What is shoulder impingement? Shoulder impingement is a condition where the bursa or rotator cuff tendons are compressed in the subacromial space, the area between the head of the arm bone (humerus) and the acromion, a bony prominence at the top of the shoulder joint. Impingement of the shoulder can be caused by bony spur on acromion, poor posture, weakness of the rotator cuff muscles or muscle tightness. Shoulder impingement may also be ...