When we ask our students to lift their arms overhead (for example, in Urdhva Hastasana [Upward Hand Pose], it may seem like a simple request, but it's actually a complex biomechanical challenge. Raising the arms requires a precisely coordinated sequence of movements of the humeri (upper arm bones), shoulder blades (scapulae), clavicles (collar bones), ribcage, and spine.
Different students accomplish this in different ways. There are thousands of possible variations and permutations of motion and timing, some of which work better than others. The coordination of shoulder blade and upper arm movement as the arms lift is called the scapulo-humeral rhythm. In this column, we will explore a small but crucial part of this rhythm—the outward rotation of the humeri—so that you can help your students move their arms more safely and effectively in yoga poses.
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Understand the Anatomical Structure of the Upper Arm
Let's start by identifying some anatomical structures. The bulging top end of the upper arm bone is called the head of the humerus. The half of the head that faces inward toward the body is a smooth hemisphere that forms a joint with the shoulder blade (the gleno-humeral joint). The half of the humeral head that faces outward, away from the body and shoulder blade, is capped by an irregular bump called the greater tubercle, which forms a point of attachment for several muscles that move the arm. The front of the humeral head also has a bump, the lesser tubercle, that is an attachment point for several arm muscles.
Now let's explore the scapula. If you reach one of your hands across your body over the opposite shoulder, you can palpate a horizontal ridge of bone that protrudes from the top-back of the shoulder blade. This is the spine of the scapula. The outermost (lateral) extremity of this spine is called the acromion process. You can find it by running your fingertips outward along the scapular spine to where it turns forward at its end. The whole spine of the scapula forms the back wall of a sort of trough that sits atop the shoulder blade. If you try to press your fingers into this trough by pushing down in front of the scapular spine, you will find that the space is filled with muscle. The muscle closest to the surface is the trapezius, but underneath it lies a second muscle that concerns us more here: the supraspinatus.
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The supraspinatus is one of the four muscles of the rotator cuff. Its tendon runs out to the side, underneath the acromion process and over the top of the head of the humerus, where it attaches to the greater tubercle. This arrangement turns out to be very significant: the supraspinatus tendon is sandwiched between the acromion (above it) and the head of the humerus (below it). When the supraspinatus contracts, it pulls the greater tubercle inward (medially) and upward toward the shoulder blade. This lifts the rest of the arm outward (laterally), away from the body, into abduction.
Prevent Pinching, Inflammation, and Tears
This is one of the early steps in the scapulo-humeral rhythm. It begins the movement of the arm from hanging downward alongside the body to reaching out away from the body on its way to lifting overhead. But this action can cause trouble if it occurs too early. If the supraspinatus contracts strongly while the arm is in its neutral, non-rotated, Tadasana position, it can lift the greater tubercle straight up into collision with the acromion process. This can pinch the supraspinatus tendon between the acromion and the humeral head. Doing this repeatedly or forcefully can fray, inflame, or even tear the tendon. This is perhaps the most common type of rotator cuff injury.
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Preventing this problem is quite simple, and is a natural part of a healthy scapulo-humeral rhythm. The first step to lifting the arm overhead is not abduction, but rather a combination of "cinching down" the head of the humerus, so there is more space between it and the overlying "roof" formed by the acromion process, and externally rotating the humerus, which moves the greater tubercle backward, so that most of it no longer lies under the acromial "roof." Two rotator cuff muscles, the infraspinatus and teres minor, are primarily responsible for these actions.
You can palpate infraspinatus by reaching your left hand over your right shoulder and pressing your fingers into the flesh a couple of inches below the spine of the scapula. If you then rotate your right arm firmly outward, you will feel the infraspinatus contract under your fingertips. Infraspinatus is primarily an external rotator; it does not cinch down the head of the humerus much. This is because its tendon runs more or less horizontally from the back of the shoulder blade, across the back of the humeral head to the greater tubercle, and therefore pulls the tubercle mostly backward rather than downward.
Teres minor is a little harder to palpate than infraspinatus because you have to reach farther over your shoulder and out to the side. It runs along the lower part of the outer border of the back of the shoulder blade, alongside the bottom portion of infraspinatus. You can feel it contracting when you rotate your arm outward, but it is not just an external rotator. Because it lies lower on the shoulder blade than infraspinatus, its tendon runs more vertically behind the humeral head to reach the greater tubercle. When it contracts, it pulls the tubercle not only backward, but also downward, producing much of the downward cinching action that prevents the tubercle from colliding with the acromion as the arm abducts.
Teach Your Students to Raise their Arms Overhead Safely
So when you ask your students to lift their arms overhead, instruct them first to rotate their upper arms outward and pull them downward. As they do this, subscapularis, the fourth rotator cuff muscle, will modify the action for better or worse. Subscapularis lies on the front surface of the shoulder blade, between the blade and the ribcage. Its tendon runs in front of the humeral head and attaches to the lesser tubercle. This arrangement makes it primarily an internal rotator, but it can also help cinch the humeral head downward. So when you tell your students to externally rotate their arms and pull them down, subscapularis must release enough to allow the rotation. In spite of this, it might be worthwhile to keep enough tension on the muscle to aid in the downward action. One way to instruct your students to do this is to tell them to pull their arms inward toward the midline of the body and keep some inward-turning resistance on them as they rotate them outward.
For a healthy scapulo-humeral rhythm, the rotating and cinching actions should start before the arms begin to lift; however, that's not the end of the story. The same actions must continue throughout the pose, both during the process of lifting and after the arms are fully elevated. Maintaining these actions will help keep the supraspinatus tendon in a safe position, away from the acromion.
Instructing your students to do this can get confusing if you're not careful. When your students' arms are at their sides, the instruction to "rotate your arms out" means to turn the outer (triceps) side of their arms backward and the inner side forward. Once the arms are overhead, the same direction of rotation turns the outer arms forward (triceps forward) and the inner arms backward. Although this is technically still "outward rotation" to an anatomist, to a student it may look like inward rotation. So avoid the terms "outward rotation" and "inward rotation" when their arms are overhead, and instead tell your students to "turn your arms so the outer side moves forward and the inner side moves backward" while you show them by example what you mean.
If you can successfully communicate these movements to your students before and during arm elevation, they will strengthen the muscles that reinforce the action, stretch those that antagonize it, and learn nerve patterns that help them do the right thing at the right time to lift the arms with safety, efficiency, and grace.