The tendons of the rotator cuff muscles, which attach to the shoulder blade and upper arm bone, travel under the bony ledge created by the acromion. As you raise your arm above 90º, the space between the tendons of the rotator cuff and the acromion decrease.
This narrowing may cause the acromion to rub against and impinge upon structures such as the Supraspinatus tendon and bursa, causing irritation and pain. Rotator cuff pain commonly causes local swelling and tenderness in the front of the shoulder.