Rotator cuff injury.
These are the most common shoulder injuries. The rotator cuff is a group of four muscles and tendons that stabilize the shoulder joint. Rotator cuff injuries happen when the tendons or bursae near the joint become inflamed from overuse or a sudden injury. They are common in people with jobs that involve overhead motions, like painters, or athletes who repeatedly reach upward, such as tennis players and swimmers.
Impingement.
This happens when the top of the shoulder blade puts pressure on the soft tissues beneath it when the arm is lifted. Tendinitis and bursitis can develop, limiting movement and causing pain. Repeated overhead movements, such as those used by swimmers, increase the risk of impingement.
Instability
Shoulder instability happens when the round end of the upper arm bone is forced out of its shallow socket, either partially or completely. Once the tendons, ligaments, and muscles of the shoulder become stretched or torn, the shoulder becomes “loose” and dislocations can occur repeatedly.