Lactic acid is fitness enemy No. 1. It builds up in your muscles, makes them ache, and, for the sake of everything that is fit and pure, needs to be flushed out of your system and massaged from your muscles. Lying on your back with your legs up against the wall will surely help. 

Or, at least, that's what a lot of people—including some trainers, physiologists, and academics—will tell you.

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But they're wrong. Or, let's call it, misinformed. The body doesn't actually produce lactic acid. It produces lactate. And, while that's mostly a matter of semantics, we're going to call it lactate because 1) that's what it produces, 2) your body's pH is too high for lactic acid to exist in the body, and 3) lactate doesn't make your muscles more acidic.

In fact, not only does lactic acid not make your muscles sore, but its production actually helps ease your muscles' mid-workout burn, explains Janet Hamilton, C.S.C.S., an exercise physiologist at Running Strong in Atlanta.

When you perform a high-intensity workout—like strength training, tennis, or a boxing circuit—your body demands a ton of fuel, which it supplies through glycolysis, breaking down carbs, glycogen, and other molecules to produce a high-energy compound called ATP, Hamilton says. ATP is then broken down, releasing energy and other molecules, including hydrogen, throughout your muscles.

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Get this: It's actually those hydrogen ions that are wreaking havoc on your workout. When hydrogen builds up in your muscles, it causes a drop in your body's pH, making it more acidic, interfering in your muscle fibers' ability to contract, and causing a mid-workout burn that will eventually cut your exercise short.

So why do people blame lactic acid (or, excuse us, lactate)? It all goes back to the 1920s, when scientists first observed that lactate was a byproduct of glycolysis. What's more, as blood levels of lactate increased, so did the muscle fiber's acidity, she says.

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Basically, high levels of lactate in your bloodstream are associated with a mid-workout "hurts so good" burn. But lactate isn't causing it. It's a common case of mistaking correlation with causality. Think about it this way: Just because you wear your swimsuit in the summer doesn't mean that pulling it on in the winter will make the temps rise 50 degrees and the water thaw.

See, apart from hydrogen, glycolysis also yields a byproduct called pyruvate, which can snatch up rogue hydrogen ions floating through your muscles, forming lactate. Why? To try to keep your body from becoming too acidic, she says. You get that? Lactate forms to rid your body of workout-wrecking hydrogen ions.

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As if that weren't enough to rehab lactate's rep, consider this: Lactate easily converts back into pyruvate, which your body can use to create even more workout-revving energy, Hamilton says. Lactate is fitness fuel.

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So what's causing the burn?

While the formation of lactate can ease the pain, pyruvate can't always gobble up the hydrogen ions fast enough, meaning you can still get some burn and fatigue when you hit it hard, says Los Angeles-based trainer Mike Donavanik, C.S.C.S. Hydrogen-caused fatigue in your muscles tapers off within minutes to hours after your workout (depending, of course, on how much hydrogen you build up), he says.

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Any soreness after that—typically called delayed-onset muscle soreness—is caused by micro-tears, as well as swelling in your muscles, says Anthony Wall, M.S., exercise physiologist and director of professional education for the American Council on Exercise. But neither hydrogen nor lactate (and especially not lactate!) has anything to do with it.

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