» For pain and/or loss of range of motion of a joint, see joint stiffness Stiffness is the resistance of an elastic body to deflection or deformation by an applied force. It is an extensive material property. Definition The stiffness, k, of a body is » k=frac where » A is the cross-sectional area, E is the Young's modulus, » L is the length of the element. For the special case of unconstrained uniaxial tension or compression, Young's modulus of elasticity can be thought of as a measure of the… (More on Stiffness)