While all four muscles work in concert to stabilize the shoulder, each muscle also helps support the shoulder individually. The subscapularis is a powerful internal rotator. Supraspinatus helps hold the ball up in its socket against the downward pull of gravity on the arm, and it initiates abduction, or lifting the arm up from your side, as in Virabhadrasana II (Warrior Pose II). Teres minor and infraspinatus are the primary muscles that control the external rotation of the shoulder. When they're strong and healthy, they help to protect the shoulder joint by positioning the ball in the socket while you raise your arm overhead. Conversely, their weakness can contribute to common shoulder problems such as shoulder impingement, tendinitis, and bursitis.
These important external rotators, infraspinatus and T. minor, are the part of the rotator cuff that is strengthened in Downward Dog. It's a good thing, too, because these days, the laborsaving products and devices we use make our arms and shoulders progressively weaker as the decades slip by. A weakened rotator cuff might lead to abnormal shoulder-movement patterns, which can contribute to inflammation and pain. Not only that, but weak muscles are likely to tear when you put a load on them that they aren't strong enough to handle. Sometimes the tears are microscopic and will heal on their own. But if the tears are bigger, a surgeon may have to sew the separated ends of the torn tissues together. Repairing a torn rotator cuff surgically, though, isn't a given: One doctor described the repair process as being like trying to sew up a run in a nylon stocking. The tissues of the atrophied muscles and their weakened tendons are just plain flimsy—liable to tear and difficult to repair.
So, a word to the wise: It's much easier to work your rotator cuff muscles, make them strong, and keep the tissues healthy than to have to see a physical therapist like me for shoulder treatment and rehabilitation or, worse still, to have to visit a surgeon. And it's in this way that your daily Downward Dog practice will really pay off—if, that is, you know how to engage infraspinatus and teres minor.!--page-->