The Rotator cuff is a group of muscles and tendons that surround the shoulder joint, keeping the head of your upper arm bone firmly within the shallow socket of the shoulder. A rotator cuff injury can cause a dull ache in the shoulder, which often worsens when you try to sleep on the involved side. Muscles : 1. M. Supraspinatus 2. M. Infraspinatus 3. M. Subscapularis 4. M. Teres Minor