Meeting Abstract

40.6  Friday, Jan. 4  Functional diversification within and between muscle synergists during locomotion HIGHAM, T.E.*; BIEWENER, A.A.; WAKELING, J.M.; Harvard University; Harvard University; Simon Fraser University thigham@fas.harvard.edu

Locomotion arises from the complex and coordinated function of limb muscles. Yet muscle function is dynamic over the course of a single stride, and between strides for animals moving at different speeds or on variable terrain. The considerable diversity in locomotor behavior hinges on this ability of muscles to act in a time-varying fashion. While it is clear that muscle recruitment can vary between and within muscles, we know little about how work is distributed within and between muscles under in vivo conditions. Here we show that the lateral gastrocnemius (LG) of helmeted guinea fowl (Numida meleagris) performs considerably more work than its synergist, the medial gastrocnemius (MG), and that the proximal region of the MG (pMG) undergoes greater changes in length compared to the distal region (dMG). Positive work done by the LG was approximately twice that of the proximal MG when the birds walked at 0.5 m s^-1, and four times when running at 2.0 m s^-1. This is likely due to different moments at the knee, as well as differences in motor unit recruitment. The increase in work with increased speed was mediated by an increase in muscle stress rather than fascicle strain. The dMG performed less work than the pMG because its dynamic stiffness was greater, and because it exhibited greater recruitment of slow twitch fibers. Thus, the pMG functions to counter knee flexion that occurs after footfall by first absorbing energy and then expending energy, whereas the dMG generates force economically by remaining relatively isometric. Our results reveal the functional diversity between and within muscle synergists, which increases with changes in gait and speed.