Four tendons attach muscles from the shoulder blade and ribs to the upper arm bone (humerus). Because these tendons help to rotate the arm within its socket, this sleeve of tendons is called the rotator cuff.
Tendons in the rotator cuff can be injured easily because they move within a tight space. When the shoulder is turned or lifted at the limit of its natural range of movement, the tendons in this tight space are moved, too. Occasionally, the rotator cuff tendons can bump or rub against a bony knob (the acromion) above them or against a ligament at the front of the shoulder.
This friction is known as impingement syndrome and causes inflammation in the rotator cuff. Rotator cuff friction is most likely to cause inflammation if your shoulder movement is rough or repetitive. Inflammation can cause three problems:
Rotator cuff tendonitis — Inflammation of a single tendon causes pain only during specific movements, when the muscle that pulls against that tendon is being used or when you are reaching upwards.
Shoulder bursitis, also called subacromial bursitis — Bursitis occurs when inflammation spreads into the pocket of fluid that lubricates the rotator cuff tendons. Pain is often worse at night and occurs when you move your shoulder in almost any direction, particularly if you are reaching upwards.
Rotator cuff tear — The tendon may tear after it has been weakened by inflammation.
Several types of shoulder use commonly trigger rotator cuff injury:
Pushing off with your arms — People with arthritis of the knee, other painful conditions in the legs, or weak quadriceps muscles in the thighs often compensate by pushing off with their arms when they rise from a chair. The shoulder is not built for this use. During the push off, the shoulder's socket and humerus function like an upside-down mortar and pestle, crushing and grinding the rotator cuff tendons. Falls onto an outstretched arm, head-on automobile accidents and sports collisions also can crush the tendons.
Repetitive reaching — Overhead arm positions narrow the tight space that the rotator cuff tendons must pass through. Pushups, swimming, house painting, filing, building construction, auto mechanic work and other activities can cause injury of the rotator cuff.
Forceful or abrupt overhead arm movements — Tears are particularly common in athletes in throwing sports, racquet sports and wrestling. Abrupt movements, such as pulling to start a lawn mower, can tear a weakened tendon.
In addition, your shoulder can be injured more easily if it is out of shape. The narrow space that envelops the rotator cuff tendons becomes even narrower if your shoulder muscles are weakened or tight. When this happens, routine shoulder movements are more likely to cause tendon friction.