Delayed muscle soreness 12–72 hours after unfamiliar load. Not a measure of training quality.
Soreness mainly follows braking (eccentric) work — running downhill, lowering the bench press — and comes from microscopic structural damage plus an inflammatory response, not lactic acid. The most stubborn myth is 'no soreness, no progress'. Wrong: soreness measures how unfamiliar a load was, not how effective. Experienced athletes grow without it. Conversely, never train hard into heavy soreness — technique degrades and injury risk climbs. Light movement helps more than rest, and if pain hasn't eased after five days or your urine turns dark, get it checked medically.