There’s a longstanding misconception in the fitness world that lactic acid in muscles is what leaves you tired and achy after a tough workout. Newsflash: This was actually debunked decades ago.

So why aren’t more fitness fans in the know? Probably because the real story is a nuanced, scientific one: While lactic acid can cause an immediate burning sensation during exercise, it doesn’t stick around long enough to deserve the blame for muscle soreness after a workout. 

Here, we’ll dig into what exactly lactic acid is, what it does in your body, and what really makes your muscles sore after exercising.  

What Is Lactic Acid?

Lactic acid, also called lactate, is a substance your body naturally makes in the process of breaking down carbohydrates to use for energy. Think about how a car runs off gasoline. “A byproduct of burning gasoline is exhaust. You can think of lactic acid as almost metabolic exhaust in the human body,” says Mike Stack, an exercise physiologist, certified strength and conditioning specialist, and president of the Physical Activity Alliance.

Higher-intensity exercise, like sprinting or lifting heavy weights, generates more of this “exhaust,” he says. These types of intense, explosive activities are also impossible for your body to power with oxygen alone. Instead, sprinting, heavy weightlifting, and other high-intensity workouts are considered anaerobic exercise, meaning “without oxygen.” Your body taps into a form of sugar stored in your muscles (glycogen) during anaerobic exercise and breaks it down into energy. Lactate gets made in the process.

But lactate isn’t just a byproduct: It’s also used as part of the fuel for those anaerobic bursts of exercise. “Think of it as a quick fuel source your muscles can tap into when you’re exercising hard and your body can’t keep up with the oxygen demand,” says doctor of physical therapy and certified strength and conditioning specialist Jessica Chellsen, owner of Vibrant Coast Physical Therapy and Wellness. “Lactate is not bad: Your muscles, heart, and even brain can use it as fuel.”

Lactic Acid’s Role In the Body

A byproduct of your body’s energy system probably doesn’t sound like something you want building up in your muscles. But it’s nothing to be afraid of—and it's part of the equation when you’re generating a lot of energy at a rapid pace to fuel a hard workout. At a high intensity, often around 70 percent of your maximum heart rate, Stack says, “your body can’t get enough oxygen to the muscle to be able to produce energy, so it ends up providing energy in a more accelerated state, and it has to produce this byproduct.”

This point is called your anaerobic threshold, and it’s where lactic acid starts to build up in your muscles. And components of the lactate, like hydrogen ions, “change the muscle’s environment and cause that burning sensation,” Chellsen explains. This temporarily changes the pH in your muscles; the more acidic environment may also contribute to the burning sensation. 

But these higher-than-normal levels of lactic acid in your muscles don’t last long. While experts once thought lactic acid was only a waste product and hung around in the muscles until it was cleared (often with recovery workouts), today, there’s a deeper understanding of the way it gets reused. After exercise, lactic acid is cleared quickly (some experts estimate within an hour) and recycled back into energy, Chellsen says. 

Think about the last time you sprinted up a flight of stairs: Your quads are burning by the time you reach the top, but two minutes later, the sensation is gone, Stack says. That’s lactic acid: here for a brief flash of discomfort, and gone shortly after. And that’s a good thing: Can you imagine feeling that burn for hours or days longer?

Some elite endurance athletes put this physiology to use for their training, using something called the lactate threshold to gauge their workout exercise intensity, Stack says, where they’ll dial up the intensity in their workouts until reaching a certain concentration of lactic acid in their blood. They start with a simple finger prick, depositing a couple drops of blood on a lactate test strip. The strip is read by a lactate analyzer, a small meter that tells you the concentration of lactic acid in your blood. “This measurement is done very frequently with endurance athletes and metabolic athletes who are looking for a more precise way to prescribe training intensity other than heart rate,” he says. 

That’s a pretty scientific process, and, as you can see, has nothing to do with muscle soreness. In fact, judging any workout on post-exercise muscle soreness isn’t likely to tell you much of value. “You can have a very effective, progressive, and well-designed strength program and not feel sore afterward,” Chellsen says. “Consistency and progressive overload are what drive strength and fitness gains, not whether you ‘crushed yourself’ to the point of soreness.” In other words, you can absolutely gain without pain.

Does Lactic Acid In Muscles Cause Soreness?

No, lactic acid in muscles does not lead to soreness. But despite strides in research since that belief first came to be, the myth has stuck around for decades.

“For years, lactate got blamed because scientists noticed it was present when people felt that ‘burn’ during hard exercise,” Chellsen explains. It seemed logical that it might also be to blame for persistent muscle soreness after a workout.

But way back in the 1980s, this theory was debunked with experiments that showed runners experienced muscle soreness without showing any elevation in their levels of blood lactate and vice versa. 

That said, the myth that lactic acid buildup causes muscle soreness persists, likely because it’s “a simple, catchy explanation, and those tend to stick around in gyms, fitness classes, and pop culture long after the science has moved on,” Chellsen says. 

So What Makes Muscles Sore?

First, let’s clarify: There’s a big difference between that immediate burn you feel in your muscles smack-dab in the middle of a tough workout and the achy soreness that settles in a day or two afterward.

The immediate burning sensation is due to the hydrogen ions in lactic acid that temporarily enter muscles while you’re exercising intensely and producing energy anaerobically. 

But once that initial burn disappears, any other discomfort is due to tiny (and normal!) tears to your muscles caused by exercise. “It doesn’t come from lactate or metabolites; it comes from microscopic damage to muscle fibers, especially when you do new or challenging movements,” Chellsen says. “That damage sparks an inflammatory response and a bit of nerve sensitization, which is why you feel stiff or sore.”

Even though it’s uncomfortable, this inflammation is all part of the normal recovery process that allows your muscles to repair and grow stronger in the process. You do a tough workout and it creates those microscopic tears. Then, your body builds muscle to repair the tears in a process called muscle protein synthesis, making your muscles bigger and stronger over time. 

And that’s exactly what you want to happen, Stack points out. It means the next time you face that same stressor—like repeating that sprinting workout or trying to set a new personal best in the back squat—you’ll be better equipped to handle it. “This is a totally normal consequence of any type of anaerobic training,” he says.

Sometimes that discomfort doesn’t kick in until a whole day or two after you’ve worked out, and that’s totally normal too. That experience is called delayed onset muscle soreness or DOMS, Chellsen adds. DOMS is a specific type of post-workout muscle soreness that’s more common if you’re getting back into exercise after a long break or trying something new for the first time. It can show up anywhere from 12–24 hours after exercise (and stick around even longer), and it has nothing to do with lactic acid.

Remember, a little muscle soreness is totally normal and usually nothing to worry about. But pain, bruising, swelling, or any difficulty moving naturally after exercise are warning signs you’ve overdone it. It’s worth talking to a healthcare provider about any lingering discomfort you think could be more than muscle soreness, especially before working out again, in case you’re actually injured.

The Takeaway

Lactic acid, or lactate, is a compound made by your body when it breaks down sugar stored in your muscles for energy during intense exercise that can’t be fueled by oxygen alone. If you keep up this kind of high-intensity exercise long enough, lactic acid may build up in your muscles, causing a temporary burning sensation.

But lactic acid is cleared relatively quickly and doesn’t contribute to post-workout muscle soreness. Instead, that type of longer-lasting discomfort is due to tiny tears in the muscles created by the exercise, nerve sensitization, and the inflammatory process that helps repair these tears. This microscopic damage is normal and expected; as your body repairs the damage, your muscles grow bigger and stronger so you continue to make progress toward your fitness goals.