What is Lactic Acid and Does It Really Cause Muscle Soreness?

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There’s a lot of misinformation out there about lactic acid. Some people claim lactic acid builds up in your body when you work out, causing you to feel tenderness in your muscles days after intense exercise, while others may offer advice on how to alleviate “lactic acid pain.” We wanted to wade through the myths around lactic acid to understand what it really is, so we talked to experts and asked them if—and how—it affects our bodies during a workout. Here’s what they had to say.

What exactly is lactic acid?

First off, while the body technically does produce lactic acid, it quickly dissociates into lactate and a free hydrogen ion. (Did we lose you already?) Even though lactic acid and lactate are often used interchangeably, they aren’t the same thing. For the sake of this story (and scientific accuracy), we are going to stick with the term lactate.

When you engage in strenuous exercise, like HIIT or strength training, your muscles need more of the usable fuel known as adenosine triphosphate (ATP) than they would if you were sitting at a desk—and at a faster rate, too.

What is ATP?

The way ATP works is that your body takes food products, breaks them down, and turns them into this fuel. This process can happen with or without oxygen. But ATP is actually produced a bit quicker in the absence of oxygen.

Because your body demands fast fuel when you’re getting a serious sweat on, working muscles go from using aerobic metabolism (which requires oxygen) to anaerobic metabolism (which does not use oxygen). And that’s where lactate comes in.

How does lactate relate to ATP?

According to Janet Hamilton, a registered clinical exercise physiologist and run coach at Running Strong in Atlanta, lactate—which is a byproduct of stored carbohydrates—is there for a reason.

Hamilton explains that when you break down carbohydrates to make ATP, your body also creates something called pyruvate. In most instances, pyruvate goes onto the next stage of metabolism, known as the krebs cycle, to create more ATP. This is a process that requires oxygen. It usually happens when you’re carrying out regular daily activities like walking to the grocery store, for example. When you work out at a high intensity, however, your need for ATP increases, so your body produces ATP anaerobically (i.e. without oxygen), because it’s a faster process with fewer steps.

Pyruvate becomes lactate, Hamilton explains, when there isn’t enough oxygen to create more ATP. But don’t stress: that lactate can eventually be metabolized for energy. This means, despite what most people believe, lactate isn’t really a bad thing.

“If there’s enough lactate in an area that it’s starting to build up, then the body naturally recognizes that and it will shuttle the lactate either to another cell nearby. Or it will send it off to the heart, where it can be utilized as fuel,” Hamilton says. Plus, she explains, pyruvate and lactate can flip back and forth pretty easily. This means that lactate can turn into the post-workout fuel that your body needs.

How does lactate affect our bodies?

You have too much lactate when your lactate production exceeds its clearance rate. This can then cause a buildup. This isn’t the end of the world, but it can throw off pH levels in your body. Hamilton explains that the hydrogen ion that accompanies lactate changes the acidity of an environment. This in turn causes those burning sensations you get while doing reps on a leg-extension machine. Now you see where the name lactic acid comes from?

“That god-awful burning…that is the buildup of hydrogen in the form of—for a lack of a better word—lactic acid,” Hamilton says. “That immediate soreness, that burning sensation, has something to do with the acidity of the environment.”

Although you may feel lactic pain mid-workout, Registered Dietitian and Certified Fitness Trainer Justine Chan of Ever After Health says lactate does not cause long-term discomfort. “While [lactate] may cause you fatigue during your workouts, it does not cause sore muscles, because it does not stay in the blood for long,” she says. “Instead, it quickly travels in the blood to the liver to be converted back to glucose.”

Bottom line? When you exercise vigorously and get a buildup of lactate, that feeling may be uncomfortable, but it won’t last long. And the soreness you experience days after a workout has nothing to do with it.

So what does cause sore muscles?

Now that we know that lactate only causes temporary aches, you’re probably wondering what causes the muscle tenderness you may feel two days after a ten kilometer run or an intense kickboxing session.

“The soreness that you feel after your workout has nothing to do with lactate,” Hamilton reiterates. “The delayed onset muscle soreness is actually due to micro trauma within the muscle fibers—tiny little tears.”

In conclusion, lactate is different from lactic acid, and neither really have to do with the muscle soreness that comes after an intense sweat session. Now that you know exactly how our body processes the energy needed to get through a workout, make sure that you’re giving it the proper fuel and rest it needs.

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