Meeting Abstract
Skeletal muscle fibers can only generate high forces and powers over a narrow range of lengths and velocities. Many morphological and physiological properties of muscle are thought to have evolved to allow fibers to utilize these optimal lengths and velocities across a broad functional range. However, these optima are not constant; but rather depend on the activation state of the muscle. This study used the cane toad plantaris muscle to determine if and how muscle lengths and velocities changed with activation level in vivo, and how this related to changing optima. Muscle ergometry was used to determine the effect of activation level on force-length and power-velocity relationships. Electromyography and sonomicrometry were used to determine operating lengths and velocities at a range of in vivo activation levels. Operating velocities decreased in line with decreased optimal velocities; suggesting that operating velocity can be modulated and optimal velocities used at all activation levels. However, fiber operating lengths did not increase in line with increases in optimum length. At the highest in vivo activation levels, fibers shortened over the plateau of the maximally activated force-length relationship. However, at lower in vivo activation levels, fibers appeared to operate entirely on the ascending limb of sub-maximally activated force-length relationships. This suggests that the ability to change operating lengths is constrained, and that muscles are tuned to produce maximal performance when demand is highest. This study provides insight into the factors which have shaped muscle performance, and informs our understanding of how muscle operating lengths and velocities are modulated during locomotion.
