The shoulder is a ball and socket joint. It is one of the most complex joints in the human body. In a healthy shoulder joint, the ball, or humeral head, and socket, or glenoid, remain stable, while allowing maximal range of motion of function. 

Shoulder impingement syndrome is a common cause of shoulder pain that may affect anyone who performs repeated reaching and lifting over their heads.

Also known as subacromial impingement syndrome, it occurs deep inside the shoulder, when the bones of the shoulder blade and upper arm chafe against the top of the rotator cuff.