Phases of Throwing / Throwing Biomechanics

Overhead throwing is divided into six phases: wind-up, stride or early cocking, late cocking, acceleration, deceleration and follow-through. The crucial concept is that throwing is a kinetic chain - energy is generated in the legs and pelvis, transferred through the trunk, and delivered to the shoulder, elbow and hand - so efficient, well-timed transfer from the lower body protects the arm, whereas a deficient chain forces the arm to make up the energy and increases its load. The peak loads occur at specific phases. In late cocking the shoulder reaches maximal abduction and external rotation, the arm 'laid back', generating large anterior shoulder forces and a high valgus load at the elbow - this is the phase of internal, posterosuperior, impingement and of ulnar collateral ligament strain. In acceleration there is explosive internal rotation with the peak elbow valgus torque and shoulder distraction, which stresses the UCL and the rotator cuff and labrum. In deceleration, the posterior cuff and scapular muscles eccentrically and violently slow the arm, generating the highest distraction forces - the phase associated with posterior cuff and labral tears and SLAP lesions. So the elbow UCL is injured by the repetitive valgus torque concentrated at late cocking and acceleration, and the shoulder labrum, particularly the superior labrum, is injured by the peel-back and distraction forces of late cocking and deceleration, often in the context of a glenohumeral internal rotation deficit, GIRD, and scapular dyskinesis. Prevention follows from this: optimise the kinetic chain and trunk mechanics, restore any GIRD and scapular control, and manage workload.