Eli5: Why does holding something even moderately heavy above your head for as little as 10-15 seconds cause so much discomfort in the shoulders?

Question

Eli5: Why does holding something even moderately heavy above your head for as little as 10-15 seconds cause so much discomfort in the shoulders?

In:

Answer 1

If you feel as if you should have more than enough strength, it probably means that you don’t have appropriate mobility in your shoulder joints.

If your shoulder muscles are tight it takes more muscles to get them overhead in the first place, fighting against the tight muscles. Then, if your arms can’t get directly overhead it becomes more and more like you are holding an object out at arms length, away from your body.