On the back of the shoulder there is 3 small muscles. These 3 muscles are 3 of 4 muscles that make up the Rotator Cuff. The Rotator Cuff is the main muscle group responsible for keeping the head of the humerus stable in the shoulder capsule and secured during complex manoeuvres during daily life and sports activities. The 3 External Rotators of the Rotator Cuff are the most important important to remember to exercise and maintain as all 3 are responsible for externally rotating the head of the humerus in the shoulder capsule to prevent breakdown of the cartilage, ligaments and tendons. In particular, preventing breakdown of the infraspinatus tendon between the head of the humerus and the acromion. Which internally rotated shoulders produce bursitis, tendonitis or both. Due to the greater tubercle, a notch on the head of the humerus, the infraspinatus tendon, bursa and the acromion become too close together, pinching both the bursa and infraspinatus tendon between 2 hard bones. This collision produces symptoms of both bursitis and tendonitis. As the bones compressing soft tissues combined with abrasive movement causes injury to the soft tissues. External Rotator Exercises Strengthen the the 3 External Rotator Cuff Muscles Supraspinatus, Infraspinatus and Teres Minor If Strong, Realign the Shoulder Joint which widens the space between the head of the humerus and the Acromion. The Acromion is the end of your shoulder girdle, in combination a bone segment of the clavicle and scapular, or simply your shoulder blade and collar bone.