You've probably heard a million times that you should externally rotate your shoulders in Adho Mukha Svanasana (Downward-Facing Dog Pose). If you thought that was just your yoga teacher nitpicking, it's time to reconsider. Learning to engage and strengthen the rotator cuff muscles is crucial to preventing common shoulder injuries that plague yogis and non_yogis alike. If you know how to use these muscles the right way, your Down Dogs can help keep your shoulders strong and healthy for a lifetime.
The rotator cuff is one of the most important but widely misunderstood structures in the body. It gets damaged often enough that its name has become synonymous with injury. It's a group of four shoulder muscles that surround each shoulder—like a cuff. Boiled down to the essentials, its job is to support and position the ball that forms the head of the upper arm bone and fits in the socket of the shoulder joint. The shoulder is inherently an unstable joint, so building the strength of these supporting muscles is crucial. If they're weak or deconditioned, as is often the case, the shoulder is vulnerable to injury and pain, and the rotator cuff itself may tear.
You can remember the four rotator cuff muscles by the acronym SITS, for subscapularis, infraspinatus, teres minor, and supraspinatus. They all originate on the scapula (shoulder blade) and insert on the humerus (upper arm bone), near the humeral head (the ball that fits in the shoulder joint). The names of three of the muscles give you a clue to their location: subscapularis sits under the scapula, between the ribs and the front surface of the scapula. Supraspinatus sits above and infraspinatus sits below the spine of the scapula. You can feel them with your fingers: Touch one of your collarbones with the fingers of the opposite hand and slide the fingers straight up over the top of the shoulder. Then reach down the back about an inch or two; you'll find a ridge of bone that's more or less parallel to the ground. That is the spine of the scapula, which separates the supraspinatus and infraspinatus on the back surface of the scapula. The teres minor gives you no clues about its name; it just sits on the outer edge of the scapula, near the posterior fold of the armpit.