A common cause of shoulder pain is soreness of the tendon (a cord that attaches a muscle to a bone) of the rotator cuff. This is the part of the shoulder that helps circular motion. Another common cause is soreness of the subacromial bursa.
You might experience soreness after painting, lifting items, or playing a sport—anything that requires you to lift your arms. Or you may not remember any specific injury, but you still feel pain in your shoulder.
The main joint in the shoulder is formed by the arm bone and the shoulder blade. The joint socket is shallow, allowing a wide range of motion in the arm. The rotator cuff is made up of 4 muscles that surround the arm bone. This cuff keeps the shoulder steady as the arm moves.
The supraspinatus muscle rests on top of the shoulder. Its tendon travels under the bone on the outside of the shoulder (the acromion). This tendon is the one most often injured because of its position between the bones. As the tendon becomes inflamed, it can become pinched between the two bones. The sac of fluid that cushions the tendon can also be damaged.
If the rotator cuff is involved, the pain is usually in the front or outside of the shoulder. This pain is usually worse when you raise your arm or lift something above your head. The pain can be bad enough to keep you from doing even the simplest tasks. Pain at night is common, and it may be bad enough to wake you.