The baseball pitch has been extensively studied and can be divided into several stages: windup, early cocking, late cocking, acceleration, deceleration and follow-through. During throwing, there is a transmission of forces from the lower extremities and core to the upper extremity. The transition point between the late cocking and acceleration phase is the most critical point in the throwing motion and the most common point at which injuries occur.

In late cocking, the shoulder is abducted and the ligaments or capsule in the front of the shoulder are under tremendous strain as the forces are pushing the humeral head out the front of the shoulder. Over time, the ligaments in the front of the shoulder joint can become stretched out. This is a normal adaptive change in elite throwers but it can lead to instability in the shoulder.

With the arm in an abducted and externally rotated position, the rotator cuff tendons can become pinched between the back of the socket and the greater tuberosity of the humerus. This can cause tearing of the rotator cuff tendon and labrum over time. This is called internal impingement. Throwers with internal impingement complain of pain in the back of the shoulder, and the pain is usually reproduced when they hold their arm in the late cocking position, namely with their arm in abduction and external rotation. Patients complain of mechanical like symptoms resulting from partial tearing of the rotator cuff tendons somewhat posterior in the shoulder, as well as possible tearing of the biceps and posterosuperior labrum.

After the ball is released during deceleration of the arm, the rotator cuff contracts to hold the humeral head in the socket as the forces are such that the arm is literally being thrown out of the socket. There is also high stress on the posterior structures of the shoulder, leading to muscle fatigue of the posterior musculature of the shoulder. The ligaments in the back of the shoulder joint may develop chronic micro tears. This incites a healing response whereby the posterior capsule thickens and becomes less compliant resulting in a stiff posterior capsule and posterior rotator cuff. This can result in glenohumeral internal rotation deficit (GIRD).

Injury to a thrower’s shoulder is usually “acute on chronic” meaning that there was chronic overuse and there are adaptive changes where the body has attempted to repair the shoulder, but the shoulder is not normal. Then, then there is one acute event whereby the shoulder is suddenly worse and symptomatic. For example, one study looked at MRI’s of the shoulders of 21 professional pitchers who did not report any shoulder problems and about 50% had rotator cuff tendon tears and 50% had labral tears even though their shoulders were reported as being asymptomatic. Those who pitched more innings were more likely to have these findings.

Most throwing injuries can and should be treated non-surgically. This consists of a good stretching program. One good stretching exercise is known as the “sleeper stretch.” This stretches the posterior capsule in those throwers with a lack of internal rotation and a decreased arc of motion of the shoulder joint (GIRD). Having rest periods away from throwing is also important. Scapular stabilizing exercises helps prevent scapular dyskinesia. Keeping the rotator cuff musculature strong and balanced is also encouraged.