system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we grunt/scream when doing strenuous exercise?
gpt
Everybody uses their tummy muscles. But sometimes, when you squeeze really hard while you poop, you squeeze some air out! Same thing if you are lifting heavy ass weights. You're squeezing air out. Even though you are A. Trying to keep it in or B. Go for an extra bit of power by forcing all the air out and really *really* using your abdominals.

Edit: its the same reason that people doing martial arts either yell or do that hissing noise. they are tensing their midsection to help squeeze every bit of power out of their body.