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Lactic acid causes next-day muscle soreness

Lactate does not cause the soreness you feel a day or two after training. Exercise that raises lactate a lot need not cause soreness, exercise that barely raises it can, and delayed soreness comes from unaccustomed, especially eccentric, loading.

What the science says

  • Lactic acid build-up causes delayed soreness Limited evidence

    Schwane's 1983 study had people run 45 minutes on the level and downhill. Level running raised blood lactate but caused no significant soreness; downhill running never raised lactate yet caused significant delayed soreness.

  • Lactate production causes muscle acidosis Limited evidence

    Robergs' 2004 review argues that the acidosis of intense exercise comes from ATP breakdown and non-mitochondrial energy supply, and that lactate production actually slows acidosis rather than causing it. Lactate is a marker of those conditions, not the culprit.

  • Delayed soreness comes from unaccustomed and eccentric work Moderate evidence

    Cheung's 2003 review lists lactic acid among several proposed mechanisms but places the cause in micro-injury, connective tissue damage and inflammation, most common with eccentric actions and when returning to or starting new training.

What to do instead

Limit severe soreness by easing into new exercises and eccentric-heavy work, and manage it with lighter activity rather than trying to "flush out" lactate.

New program or exerciseIntroduce volume and eccentric work progressively over 1-2 weeks
While soreLight activity of the sore muscles, or train other muscle groups
Intense sessionsReduce intensity and duration for 1-2 days after very damaging work
Not neededCool-downs or massages aimed at "clearing lactic acid" to prevent next-day soreness

Who should be careful

Normal soreness fades within a few days. Severe pain, swelling, dark urine or marked weakness after exercise can indicate serious muscle injury and need prompt medical attention.

Frequently asked questions

Does lactic acid make me sore the next day?

No. A 1983 study found level running raised lactate without causing soreness, while downhill running caused soreness without raising lactate.

Then what causes delayed soreness?

Unaccustomed, especially eccentric, exercise that causes micro-injury and inflammation, according to a 2003 review.

How do I get less sore?

Introduce new exercises and eccentric work gradually over a week or two, and train lighter or train other muscles for 1-2 days after very damaging sessions.

Scientific references

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