The most common causes of shoulder weakness are either rotator cuff tears or shoulder injuries that cause inflammation. Rotator cuff tears cause shoulder weakness by altering the mechanics of the joint. Inflammation can lead to shoulder weakness because the body will involuntarily protect itself from pain.
Rotator Cuff Tears
The rotator cuff is a group of four muscles and tendons that surrounds the ball of the ball-and-socket shoulder joint. These muscles and tendons help to lift the arm and perform overhead activities. A rotator cuff tear is a tear in one of the tendons of the rotator cuff. This tear weakens the ability to lift the arm and perform overhead activities. Over time, the muscle may irreversibly weaken if the rotator cuff tear is left untreated.
Other injuries that cause inflammation around the shoulder, including shoulder bursitis, can lead to the symptoms of shoulder weakness. When the body experiences pain, your reflexes take over and prevent you from causing further pain. In conditions of shoulder inflammation, your body may not cooperate and allow your to lift heavy objects or reach high overhead. This is why some patients with normal rotator cuffs may experience weakness of the shoulder.