DEAN, M.N.*; AZIZI, E.; SUMMERS, A.P.; Univ. of California Irvine; Brown University; University of California Irvine: Uniform strain in broad muscles: A new twist on tendons

Myofilament overlap determines tension generation in all vertebrate skeletal muscle. The range of muscle fiber strains used to generate a given movement (i.e., sarcomere lengths) is therefore linked to force production. As a result, regions of a muscle experiencing different strains operate in different regions of the length-tension curve, likely decreasing whole-muscle force output. The anterior jaw adductor muscle of the cartilaginous fish, Hydrolagus colliei, exhibits a morphological solution to ensuring similar strains. The muscle�s tendon flips 180 degrees on its longitudinal axis, such that anterior fibers insert more posteriorly and vice versa. Since insertions closer to the jaw joint experience smaller excursions during mouth opening, the anterior face of the muscle strains less than in an unflipped tendon system (the inverse is true for the posterior face). This results in nearly homogenous strain across the muscle with a flipped tendon, compared with a 10% inhomogeneity between anterior and posterior faces in the unflipped condition. We illustrate that Hydrolagus� morphology functions effectively in strain homogenization. The human latissimus dorsi muscle exhibits a similar morphology, indicating that this may be an ideal anatomical mechanism for strain homogenization in broad muscles attached to rotating structures and inserting relatively far from the joint.