When I used to play rock guitar decades ago, we always joked about a phenomenon that some lead guitarists had that we called "lead mouth." When the guitarist was really into playing a lead break, his mouth would start to distort like he was chewing on the air. Now "true lead mouth" was a purely involuntary thing. The person could not help it and may not have ever realized he was doing it. "Fake lead mouth" was a contrived thing that the guitarist was doing to try to look cool. The fake version is characterized typically by pursing the lips, like Billy Idol does a lot.
Anyhow, I was reminded of all this when I jammed with a local clawhammer banjo player. He is great player and a good guy, but he has a serious case of lead mouth and I don't even think he knows it. He bares his teeth and looks like he is about to bite someone when he plays. When people jam with him for the first time, it startles them. So I got to thinking about involuntary physical reactions when playing.
Here is mine: When I am learning a new tune and really concentrating on getting it right and perhaps struggling a little bit, my nose itches. It is not a little itch either, it comes on all of a sudden and it is really intense. Scratching doesn't help, but it goes away when I stop playing. Very wierd.
So, for discussion, what purely involuntary physical reactions do you have, or have you ever encoutered in someone else, which is brought on by playing?