The elasticity of materials is described by a stress-strain curve, which shows the relation between stress (the average restorative internal force per unit area) and strain (the relative deformation).
The acceleration phase of the overhead throw causes the greatest amount of valgus stress to the elbow.Extension occurs at a rate of up to 2500 º per second and continues to 20 º of flexion. During this phase, the forearm lags behind the upper arm and generates valgus stress while the elbow is primarily dependent on the anterior band of the UCL for stability. During the acceleration phase, valgus stress can exceed 60 Newton meters (Nm), which is significantly higher than the measured strength of the UCL in cadavers. The valgus force can, therefore, overcome the tensile strength of the UCL and cause either chronic microscopic tears or acute rupture.
Elasticity, Stress, and Strain graph of Ligament