZI, E.*; ROBERTS, T.J.; Brown University; Brown University manny_azizi@brown.edu

Pinnate muscles are characterized by short fibers that attach at an angle relative to the muscle�s line of action. Recent studies have shown that as obliquely oriented muscle fibers shorten they also rotate. Fiber rotation during a muscle contraction changes the relationship between the shortening velocity of a fiber and the output velocity of the muscle tendon unit. When fiber shortening is combined with rotation the gear ratio of the muscle (muscle velocity/ fiber velocity) increases. Although fiber rotation results in a velocity advantage, the proportion of fiber force directed along the muscle�s line action declines. This direct trade-off suggests that the relative force and velocity output of a muscle depend in part on the magnitude of fiber rotation. Our computer simulations suggest that dynamic muscle shape changes during shortening influence the magnitude of fiber rotation and therefore modulate the nature of this force-velocity trade-off. In this study we used an in situ muscle preparation of the lateral gastrocnemius of wild turkeys (n=5) to determine whether gearing can vary across mechanically diverse contractions. We used a servomotor to measure whole muscle force and velocity and sonomicrometry to measure fascicle velocity and muscle bulging during a series of isotonic contractions at varying force levels. We find that gear ratio varies dynamically with muscle load such that at high muscle forces, fiber rotation was minimized. These results suggest that architectural gear ratio is determined not only by static muscle anatomy, but also by the mechanics of force production during contraction. Variable gearing may be one way that the mechanical behavior of pinnate muscles can be modulated to meet a range of mechanical demands.