Meeting Abstract

P1.72  Saturday, Jan. 4 15:30  Determinants of muscle shape change during lengthening and shortening contractions GIDMARK, NJ*; KONOW, N; ARELLANO, C; ROBERTS, TJ; Brown University gidmark@uw.edu

As a muscle contracts, it can change shape. In pennate muscles, the line of action of a muscle fiber diverges from that of the whole muscle-tendon unit (MTU). Therefore, shape changes can alter the force and velocity of contraction by modulating how fiber force and length change translate into MTU force and length change. This behavior can be characterized by the MTU:fiber speed ratio, or “gearing”. We explore the drivers and constraints of muscle shape changes by measuring several aspects of muscle shape, length, force and fiber length as muscles undergo shortening and lengthening contraction in vivo and in situ. We use high-speed x-ray video recordings to track radiopaque markers implanted along muscle fibers, at the muscle belly margins, and within the aponeuroses of turkey lateral gastrocnemius during jumping and landing behaviors. Muscle force was measured from strain gauges glued to the bony tendon. We digitally fit a plane to the aponeurosis markers and used fiber markers to calculate fiber pennation angle relative to this plane. We compared contractions involving two contrasting fiber length change patterns: The gastrocnemius shortened during jumping and mainly lengthened during landing. In situ, we recreated these patterns using an ergometer and nerve stimulation to elicit lengthening and shortening contractions, while measuring the same set of variables. Preliminary analysis of six individuals shows that: 1) changes in muscle width were strongly predicted by changes in fiber length (widening with fiber shortening and vice versa); 2) changes in aponeurosis width were weakly predicted by changes in fiber length and reflect a force-dependent pattern; 3) gearing is typically higher during lengthening than shortening contractions. Supported by NIH grant AR055295.