But continuously working with the arms raised overhead, repeated throwing activities, or other repetitive actions of the shoulder can cause impingement to become a problem. Impingement becomes a problem when it causes irritation or damage to the rotator cuff tendons. Raising the arm tends to force the humerus against the edge of the acromion. With overuse, this can cause irritation and swelling of the bursa. If any other condition decreases the amount of space between the acromion and the rotator cuff tendons, the impingement process may get worse.
Bone spurs can reduce the space available for the bursa and tendons to move under the acromion.
The AC joint is directly above the bursa and rotator cuff tendons. In some people, the space is too small because the acromion is oddly sized . In these people, the acromion tilts too far down, reducing the space between it and the rotator cuff. Impingement syndrome causes generalized shoulder aches in the condition's early stages. It also causes pain when raising the arm out to the side or in front of the body.