Architectural gear ratio and muscle fiber strain homogeneity in segmented musculature

In the segmented axial musculature of fishes and amphibians, the patterns of muscle fiber shortening depend on both the orientation of muscle fibers relative to the long axis of the body as well as the distance of fibers from the neutral axis of bending (vertebral column). In this study we use the relatively simple architecture of salamander hypaxial muscles to explore the separate and combined effects of these morphological features on muscle fiber strains during swimming. In Siren lacertina the external oblique (EO) muscle has more obliquely oriented muscle fibers and is located further from the neutral axis of bending than the internal oblique (10) muscle. To examine the effect of muscle fiber angle on strain patterns during swimming, we used sonomicrometry to quantify architectural gear ratio (AGR = longitudinal strain/fiber strain) in these two hypaxial muscles. By comparing the muscle fiber strains and shortening velocities of the EO and 10 during swimming, we test whether variation in mediolateral position of the muscle layers is counteracted by their differences in AGR. We find that despite substantial differences in mediolateral position, the EO and 10 undergo similar fiber strains and shortening velocities for a given amount of axial bending. Our results show that variation in muscle fiber angle acts to counteract differences in mediolateral position, thereby minimizing variation in muscle fiber strain and shortening velocity during swimming. These results highlight the significance of both muscle architecture and muscle moment arms in determining the fiber strains required for a given movement.