What is DOMS and why does soreness peak a full day or two later?
Delayed onset muscle soreness, DOMS, is the stiff, tender feeling that shows up in a trained muscle roughly 24 to 72 hours after a workout, typically peaking somewhere around the 48 hour mark rather than immediately after training.
The delay itself is the defining feature. Acute pain during a set comes from metabolite buildup and mechanical strain in real time. DOMS is a separate, slower process tied to the actual microscopic damage and subsequent inflammatory repair response happening inside the muscle fiber over the following days.
Early theories in the mid 20th century assumed DOMS was caused by lactic acid buildup lingering in the muscle for days, a theory that has since been thoroughly disproven, since lactate clears from muscle tissue within roughly an hour of exercise ending. Research through the 1980s and 1990s, including work by Priscilla Clarkson, instead identified microscopic disruption to the muscle fiber's internal structure, specifically at the Z disc, alongside a resulting inflammatory response, as the real underlying mechanism.
This means the ache you feel two days after leg day is not old lactic acid finally catching up with you, it is your immune system and repair machinery actively working through microscopic structural damage that the workout itself created.
This misconception about lactic acid has proven remarkably persistent in gym culture despite being thoroughly settled science for decades, likely because the phrase itself sounds intuitively plausible and had already become deeply embedded in fitness vocabulary well before the more accurate inflammatory explanation became widely known outside academic circles.