Is it an intentional muscle movement or a natural spasm/clench? ?

Question

I've noticed that a man's cheek/jaw area will sometimes clench; especially seen among the strong jawed of the species. I've tried performing this little tic myself, yet to no avail. Is it intentional? Or is it summoned by a teeth clench? What is this phenomena?

Question inspired from seeing that dude in Burn Notice.

Thanks in advance.

Answer 1

I've got a strong jaw, and yes that happens. I can do it intentionally by simply biting down, but I suppose that if I was stress out or something I might grit my teeth a bit, which would in turn flex that muscle. So, it's both.