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Lactic acid does not cause soreness or fatigue

 •  • by Paul Ingraham
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If you’re still thinking about lactic acid painfully accumulating in muscles during exercise… time to adjust that mental image! It’s a myth, and the myth contains myths within myths.

Today’s debunking surely isn’t news to many readers, but it undoubtedly is for many — 40% in a Twitter poll I took — or at least a good reminder. You know that thing where you remember that there is a myth about something, but you can’t remember what the myth is? Which version is the truth and which is the myth? Confession time: that was me when I picked this topic again for the first time since 2006. I definitely needed to brush up on the biochemistry! So I did, and you get the readable summary…

Update: This post originally asserted that “lactate literally reduces acidosis” as a part of a larger argument that lactic acid “does not cause fatigue.” This was cribbed directly from the work of Dr. Robert Robergs. I was unaware that this view is opposed by at least some other experts. There were vigorous rebuttals and rebuttals to the rebuttals, and a rather technical debate continues to this day: Robergs published again just a few months ago.

Robergs’ position might be largely unopposed, or the reverse — I have no way of knowing. All I can do is report the debate. Importantly, this controversy is only relevant to the biology of fatigue, not soreness.

This post remains mostly unchanged except for this update. I have made more changes to acknowledge the nuances in the permanent version of this content in my article on delayed-onset muscle soreness.

Lactic acid is not a garbage molecule, not a “waste product” that “builds up” and causes fatigue and then soreness. It’s not even an acid! It’s just lactate in muscle metabolism, which is not an acid. But calling it an “acid” harmonizes nicely with its bogus reputation for being a troublemaker, especially the idea that it causes “the burn” in exhausted muscles. It is only partially involved in that (and not because it’s an acid, see Pollak et al).

Lactic acid molecule — three red oxygen atoms on a skeleton of black carbon, with some white hydrogen at the edges. But it’s lactate muscles produce — not an acid. This would have to lose the white on the right to be lactate.

These pernicious, unkillable myths have been with us for many decades. They originated with “one of the classic mistakes in the history of science,” according to George Brooks, a Berkeley physiologist (see Kolata). A simplistic misinterpretation of a simple experiment on frog muscles produced the theory that lactic acid causes fatigue… and coaches and athletes and popular health articles and infographics have been repeating it ever since.

Lactate actually fuels muscles when the standard cellular engines hit their limits — which they often do. This backup system is “an essential feature of repeated intense muscle contractions, and without lactate production such repeated contractions could not occur” (Robergs 2012).

Lactate does indeed accumulate in exhausted muscles, but only because we churn it out faster than muscles can use it. (Even so, it’s gone within an hour, not lingering for days.) Fatigue is complex and has many causes (understatement), but it is not caused by muscles drowning in lactate. One reason elite athletes can do more than the rest of us is that their intense training teaches their muscles to use more lactate.

The lots-of-lactate state is routinely called “acidosis.” However, lactate literally reduces acidosis (according to Robergs 2004; not everyone agrees with this technical point; even if false, it remains the case that lactic acid is not “the fatigue molecule”).

Conventional wisdom is often a Russian doll of errors within errors within errors. Almost anything that lots of people believe is probably wrong. Or at least egregiously oversimplified!

This is a slightly abridged version of an updated section of my full article about delayed-onset muscle soreness. The full version has somewhat more detail, especially about the acid difference, plus a great deal more about soreness:

A Deep Dive into Delayed-Onset Muscle Soreness — The biology & treatment of “muscle fever,” the deep muscle soreness that surges 24-48 hours after an unfamiliar workout intensity. (17,000 words, 70-min read)