So, first I want you to reach overhead in a slouched posture, look how far your arm goes up. Now, stand up straight, head high, chest up and tight core, then reach up overhead. Does your arm move about 30 to 40 degrees more when you actually use good posture?
I see people in my office with impingement pain in their shoulders frequently. Why do they have pain? Forward flexing your arm up and overhead, naturally will cause slight compression into the shoulder. Normal motion is that your humeral head(or the ball of your shoulder) will rotate externally (or outwardly) rotate and this clears the rotator cuff muscles from compressing sharply up into the acromial clavicular(AC) joint.
This happens cleanly when your glenohumeral (or ball and socket) joint is sitting in the correct position. If you slouch and move your arm up, the glenohumeral joint is angled incorrectly and you will cause compression or impingement of your rotator cuff into the acromial clavicular joint. Thus, every time you do this, you pinch those muscles and they can get inflammed and/ or microtears occur in the tendons.
As this goes on, you can also pinch the bursa sac in the shoulder as well. So, the cycle is pinching pain, then inflammation, then tendonitis/bursitis, and then you have shoulder pain. Sometimes, you just stop using that arm as much because of pain and slowly the mobility decreases and you have a frozen shoulder, too. It’s an ugly cycle. So, please sit or stand up straight when you are reaching overhead or doing repetative motions of your arms. This should help you avoid impinging those little muscles in your shoulder and avoid pain. Didn’t your mother always tell you to sit up straight!