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.
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