Arm Yourself Against Injury
Besides being external rotators, the teres minor and infraspinatus are very important in positioning the head of the humerus as you flex the shoulder (when you bring your arm forward and up overhead) and abduct the shoulder (when you bring your arm straight out to the side and up). The two muscles actually pull down on the head of the humerus as the arm elevates, to prevent the ball from crashing up into the acromion--the projection of the scapula that, like a carport roof, protects the top of the joint.
If the teres minor and infraspinatus are too weak to do their jobs or there are other problems with the movement of the scapula, you can develop impingement syndrome, in which soft tissue gets compressed between the head of the humerus and the acromion. If the damaged and inflamed tissue is a bursa, one of the cushioning pads between tendon and bone, you have bursitis. If it is a tendon (commonly the supraspinatus tendon), you have tendonitis.
The subscapularis is the hardest part of the rotator cuff to locate, and its actions are the hardest to understand. It originates on the front surface of the scapula and runs forward, forming part of the posterior fold of the armpit. It then wraps around to the inner upper shaft of the humerus and inserts there. The subscapularis is a very strong internal rotator of the shoulder, so its action opposes the actions of the infraspinatus and teres minor. Along with the rest of the rotator cuff muscles, the subscapularis helps stabilize the ball of the humerus in the shoulder socket.
No More Tears
Unfortunately, it's not uncommon for the muscles of the rotator cuff to be underworked and therefore weak. This weakness can lead not only to impingement syndrome but also to tears in the rotator cuff muscles themselves, usually near where three of them insert on the greater tuberosity of the humerus. These tears are quite painful and can severely limit your ability to move your arm. Rotator cuff tears can occur during vigorous shoulder activities for which you're not conditioned--throwing a Frisbee for the first time in years, for instance--but are most common in elderly deconditioned folks, especially women, and younger, active people who fall while cycling, running, or playing a sport.
If you have an acutely injured rotator cuff, or one that hasn't fully recovered from an injury or surgery, it's best to leave the exercise prescription to a health care professional. Mild to moderate rotator cuff tears can be treated conservatively with medication and carefully selected exercise, while more severe tears may require surgery.
But given the challenges of healing a torn rotator cuff or recovering from impingement syndrome, it's obviously better to build and maintain the strength of the rotator cuff muscles to prevent these problems in the first place. Practicing yoga, in which you often bear weight on the arms, is a wonderful way to do this. If you come to yoga with weak rotator cuff muscles, however, it's best not to immediately put big loads on them, as you would in Surya Namaskar (Sun Salutation) or inversions. So if you are new to yoga, are weak in your upper body, or have a previously injured (but now healed) rotator cuff, begin building strength with postures in which you bear only light weight on your arms.