59.1 Friday, Jan. 6 Muscle properties are tuned to mechanical function: lessons from hopping toads AZIZI, E; Univ. of California, Irvine email@example.com
Variation in the basic properties of muscles has likely been shaped by the mechanical tasks muscles perform. However, most muscles cannot be easily fit into a functional category and often perform a range of mechanically diverse tasks. Therefore, the relationship between the basic properties of muscles and the mechanical conditions in which they are used remains tenuous. Moreover, comparisons of functionally distinct muscles from distantly related species are limited by the confounding effect of phylogenic history. We use hopping in toads as a model system for comparing muscles that primarily produce mechanical energy with muscles that primarily dissipate mechanical energy. In this system the hindlimb muscles undergo active shortening to produce the mechanical energy used to propel the animal into the air whereas the forelimb muscles undergo active lengthening to dissipate mechanical energy during landing. Therefore, we observe distinct mechanical tasks performed by different muscles of the same individual, during the same locomotor task. We hypothesize that in order to avoid muscle damage common to energy dissipation, forelimb muscles will have higher passive stiffness and therefore operate over significantly shorter lengths. We use a combination of in vivo measurements during hopping and in vitro measurements of isolated muscles to compare the plantaris muscle, an ankle extensor, to that of the anconeus muscle, an elbow extensor. We find that the anconeus muscle has significantly higher passive stiffness and operates at much shorter lengths than the planataris. These results show that variation in passive elasticity is an important determinant of a muscle’s operating length and that such variation is likely driven by a muscle’s mechanical function. Funded by NSF IOS grant #1051691.