Subscapularis is the largest of the rotator cuff ("RC") muscles; it lies inside/under the shoulder blade itself, and is responsible for internal rotation.
Our research and experience shows that, in the vast majority of the population (excepting two groups, mentioned below) the shoulder--arm function we describe as 'internal rotation' is much stronger than external rotation. Loosening the internal rotator cuff muscle (subscapularis) and strengthening and stretching the external rotator cuff muscles (infraspinatus and teres minor) can have an almost magical effect on some kinds of shoulder pain.
Symmetry of functional strength is needed in the rotator cuff muscles, to avoid having the humerus pulled forwards in the joint capsule (the capsule and its associated fascia are infused with pain-signalling mechanisms; if the capsule is stretched by this movement, you feel pain). Together with long head of biceps, unbalanced RCs are a hidden cause of shoulder pain and/or dysfunction for many.
The two groups in the population who do not conform to this trend are, perhaps unsurprisingly, among the strongest and most mobile of athletes too: gymnasts and rock climbers.
I'm going to show you a fragment of a yoga pose, called Garudasana, which means Eagle Pose. And I'm assuming it means that because of the wings, shape of the wings and so on, but it's one of the absolute best pose for a muscle inside the shoulder blade called subscapularis, which in fact is an internal shoulder rotator. But it also has a fantastic effect on the very upper part of the upper back and some muscles around and behind the shoulder blades, which you simply can't get any other way. This is what it looks like. I'm going to swing my arms across like this and catch one arm above the other. I'm just going to turn side on so you the sequence from here on? I then reach up and reach the fingers around the back of the wrist like this, and pull the wrist down to the elbow.
Now I'm going to turn and face you again to show you what that looks like. You can see how I'm holding the arm here and notice I'm going to open out the held arm, so that the elbow is more open than 90 degrees. I'm going to turn and face you now. And from here, the arm that I'm holding, I'm going to press away from the midline of the body, like so. That's what stretches the internal rotator, because this is external rotation. Now the contraction is, I'm going to press the held arm back to the midline of the body, which I'm doing now, for a count of five. I stop, I breathe in, I make sure that the elbows are still open out at 90 degrees, and then I take the arm further away from the midline like that. And that's what stretches the muscle inside the shoulder blade.
Now I'm going to turn away from you and show you by adding some little slumping and twisting movements like this, I can stretch all sorts of muscles across and above the upper back. And then if I lift the arms up like this, I can stretch all sorts of muscles on the outside of the shoulders as well. And now I'll show you what that looks like from the other side. Just release that arm, give it a bit of a shake. And then catch above the elbow like this, reach up – ou might need a hand to help you hold your own hand, so a partner could be helpful here. Face away from you to show this angle. We're opening the elbow out past 90 degrees, and then we're going to press the arm away from the midline of the body.
Now you'll notice that I'm holding my shoulders still, when a lot of people do this exercise they do this, they actually twist at the waist. Don't do that; instead, press the arm away from the midline of the body like that, so it's actually a shoulder movement. Then press the held arm back to the midline of the body like I'm doing now, stop, breathe in, and on a breath out go further. Notice that the elbow angle is open past 90 degrees. And if I turn and face away from you, we'll just repeat those little twisting movements, which feel so nice in the upper back. And then lift the arms up above your shoulders like this and twist around a little bit as well. And then let the arms go. I normally find that just by giving a bit of a shake and a shrug like this it makes everything feel completely comfortable. No exercise will make your upper back feel as good as this one does. So try it. You're going to love it.