system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people make weird faces while working out/carrying heavy objects?

As I was doing my reps, I realized I was making a weird face, and that everyone else were too, and had no idea why.
gpt
Because under extreme stress, contraction of more muscles than are being targeted, works to maintain tightness and prevents you from being exposed to injury.
And because the body is one big unit, muscles work synergistically, even facial and neck muscles. Try tightening your fist as tightly as possible, you may notice more muscles than just your forearm will contract.