The most common cause of shoulder pain that I see is due to a weak shoulder stabilizer muscle. The infraspinatus muscle covers most of the shoulder blade and attaches to the top of the arm bone (the humerus). Its job is to keep the humerus in place when you move your arm. In other words, it keeps the ball in the socket. If this muscle is weak and not doing its job, the head of the humerus will be forced out of the socket when you raise your arm, and it will hit a roof of bone above the shoulder joint. Other muscles between these bones will be pinched, causing shoulder and arm pain. If this pinching continues, those pinched muscles (called the rotator cuff) will become threadbare and tear. I think most people have heard of a torn rotator cuff. When this occurs, it’s surgery time, and the rehab afterward is not a lot of fun.
This dire scenario can be avoided most of the time by strengthening the infraspinatus muscle. It’s easy to do and takes about five minutes twice a week for about six weeks. While lying on your side, keep your top arm against your side with your elbow bent at 90 degrees and a light weight in your hand. You lift the weight toward the ceiling in slow motion with your elbow never leaving your side. When you have lifted as far as it will go, then slowly lower the weight. Continue this sequence in slow motion until you are attempting to lift with all your might, but the arm will not move. At this point, I would still push it as hard as you can for five seconds. You should burn out in about five or six repetitions. If you go beyond eight, you need to find a heavier weight. When you are done on one side, flip over and do the other. If one side is weak, the other is probably close behind, so you always strengthen both sides. Give yourself three or four days before repeating this exercise.
Infraspinatus weakness and its accompanying shoulder pain are common in people over 50. It occurs much earlier, though, in people who frequently have their arms above their shoulders, such as athletes, construction workers who work over-head, or even people who sleep with their arm tucked up under their pillow.
Once you have strengthened these muscles over six weeks, you will still need to maintain them by doing the exercise about once every two or three weeks. In most cases, this is an easy solution to shoulder pain and its consequences.