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Muscle fiber angle and strain amplification in segmented musculature.

Brainerd, E.L. and Azizi, E.

The three-dimensional complexity of myomeres and myosepta has made it difficult to develop a comprehensive understanding of the relationship between muscle fiber angulation, connective tissue mechanics, and locomotor function of axial musculature in fishes. Our approach to this problem has been to study a less complex segmented muscle system -- the lateral hypaxial musculature (LHM) of salamanders -- in which obliquely-oriented muscle fibers are organized into 2-4 planar layers attached to vertically-oriented myosepta. We have developed a planar, isovolumetric model of the LHM, and tested our model using sonomicrometry during steady swimming in an aquatic salamander, Siren lacertina. Predicted segment strains did not deviate from mean measured strains by more than 5%, indicating that our model indeed provides an adequate representation of this relatively simple system. Our model differs from previous models of segmented and pinnate musculature by allowing the segment strains in the two dimensions orthogonal to shortening (Poisson's ratios) to vary within the isovolumetric constraint, i.e. we allow the segment to bulge in either the dorsoventral, mediolateral or both dimensions in response to longitudinal shortening. Our model shows that the ratio of these two orthogonal strains strongly affects the magnitude and speed of segment shortening -- greater dorsoventral bulging leads to greater longitudinal segment strain for a given amount of muscle fiber strain (strain amplification). The orthogonal bulging ratio is likely to be affected by the stiffness of the myosepta and skin; therefore our model provides an explicit mechanical link between muscle fiber angulation and connective tissue mechanics.

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Functional Morphology & Biomechanics Laboratory
Ecology & Evolutionary Biology - Brown University

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