system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do humans use face muscles (clenching teeth, making angry eyes, eyebrows going down) when lifting something heavy, even tho it's unnecessary?
gpt
It's because tension equals strength. Clenching teeth especially increases force production because it increases tension in the body. The same way that doing the valsalva maneuver increases tension and strength. The face muscles is not something everyone does (even the teeth clenching) but tensing them usually means tensing the neck too which causes more stability.

The more tension added to the body the more you'll lift. You can see this during a bench press where leg drive doesn't actually lift anything but using your quads to produce tension in the legs increases your force production by a large margin. Another example is an overhead press where squeezing the hell out of your quads and glutes will make you lift overhead more.

But yeah adrenaline also increases if you're getting in the zone by clenching and tensing your face.