Tissue Stiffness

Tissue stiffness describes how strongly a biological tissue resists deformation when a mechanical load is applied. A stiffer tissue generally deforms less under comparable loading conditions than a more compliant tissue. Stiffness is relevant to tissues throughout the body, including muscle, tendon, cartilage, blood vessels, skin, and internal organs .^1,2

In mechanics, stiffness can refer specifically to the relationship between an applied force and the resulting displacement of a structure. Material properties are often described instead using quantities such as Young’s modulus, which relates stress, or force per unit area, to strain, the relative deformation produced by that stress. These concepts are related but not interchangeable because structural stiffness depends on factors such as a tissue’s dimensions and geometry as well as its material properties .¹

Biological tissues are mechanically complex. Many are viscoelastic, meaning their response depends partly on time or the rate at which they are deformed, and some are anisotropic, meaning their mechanical response differs with the direction of loading. Tissue stiffness may also change with the amount of deformation. For these reasons, a tissue cannot always be described accurately by a single fixed stiffness value .^2,3

Measured tissue stiffness also depends on how and where it is tested. Techniques such as tensile testing and indentation can produce different estimates of mechanical properties because they deform tissue in different ways and at different spatial scales. A reported stiffness value therefore needs to be interpreted in the context of the measurement method and loading conditions .^2,3