The Youngs modulus of elasticity is a material constant. A strip of material subjected to a force will lengthen. Force F, over cross-sectional area, A = ¼ πd02, is stress, σ = F/A, and the relative length change, ε = Δl/l0, is the strain. If the stress-strain relationship is linear, the material obeys Hooke’s law and the slope of the relation is called the Youngs modulus of elasticity, E. The stress-strain relation of biological tissues is curved, and the local slope is the incremental elastic modulus, Einc, which is strain (or stress) dependent. The Youngs modulus of elasticity is actually a stiffness modulus.