“…Tissues may show greater or smaller viscoelastic behavior depending on the presence and quantity of certain components, including fluid flow at the site and the ability of fibers within the tissue to undergo stress relaxation. Thus, most soft tissues appear isotropic when a mechanical stress force is applied (Parker et al, 1990;Sarvazyan, 1993;Palmieri and Nightingale, 2011), although there is evidence that some soft tissues, for example, muscles, possess anisotropic mechanical and ultrasonographic properties (Levinson, 1987).…”