I thought I would share my recent experience with everyone here because it could help you. I just spent 5 months away from the gym, losing tons of mass and strength and degenerating into a normal person. The problem I suffered from is incredibly complex and not very well-understood, even by physicians, orthopedists, chiropractors or sports therapists.
Do a google search for "serratus posterior superior." You'll get a few definitions and a couple of pictures. It takes some real digging to find the trigger point article on it. And there's only one I know of. If the internet doesn't have a lot of info on it, you can bet your average backwater sports therapist has no idea.
The serratus posterior superior is a DEEP muscle attached to several of your top ribs and responsible for lifting the ribcage during breathing. It is covered by your shoulder blade, but that means it has three big layers of muscle and one layer of bone sitting on top of it. It's not all that difficult to get to if you move the shoulder blade out of position, though.
For those of you who haven't heard of trigger point therapy or foam rolling/tennis ball, I suggest you stop reading this now and immediately read some stuff on it. For those of you that have, you know that trigger points develop in muscles that are overused and must be manually destroyed in order to maintain proper function of the muscle. There are trigger point charts out there to show you where the most common locations for these buggers are, and you can use them to get an idea of where your latent ones might be. If you're actively tight and feeling pain, you already have one anyway and you should be able to locate it.
The point is, when trigger points develop in the serratus posterior superior (SPS), the symptoms are complicated and the condition is extremely difficult to diagnose. The principle pain felt is a deep ache seemingly behind the shoulder blade. However, the true pain may be masked by referred pain. Some side effects that trigger points here can cause include numbness or pain in the rear delt, elbow or fingers. It can also affect the triceps region and it can cause pain in your chest. Sounds a lot like, well, everything else that can go wrong, no? Symptoms of TOS are mimicked to some degree by trigger points in the SPS. For the longest time, I was sure I had TOS because any time I put my arm above my head, I ended up with numbess and tingling. I also found it 100% impossible to use my rear delt, and dermal sensation through the back of my shoulder was minimal (for example, I was hooked up to a tens unit and they put enough juice through me to cause my whole arm to move, but I still felt nothing).
You could go see a therapist who could diagnose you with some kind of elbow problem. The numbness in your hands might be interpreted as carpal tunnel or TOS. There are a multitude of other maladies and misdiagnoses that could happen too. I went through almost every one of them myself, poured thousands into therapy, herbs, massage and chiropractic care and spent a ton of money on equipment I didn't need to buy. The more I searched on the net, the more conditions I found and I kept eliminating them one by one, either by symptom lists or by negative treatment.
Finally, I found this site: http://www.pressurepointer.com/pain_reference_chart.htm
This site is the only site I know of that mentions the SPS or talks about trigger points that develop there (it is also extremely handy for other muscles so check it out). It tells you you need a special cane to get at the SPS, but I can tell you that simply laying on a tennis ball works just as well and doesn't require a mail order that takes weeks to get to you.
So what's the point of this post? If you're having chronic shoulder problems that seem difficult or impossible to fix, give this a shot. Try running some sprints and see if your shoulder bothers you then. If it does, I'll bet you have trigger points on your SPS. Odd that your shoulder should hurt when you run no?
And the treatment: get your tennis ball ready. Find a good place to lay down on the ground, shut the doors and windows and warn everyone within earshot that they may hear you crying from pain. You put the tennis ball somewhere around your shoulder blade area. But, just before you put your weight on the ball, pull the affected side's arm over the front of your body. Your objective is to move the shoulder blade out of position to the side, exposing the SPS (look for a few pictures if you haven't already). It's pretty obvious how to get at it once you see how it's located. Now, put your weight down on the ball right where your shoulder blade used to be. Ease onto it, if you're tight this is going to hurt just as bad (if not worse) than the ITB on the foam roller. Once your entire weight is down (this may take a few minutes of grinding pain and eyes tearing up), roll around a bit to start really taking out those trigger points. My SPS has been tight forever, so it hurt like the end of the world for me, but your pain is still going to be pretty bad with any degree of tightness.
Repeat this three times, twenty minutes apart or so for 3-5 minutes at a time. Then go to bed and prepare to be extremely sore tomorrow. With this simple treatment, along with my routine tennis ball treatment and foam rolling, my shoulder pain was gone literally overnight. I am back to putting my arm above my head with no bad biomechanics and I feel much lighter and more flexible.
Like I said before, this muscle has barely been given consideration by most therapists and diagnosing problems in the SPS is extremely difficult to do. For those of you who are suffering from anything in the following list, give this a try (this list is the progression of diagnoses I went through to finally realize relief):
Shoulder impingement due to...drum roll...kyphosis (normally that's a safe bet but not this time) -> recommended: rotator cuff work (I know dumb)
Scapular winging due to serratus anterior atrophy -> recommended: serratus anterior strengthening
Thoracic outlet syndrome due to poor posture (forward head) and misaligned cervical vertebrae -> recommended: 12 sessions of intense (and costly) chiropractic adjustment
At this point, I was 4 months out of the gym and absolutely fighting tooth and nail for anything that could make me feel better. I looked up everything on the net about thoracic outlet syndrome, including forums here and on tnation. What I found is that some guys end up getting bad TOS from having a leg too tight or something. I foam rolled the begeesus out of my right leg as I knew it was tight, but I had no idea how bad it was. The fascia was so tight it was impossible to stretch the muscle and my pelvic tilt was, for all practical purposes, intensely incorrect. However, after foam rolling all of the tightness out, stretching out the muscles and realigning the hips, still no relief in the shoulder area (of course I rolled/tennis balled the shoulder area too - everything I could think of).
FINALLY I found the SPS and its trigger points. In less than an hour I was completely and utterly relieved of almost all of my pain. Trigger point therapy has been called "not quite a miracle, but damn close." The only reason it isn't a miracle is because it takes at least a few days to relieve the worst of symptoms. But come on, a few days? Relative to 5 months of average American sedentary lifestyle completely against my will, I'll take three days of treatment to fix permanently a problem I've been living with since I started lifting years and years ago.
So take my advice: If you have a chronic problem that seems impossible to fix, give this a shot. At the very least, if you aren't already foam rolling or tennis balling, you ought to be. Maybe your problem can be solved by rolling a different area out, who knows. Even if you have NO apparent problems whatsoever, I guarantee foam rolling and tennis ball therapy can make you feel better.
Thanks all, and I am happy to bring you info on this obscure, incredibly-difficult-to-diagnose problem. I hope you can get something from it.