system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does it take >24 hours to "feel" a hard workout?
gpt
It's called DOMS, or delayed onset muscle soreness.  People often reach peak soreness the day after a workout, or even the day after that.  We aren't entirely sure about the specifics what drives muscle soreness.  People historically said "something something lactic acid something", but that's probably not what is really doing on.  Eccentric exercise seems to cause more soreness than concentric.