On the Bidirectional Viscoelastic Behavior of the Human Achilles Tendon

Soft tissues such as tendons and ligaments are made up of groups of collagen fascicles surrounded by a sheath of epitenon. The friction between these structural fibers and their surrounding ground substance has been suggested to be the main contributor to the observed viscoelastic response of the tissue[1]. During normal daily activities such as walking, climbing stairs or jumping, these tissues are subjected to alternating loading and unloading conditions. Depending on the load and tissue orientation, this alternating loading condition may not be uniformly applied over the entire volume of the tissue. In some instances, certain fiber bundles are in tension (loaded) while others might be unloading. Hence, the development of accurate predictive models requires characterization of not only the loading behavior, but also the unloading behavior. To our knowledge, there are few models that specifically address the unloading behavior of the tissue.