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The teres major is the most ignored, least well known muscle that you need to know about. It's on your back, so it's already getting less love than the "mirror muscles," and it's under your arm and sandwiched between your lats and your arm. It's not even a part of the rotator cuff. It's just left out in the cold.
But there's a reason why knowing about your trees major is important. The teres attaches the humerus to the scapula. Specifically, it ties the bottom point, known to anatomists as the inferior angle, of the scapula to the shaft of the humerus about a tenth of the way down.
Well, yes, actually.
What happens when you do that is that your humerus rises away form your scapula. The scapula rotates upward, with the tip of it (the part the teres major attaches to) moving away from the spine, too, but that shouldn't start to happen until the arm is about 30° abducted in the scapular plane. After that, every additional two degrees of humeral abduction is usually accompanied by one degree of scapular superior tilt.
Hmm. What does all this mean in English?
When you raise your arm out to the side, not quite straight out, the movement begins with just your arm. Once there's about a foot to eighteen inches of space between your arm and your body, your shoulder blade starts to move and from there on, it moves as you raise your arm overhead, but it's only supposed to move a certain amount. What happens with a tight teres major is that your scapula moves too much, too fast, in the wrong part of the process of abducting (moving away) your arm.
At first glance, that sounds like something they might not matter too much. But it's not, because when your scapula is in he wrong place your whole upper back is in the wrong place.If you have a teres major injury,which will almost certainly be a result of overuse in overhead positions or of excessive throwing, you can bet it's going to give you problems all over your upper back, neck and shoulder. And it's going to stop you throwing too.
How To Assess It
It's pretty hard to do on yourself visually, because of course, you can't see your own back. (If you can see your own back, don't let me stop you, but advice about our anatomy might not be useful to your species.) But it's not that hard to do it by feel. Put one hand under the other arm and find your scapula with your fingers. It should be lying flat against the thoracic wall. If it's not, that could be partly caused by a tight teres major pulling it up and away, as well as poorly activated upper back musculature. Now you've found the point, or inferior angle, of your scapula, raise your arm. Remember you want it slightly in front of you, not right out to the site. If you imagine a line that runs from side to side right through your body, that's called the coronal plane and you want to be moving your arm in the scapular plane, which is about thirty degrees forward of the coronal plane. So slightly in front, then. If your scapula starts moving as soon as you move your arm you have scapular dyskinesia. If it starts when your arm is about a foot to eighteen inches away, or abducted by thirty degrees (feel free to use a protractor) you have normal scapulohumeral rhythm and your certificate will be mailed to you. But something in between could mean your teres major has begun to tie your scapula a little too tightly to your arm.
Once that starts, you'll often find you';re moving your scapula as soon as you want to move your arm. Your raised arm is more like a kind of mutant shrug, so it's hard to move your arm around when it is raised. That's going to make your overhead press look funky, but over time, that will be the least of your worries as your upper back warps out of shape, developing permanent compensatory rotations and kyphosis. And all because you didn't take that teres major to task.