What causes muscle soreness after exercise?

Delayed-onset muscle soreness comes from microscopic muscle damage and inflammation, peaking 24–72 hours later — not lactic acid, which clears within an hour. Light movement, sleep, and protein help; it fades as you adapt.

Lactate clears within about an hour of stopping exercise, so it cannot explain soreness that peaks on day two. The cause is microscopic damage and the inflammatory repair response.

Soreness fades as you adapt to a movement. Sharp pain during the exercise, swelling, or dark urine after intense training is different and needs medical attention.