Most people have never heard of their supraspinatus muscle (much less the supraspinatus tendons that attach to it), but it's one of the small cookie-cutter muscles located in the upper area of the back. If you take either hand and reach up to grab the area between the side of your neck and your opposite shoulder (like you're giving yourself a little massage), your fingertips should be resting more or less directly on it.
It's part of the rotator cuff, which helps stabilize the shoulder and keep it in the right position, and the function of the muscle is to help the shoulder raise your arm out to the side and overhead. Contrary to what a lot of people think, the supraspinatus is only the prime mover, so to speak, for the first few degrees of the motion when you raise your arm away from your body. (Beyond about 15 or 20 degrees, it's mostly the deltoid that performs the action.) Athletes like pitchers who do this motion a lot tend to develop tendon pain in the supraspinatus tendons, which is usually diagnosed as supraspinatus tendonitis.
I'll discuss whether or not this pain is actually tendonitis or something else a bit later down the page, but first, let's take a look at the supraspinatus itself.
As you can see, the muscle isn't very large, and the tendons that attach to it aren't either. In fact, one of the most common types of rotator cuff tears is a tear in the supraspinatus tendon, which comes right over the top of the shoulder. Due to its position, along with the fact that it helps to stabilize the shoulder and hold it in place against the constant drag of gravity on the attached arm, it's at quite a bit of risk, especially as you get older. Overall, most tears are experienced by people 40-70 years old.
The shoulder is a complex joint, and I'm not going to try to get into all the possible problems that can occur with it here. If you're experiencing pain, it could be anything from a muscle imbalance to an overgrown bone protrusion to something wrong with one of the tendons. There's really no way to tell without seeing a specialist, so if you have pain anywhere in the rotator cuff, I suggest that you go consult one. Again, this would be a specialist, not a regular doctor. You want someone with a lot of experience dealing with shoulders, and you should be able to tell whether or not your doctor knows what's going on by the variety of tests s/he performs. It's difficult to diagnose a supraspinatus problem as opposed to any of the three other muscles of the rotator cuff, and it takes testing from various angles to do it right.
As far as the supraspinatus tendon goes, the tendon doesn't have to actually tear in order to experience pain. Just overuse and pattern overload can cause it to become inflamed (tendonitis) or, in cases that last longer than a couple of weeks, actually cause clumps to appear in the collagen fibers that make up the tendon (tendonosis). This second condition is much more common than most people (even doctors) realize; it's been estimated that as much as 95% of long-term "tendonitis" cases are actually tendonosis.
If you have a full tear of the tendon, you'll need to see a specialist for treatment. But if you've been experiencing pain that (a) isn't a tear, or is only a minor tear (get an MRI to make sure) and (b) doesn't seem to be responding to the usual icing, rest and NSAIDs, then it's very likely that you have tendonosis rather than tendonitis. In that case, a program of specific exercises that target the supraspinatus is your best bet to get at the collagen fibers and straighten them out.
The Target Tendonitis ebook gives a thorough explanation of the best way to approach this, and the companion videos that come with it show how to do exercises for all the various parts of the body where tendonosis is likely to occur. For the supraspinatus tendon, take a look at the shoulder exercises that involve lateral motions and you'll be on your way to recovery.
