When one is in a good position whether he is seated or in a throwing position in standing, you would want your chest up, head neutral, and scapula set (which means a drop-down scapula not necessarily retracted). That set position will allow the acromion to open up a little bit and create space in front for your rotator cuff. But if one does the opposite like a posterior pelvic tilt that makes the shoulder roll forward, it changes the pull on the scapula and the rotator cuff and making it a little difficult to raise the arm completely overhead. Ultimately that position will allow more irritation and more impingement of the rotator cuff muscle creating bursitis. Interestingly, whether one is throwing overhead or underhand, the same types of rotator problems happen.
Throwing With Force and Body Cordination
Secondary to that is athletes trying to throw with their arms, but actually where you want to get your force is at the legs which do majority of the work. The strength of your legs gets drives the hips, then transferred to your torso then to your arms. If the timing is off or any of those chain is mistimed, then you have lose of energy, lose of power, lose of speed, and sometimes significant of pain depending on your throwing.
Sleeping on the side and the shoulder getting typically compressed in and rides up and presses on the acromion and jams the rotator cuff is another frequent source of shoulder impingement.
Back to impingement,ultimately any time you are ain an in range position – if you reach all the way across your body, or reaching overhead, or reaching all the way far back - those will create a discomfort because you are maximally stressing the rotator cuff and it can’t sustain that position and its riding up either pinchin on the bursa or pressing right on the potential tear causing irritation to that area.
If you are a throwing athlete - softball,baseball, foot ball - and your shoulder is hurting, if you think you might have an impingement, lets get you checked out and make sure it's not rotator cuff tear and lets get you feel better as soon as possible!