Impingement syndrome occurs when the rotator cuff tendons rub against the roof of the shoulder, the acromion.
Impingement occurs to some degree in everyone's shoulder. Day-to-day activities that involve using the arm above shoulder level cause some impingement. Usually it doesn't lead to any prolonged pain. 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 may get worse. Additionally, poor posture, weak shoulder blades and tight muscles may alter the way the shoulder complex works and may lead to impingement syndrome.
The most common symptom of impingement syndrome is pain, usually sharp when moving the arm and achy at rest, generally surrounding the shoulder. This pain will get worse with reaching overhead, reaching behind the back, or lying on that side.