It was never lactic acid

Ease into a new type of exercise gradually — a bit less volume or intensity than you’re capable of on the first session — rather than going all-out immediately. The soreness that shows up a day or two later isn’t lactic acid doing damage; it’s your body reacting to movement it isn’t yet adapted to, and gradual introduction genuinely blunts it.

Why this works

The lactic acid explanation for next-day soreness is one of the most durable myths in exercise, and it was scientifically discredited decades ago for a simple reason: the timing doesn’t match. Lactate produced during hard exercise clears from the muscle within roughly an hour of stopping. Delayed onset muscle soreness, or DOMS, doesn’t even begin until 8–24 hours after exercise, peaks around 24–48 hours, and can last several days — a mismatch too large to explain by a substance that’s long gone before the soreness even starts.

What actually causes DOMS is still being worked out, and that’s a genuinely honest thing to say rather than a gap to paper over with a confident-sounding wrong answer. The longstanding leading explanation involves microscopic damage — traditionally described as tiny tears in muscle fibres, particularly from eccentric contractions (the lengthening phase of a movement, like lowering a weight or running downhill), followed by an inflammatory response that sensitises the area over the following day or two. More recent research has proposed a more specific mechanism involving microdamage to sensory nerve endings within the muscle spindle rather than the muscle fibres themselves, which would help explain some features of DOMS that pure muscle-damage theories struggle with. Neither model is fully settled, and several competing theories remain under active investigation.

What is well established, regardless of which exact mechanism turns out to be right, is the repeated bout effect: the first time you do an unfamiliar movement or a much harder version of a familiar one, soreness tends to be significant; the second time, even at similar intensity, it’s noticeably less, and it continues to fade with repetition. This is the practical, actionable part — soreness is largely a function of novelty and eccentric load, not of how “hard” a session felt in the moment or how much lactate you produced.

How to do it

1. When starting a new type of exercise, or returning after a long break, hold back on intensity for the first one or two sessions rather than testing your absolute limit immediately.
2. Expect the worst soreness after movements with a significant eccentric (lengthening) component — downhill running, the lowering phase of a heavy lift, new exercises your muscles aren’t used to decelerating.
3. Don’t use soreness as your measure of whether a session “worked.” A session can be highly effective with very little next-day soreness, especially once your body has adapted to that movement pattern.
4. Light movement the day after a sore session — an easy walk, gentle activity — tends to make soreness feel more manageable, even though it doesn’t address the underlying cause directly.
5. Give a new movement two to three sessions before judging how sore it “should” make you. The repeated bout effect means the second and third exposures are a much better guide than the first.

Common mistakes

• Trying to “flush out lactic acid” with stretching or massage to prevent next-day soreness. The lactic acid is already gone within an hour — there’s nothing left to flush by the time soreness would even begin.
• Going all-out on the first session of something new. This is exactly the scenario that produces the worst soreness, and it’s also the most avoidable one.
• Treating severe, unusual soreness as always normal. Very intense pain, dark urine, or significant swelling after exercise are signs of something beyond typical DOMS — rhabdomyolysis is rare but serious, and warrants medical attention rather than being pushed through.
• Chasing soreness as a marker of a “good workout.” Some of the most productive, well-adapted training produces relatively little soreness — it’s a poor proxy for effectiveness once your body has adjusted to a movement.
• Assuming the exact cause of DOMS is fully settled science. It isn’t — several plausible mechanisms are still being actively researched, which is a fine and normal state for an area of physiology to be in.

What should change

Expect meaningfully less soreness the second and third time you do a new movement compared with the first, often a noticeable difference even within the same week. This is one of the more reliable, fast-acting adaptations in exercise physiology.

Your next step

If soreness is a recurring problem with your regular training rather than just something new, that’s more likely a sign your rest and recovery structure needs attention — the article on rest days and deload weeks covers that side of the equation.