The division between the body and the soul is completely arbitrary.

I don't think anyone can clearly define what the soul is. And unfortunately in modern medicine (and science) what ever we can't demonstrate with our senses (or tools we invent) is the soul.

Just like in the excellent book by Dr. Seus- Horton Hears a who- what you can't hear, see or touch just doesn't exist. Well, if you have a microscope and can see the "who's" in the speck of dust-that's different.

Of course emotions effect our muscles and other biological functions. This is what they are meant for.

Fear/ Anxiety (an emotion) is a protective mechanism that helps us flea from danger, or fight it. It leads to constriction of our blood vessels in the gut, and re-direction of all the blood flow to the muscles, lungs etc.
This happens if we see a tiger that is about to leap on us, but also if we see our boss, after we did something we know he is not going to be happy with. Those are all very physiological processes.

For me stress if at all , is a negative trigger (eg-makes me much stronger). In fact, the first treatment for MG-ephedrine (which is also very effective for me) is based on this. So, I wonder why so many MG patients experience the opposite (eg-that stress makes them worse). This just shows how complex this illness is. And how complex people are.