Shoulder impingement—sometimes called subacromial impingement—refers to the painful pinching of the muscles, tendons, or other soft tissues between the shoulder’s ball-and-socket joint (the glenohumeral joint) and the top-most bone of the shoulder, called the acromion. This area between bones is called the subacromial space. The subacromial space becomes smaller when the arm is raised, which can cause soft tissues to become squeezed.