Skeletal muscle is composed of muscle fibers and an extracellular matrix (ECM). The collagen fiber network of the ECM is a major contributor to the passive force of skeletal muscles at high strain. We investigated the effect of aging on the biomechanical and structural properties of epimysium of the tibialis anterior muscles (TBA) of rats to understand the mechanisms responsible for the age-related changes. The biomechanical properties were tested directly in vitro by uniaxial extension of epimysium. The presence of age-related changes in the arrangement and size of the collagen fibrils in the epimysium was examined by scanning electron microscopy (SEM). A mathematical model was subsequently developed based on the structure-function relationships that predicted the compliance of the epimysium. Biomechanically, the epimysium from old rats was much stiffer than that of the young rats. No differences were found in the ultrastructure and thickness of the epimysium or size of the collagen fibrils between young and old rats. The changes in the arrangement and size of the collagen fibrils do not appear to be the principal cause of the increased stiffness of the epimysium from the old rats. Other changes in the structural composition of the epimysium from old rats likely has a strong effect on the increased stiffness. The age-related increase in the stiffness of the epimysium could play an important role in the impaired lateral force transmission in the muscles of the elderly.