Why do people grimace when they are exerting effort?

I just opened a jar of peanut butter and I noticed my face was all squeezed up in a grimace as I applied torque. I often grimace when I’m exerting effort, like trying to stay upright on skis, or turning the mattress, or twisting beer caps. Other people do it, too. I call it face power, as if screwing up my face adds oomph! to the effort, but is there a physiological reason for it? Is there an evolutionary reason (see me be strong!)?