Arm Yourself Against InjuryWe tend to think of the use of our hands--to write, to play the piano, to perform surgery--as uniquely human. But without the shoulder joint, our use of the hands would be severely limited. Without the shoulder, our arms would be stuck at our sides. We wouldn't even be able to get our hands to our mouths. And we'd lose much of our yoga practice. We use our shoulders in virtually every pose, whether the arms are stretching out to the sides in Trikonasana (Triangle Pose), reaching up in Vrksasana (Tree Pose), bearing weight in Sirsasana (Headstand), or supporting the torso in Sarvangasana (Shoulderstand).
Considering the importance of the shoulders, it is surprising that they are relatively unstable, vulnerable joints. The shoulder is a ball-and-socket joint, like the hip, but unlike the hip socket, the shoulder socket is quite shallow. This shallow socket and the relative looseness of the joint allow wonderful freedom of movement: If you have normal shoulder mobility, you can sweep your arm left and right in front of you as well as circle it up and back. You can't make a circle like that with your leg unless you are a contortionist with hypermobile hips.
A Four-Muscle Job
With its inherent instability, the shoulder is very dependent on its soft tissues to help hold the joint together. These soft tissues include ligaments, which join bone to bone; tendons, which attach muscle to bone; and the muscles themselves, which both move and stabilize the bones. Of particular importance in stabilizing the shoulder are the four muscles that are collectively called the rotator cuff. They wrap deep around the joint from the back, from the front, and over the top.
Stabilization of the shoulder is a complex process shared among the four muscles, whose names can be remembered with the mnemonic SITS: supraspinatus, infraspinatus, teres minor, and subscapularis. Although all of them act together to hold the head, or ball, of the humerus in the shoulder socket (which is actually part of the scapula, or shoulder blade), each muscle produces its own distinct action in the shoulder.
The supraspinatus originates on the upper scapula, just above the spine of the scapula, and inserts on the greater tuberosity of the humerus, a small lump on the outer upper part of the bone. The supraspinatus initiates shoulder abduction. If you stand in Tadasana (Mountain Pose) with your arms by your sides and then lift your arms up to a T shape for Virabhadrasana II (Warrior Pose II), the supraspinatus begins that lift. In its stabilizing function, the supraspinatus helps keep the head of the humerus from slipping down and partially out of the socket, a painful condition called subluxation. Shoulder subluxation in this direction commonly occurs when the muscle is paralyzed by a stroke.
The infraspinatus originates just below the spine of the scapula; the teres minor originates near the infraspinatus on the back of the scapula. Both cross the back of the shoulder joint to insert near the supraspinatus on the greater tuberosity of the humerus, and both are strong external rotators. If you stand in Tadasana, palms facing your body, and then turn your elbow creases forward (the palms will naturally turn forward too), you've externally rotated your shoulder--and you've just used the infraspinatus and teres minor.