Always when I deadlift at the maximum load, my face looks more or less like this:
The same applies for when I do other heavy-weight exercises.
But what biological mechanism causes that? Face muscles, to the best of my knowledge don't participate in the mechanical system that contributes to the lift. Also lifting doesn't cause any uncontrollable emotions, that would cause my face to go >:( Furthermore, I don't see any biological gain my organism gets from a "deadlift face". It doesn't increase oxygen intake or improve cooling.
So why do I uncontrollably flex my face? I suppose it is something about my nervous system going into overdrive, but I have no idea what's the actual mechanism.
I did some research, but google only directs me to dubious "fitness lifestyle blogs", and I have no idea how to word my question in a way that would make sense on google scholar.