Ulnar Collateral Ligament: Throwing Biomechanics

Abstract

The overhead throwing motion is created by a complex series of coordinated movements involving different motor groups and the articulations of the upper extremity as well as the kinetic chain. The necessary kinematics of throwing place significant stresses across the joints of the upper extremity, which can lead to potential overload and injury. The throwing motion of the overhead pitch is divided into six segments or phases from wind-up to follow-through. In addition to these upper extremity motions, the kinetic chain increases power and ball velocity through a series of coordinated muscle activation progressing through the lower legs, pelvis, and trunk.

The bony architecture of the proximal ulna and distal humerus provide the majority of the overall stability of the elbow. The remaining stability of the elbow is afforded by the radial collateral ligament complex, the ulnar collateral ligament (UCL) complex, the anterior joint capsule, and the dynamic support of the flexor-pronator muscle group. Significant valgus stress from overhead throwing activities creates tensile stresses that often predispose the UCL to injury. Another phenomena known as valgus extension overload can occur due to repetitive injury to the elbow in the setting of UCL laxity. The purpose of this chapter is to define the biomechanics in the overhead athlete with a special emphasis upon the medial elbow.