Dead arm syndrome starts with repetitive motion and forces on the posterior capsule of the shoulder. The posterior capsule is a band of fibrous tissue that interconnects with tendons of the rotator cuff of the shoulder. Four muscles and their tendons make up the rotator cuff. They cover the outside of the shoulder to hold, protect, and move the joint.
Overuse can lead to a build up of tissue around the posterior capsule called hypertrophy. The next step is tightness of the posterior capsule called posterior capsular contracture. This type of problem reduces the amount the shoulder can rotate inwardly — a motion needed by pitchers to throw the ball forward before releasing it.
Over time, with enough force, the player may develop a tear in the labrum. The labrum is a rim of cartilage around the shoulder socket to help hold the head of the humerus (upper arm) in the joint. This condition is called a superior labrum anterior posterior lesion. The final outcome in all these steps is the dead arm phenomenon.
The shoulder is unstable and dislocation may come next. Dead arm syndrome won’t go away on its own with rest — it must be treated. If there’s a SLAP lesion, then surgery is needed to repair the problem. If the injury is caught before a SLAP tear, then physical therapy with stretching and exercise can restore it and return the player to the field.