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It’s one of those basic ideas you hear scientists repeating all the time: correlation does not imply causation. Just because two things happened together doesn’t mean one caused the other.

Well, back in the 1920s, they ignored their own advice. And the result was a major misconception that’s still around today. Because, despite what you probably were taught in high school, lactic acid does not make your muscles hurt when you exercise.

In fact, it might actually help. It all started around 1920, with a German biochemist named Otto Meyerhof. He helped figure out many of the steps of glycolysis, which is the series of chemical reactions that cells use to make energy from glucose — aka sugar.

He also kind of fit the “mad scientist” stereotype. If you’ve ever seen a scientist in a movie electrifying disembodied frog legs, well,. Meyerhof did that in real life.

He took frog legs — just the leg parts — and he gave them electric shocks, making them jump and twitch. But, after a while they stopped jumping, Meyerhof found that the legs were full of lactic acid — or, more accurately, lactate, which is lactic acid minus a proton. He and another scientist, Archibald Hill, correctly figured out the steps of glycolysis that led to higher levels of lactate in the muscles.

But Meyerhof also assumed that the lactate buildup contributed to muscle fatigue, stopping the frog legs from jumping around. When you exercise, your body uses up its main source of energy, ATP, within a few seconds. Then it starts to make more, using a bunch of different chemical reactions.

And during a hard workout, your muscles start to rely more heavily on reactions that also produce more lactate. When you exercise, the pH in your muscles goes down, meaning they become more acidic. That’s called — appropriately — muscle acidosis, and the drop in pH may be part of what causes the famous burn while you’re exercising.

There was a clear correlation between the lactate buildup and acidity in the muscles, and Meyerhof assumed causation, too: that lactate was responsible for lowering the PH in muscles. This became a generally accepted hypothesis, and you still see it all over the place today. Unfortunately, as we all know, assuming causation can be risky.

And in this case, Meyerhof was wrong. Even though the lactate idea was well-accepted, there wasn’t much empirical evidence for it, and over the years researchers started to question whether it was accurate. Around the second half of the 20th century, they started to pick apart what was really going on.

One important clue was the fact that unlike lactic acid, lactate isn’t an acid. That missing proton means it’s a base — it accepts, rather than donates, protons. Meyerhof’s idea assumed the reaction that produced lactate also produced a proton at the same time, making the muscles more acidic overall.

Turns out, it doesn’t. The series of reactions that generate more ATP in muscles do produce extra protons, which lowers the pH in your muscles. But the extra protons don’t come from the step that generates lactate.

And, in fact, lactate bonds with some of those protons, making the muscles less acidic. Lactate also isn’t involved in delayed onset muscle soreness — the pain you feel a day or two after a hard workout. Scientists think that comes from microscopic tears in your muscles, although they are still investigating that.

It’s weird that we don’t 100% know, but we don’t. So, despite what you hear from personal trainers everywhere and sometimes even see in textbooks, there’s no reason to be so hard on lactate. It’s just trying to help.

Also, a side note here: you’re gonna want to tell people about this. When they say “ah, the lactic acid!” Do it - but in the least pedantic way possible. We have to be accurate.

But don’t be condescending! Thanks for asking this lovely question, and thanks especially to our community on Patreon: your support keeps these answers coming! To learn more about how you can join — which will also give you access to our quick questions inbox — just check out patreon.com/scishow. [♪ OUTRO].