As a working guitarist, sooner or later you will be confronted by tension in your upper body and shoulders. I always try to teach to all my students how to avoid tension and explain how warming up before a session/gig and sitting/standing correctly can make a huge difference in how you will feel at the end of it.
We always forget how playing an instrument, lifting gear, or sitting all day to practice or teach can be a physically demanding task, and I don’t know any athlete that does not warm up before a performance, so why shouldn’t we?
The following is a great video that explains how things work in the upper body:
This is a great set of warm ups and massages by guitar master J. Petrucci:
If tension should arise, a great way to release is explained in this video:
I hope this was helpful, feel free to add a comment below if you find more useful videos!