system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5, after working out, why do muscles hurt more the next day as compared to right after the work out?
gpt
It's not studied very well but the general consensus is inflammation. The body responds to injury (micro-tearing of muscle fibers) by sending white blood cells, enzymes, and other helpful devices to the injured area to clean house and repair the damage.

However some of these helpful devices will have  side effects which includes increased pain sensitivity in the area being repaired. This is why the muscle feels fine if its stationary, but hurts as soon as you move/contract it.

A similar and maybe more relate-able condition is getting a sunburn. You also don't feel the worst of that until 24 hours later, as the inflammation and repair process takes time to get into gear.