Functional morphology of the endomysium in series fibered muscles

Many skeletal muscles, including the feline biceps femoris, are composed of short, tapered myofibers arranged in an overlapping longitudinal series. The endomysium of such muscles transfers tension between overlapping myofibers, and is thus an elastic element in series with them. The endomysium of the cat biceps femoris contains curvilinear collagen fibrils in an approximately isotropic (random) array. The collagen fibrils undergo only a modest reorientation as the myofibers shorten or lengthen within the physiological range. A geometrical model predicts no change in the thickness of the endomysium on changing muscle fiber length and quantifies the expected collagen fibril reorientation in the endomysium as a function of muscle extension. It is also demonstrated that a high proportion of the collagen fibrils will be curvilinear at all sarcomere lengths. The organization of endomysial collagen is appropriate for the transfer of loads between myofibers by means of shear.