Rotator cuff is a group of muscles and their tendons that act to stabilize the shoulder: supraspinatus muscle, the infraspinatus muscle, teres minor muscle, and the subscapularis muscle.
The injury is common in sport, and people in their forties, because of shoulder joint degeneration. Usually, the tendon of supraspinatus muscle is injured. This condition is usually the result of indirect mechanism (e.g. falling onto abducted arm, a sudden yank of the arm, throwing heavy object). It can be also a result of small repetitive injuries.