When it comes to exercise and lactic acid, there are a lot of misconceptions. For example, many people blame lactic acid for delayed onset muscle soreness (DOMS)—the soreness you feel a couple of days after a tough workout. But that’s not entirely the case.
Likewise, contrary to a more recent misconception, lactic acid itself isn’t the direct cause of the burning sensation you get in your muscles during a hard run—like when you’re striding up a steep hill or tackling a sprint interval. While lactic acid does play a role in that fiery effect, scientists are still working out the exact cause and you can’t quite place all the blame on lactic acid, specifically.
In truth, as lactic acid breaks down during high-intensity exercise, a build-up of the substrates—lactate and hydrogen ions—appear to play a role in increased burning sensations in your muscles. But that’s the catch: it’s not the lactic acid but its byproducts.
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So, if lactic acid build up isn’t at fault for burning, aching muscles (during or following a workout), you may be left wondering why people seem to hate on it. That’s an even better question when you realize the substance plays an important—and possibly protective—role in energy metabolism during exercise.
Here, we help you get a better understanding of lactic acid build up (or more specifically, lactate build up), including how it occurs and what it does, and does not, tell us about our workouts.
Understanding lactic acid build up—and why we should pay more attention to lactate
First and foremost, it’s important to note that the terms “lactic acid” and “lactate” are often used interchangeably, but they’re not the same thing.
“Lactic acid is a molecule that has a hydrogen ion that can be donated. Lactate is the molecule created after that ‘extra’ hydrogen ion is cut from the molecule,” explains Tracy Zaslow, M.D., primary care sports medicine specialist at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles and a team physician for Angel City Football Club and LA Galaxy. “Lactate is an actual fuel source for the body, provided to working muscles during exercise.”
Lactic acid gets produced in the body during glycolysis, Zaslow explains, which is the anaerobic process that produces energy during high-intensity exercise. Then, when it dissolves in water (which is what occurs when it’s produced in the human body) the ions split into a lactate ion and a hydrogen ion. “Therefore, it’s more accurate to say that lactate—not lactic acid—is the byproduct of exercise,” Zaslow says.
So from even with the most basic understanding of “lactic acid build up,” it’s lactate (along with hydrogen ions), not lactic acid that should be considered the possible culprits of the “muscle burning” sensation you may experience during exercise.
“Lactic acid is the general term used to describe the lactate and hydrogen byproducts that result from [glycolysis],” says Debbie Dy, P.T., D.P.T., orthopedic clinical specialist and director of education at Fusion Wellness and Physical Therapy in Los Angeles. In other words, lactic acid is a catch-all descriptor—but it’s not totally accurate.
Even so, it’s important to understand when and why this lactate build up occurs, and how it’s actually a protective mechanism rather than the cause of your pain and fatigue.
The role lactic acid and lactate play in aerobic and anaerobic energy systems
You see, your body needs energy in the form of ATP to fuel movement and support you as you run, jump, pull, push, change direction, and otherwise engage in exercise. This fuel is produced two ways—aerobically through the Krebs cycle, and anaerobically through glycolysis.
Without getting excessively scientific, the difference between the aerobic and anaerobic energy systems boils down to the presence of oxygen. During lower-intensity exercise, say a walk or slow jog, you’re able to breathe at a steady pace and keep up with your body’s demand for oxygen. Oxygen provides the fuel source for aerobic metabolism (hence the term “aerobic exercise”). It’s the preferred method to create energy for exercise and daily life as it can create a lot of energy. The issue is it’s a somewhat slow process.
When oxygen availability is low (like when you’re sprinting a 200-meter dash), anaerobic metabolism takes over. This system—again, known as glycolysis—doesn’t require oxygen. Instead, it uses alternate pathways to break down glucose into pyruvate, which creates energy in the absence of oxygen.
Glycolysis is 100 times more efficient than the aerobic pathways of producing energy, but what it offers in speed it lacks in production. In other words, it can produce energy fast, but not much of it. So when you’re in need of a lot of energy, but your respiration rate can’t deliver the oxygen you need, glycolysis can keep the energy flowing in a pinch—for a short period of time.
Here’s what’s important to understand: this isn’t an either/or situation. You’re almost never fully using aerobic or anaerobic metabolism. Your body uses both systems simultaneously, and adjusts the amount each system is used based on how strenuously you’re working and whether you’re able to consume enough oxygen to keep up with energy demands in a manner that’s sustainable. In other words, as your intensity increases, you start using glycolysis to a greater extent because your oxygen intake can’t keep up with the demand, and as your intensity decreases, the reverse is true.
So what does this have to do with lactic acid build up or lactate? Well, it’s during higher-intensity exercise, when your body is using glycolysis more extensively to fuel movement, that the lack of available oxygen to help clear hydrogen and lactate from the cells can lead to lactate and hydrogen build up. As these byproducts build, muscle acidity increases, leading to the familiar burning sensation.
What else you need to know about lactate build up during intense exercise
Lactate, specifically, is the end product of glycolysis. Glycolysis breaks down glucose through a series of chain-like reactions to produce ATP and pyruvate. As glucose gets broken down to help produce energy, lactate and other byproducts known as metabolites (like hydrogen) start building. When oxygen is available (during lower-intensity efforts), these metabolites are “buffered” out of the cells and put to use elsewhere—essentially, they’re recycled. But when oxygen isn’t available (during higher-intensity efforts), lactate and other metabolites start building and muscle acidity increases.
“The muscles are constantly clearing lactate as it’s made, but the muscles may not be able to clear lactate fast enough, causing lactate to build up. The point at which lactate accumulation exceeds its breakdown is called the ‘lactate threshold,’” says Zaslow. “The timing at which a person reaches the lactate threshold depends on fitness level and workout intensity. It can occur with high-intensity aerobic activities, as well as with strength training.”
This is the “burning muscle” sensation you start experiencing during intense, all-out exercise. And if it’s a sensation you’re familiar with, you know you can’t keep exercising for very long at the same level of intensity once the burning starts to build.
Zaslow also points out that this burning isn’t due to lactate, specifically, but the increased acidity in the blood.
How lactate offers a protective effect for performance
As easy as it would be to blame lactic acid build up, or lactate build up, for your pain and fatigue during intense exercise (you couldn’t keep racing as fast or as long as you wanted, after all), it’s actually more accurate to consider lactate buildup as a marker of fatigue, not a cause of it.
Lactate is produced continually, not just when you’re sprinting up a hill. The cause of the burning sensation, then, isn’t the presence of lactate itself, but your body’s inability to deliver adequate oxygen to your cells to continue clearing lactate to support aerobic and anaerobic metabolism simultaneously. As a result, lactate (and hydrogen ions and other metabolite byproducts) build and your muscles’ acidity levels increase.
When the resulting burn becomes too great, you’re forced to slow down or stop. When you slow down and stop, your oxygen consumption is slowly able to “catch up” to its previous deficit, and as oxygen is delivered to your cells, aerobic metabolism picks back up again. Lactate and other metabolites are then buffered away and put back to use in aerobic metabolism, and as the buildup is reduced, the burning in your muscles also eases and you can gradually increase your intensity levels again.
Ultimately, the build up of lactate helps “keep you honest,” so to speak, during exercise. It prevents you from pushing too hard for too long in a way that could cause more significant damage. But it’s not a condition that lasts forever, nor is it a condition that causes fatigue or damage. This is evidenced by the fact that the muscle burn you experience during intense exercise is actually fairly quick to ease up when you decrease your exercise intensity.
In addition to the reality that lactate build up isn’t the cause of poor exercise performance, Dy is quick to say it may actually be a good thing. In fact, she points to the reality that many studies indicate that the presence of lactate and hydrogen in the muscles may actually support and improve exercise performance, increasing or stimulating oxygen release to further support working muscles.
The role lactate can play in your health
The research on lactate and lactate build up is still very much active and developing. The understanding of how it works has changed significantly in the last 20 years and is likely to continue changing as scientists dive deeper into muscle physiology, exercise, and recovery. But one thing to note is that some of the more recent science indicates that lactate is good for your health during exercise and non-exercise conditions.
For example, a 2014 study published in Bioscience Horizons found that the lactate shuttle (the mechanism by which lactate leaves the cells with the presence of oxygen) helps maintain energy production during exercise, improves oxidative capacity, and enhances cognitive performance during prolonged aerobic exercise.
This is important because periods of low blood sugar availability, known as hypoglycemia, can occur during long, extended exercise (like a long run or bike ride). And when hypoglycemia takes place, cognition is affected, which can result in confusion and impaired cognitive performance, not unlike hypoglycemic episodes seen in diabetes.
There’s also recent evidence that high levels of lactate play a role in exercise-induced appetite suppression—in other words, exercising at high-intensity levels that cause significant build up of lactate may actually reduce your levels of hunger postexercise.
How lactate can inform your training
Your body already knows how to deal with lactate build up, and the burning you develop in your muscles during intense exercise will dictate when you need to reduce your exercise intensity. That said, there’s some evidence that using the lactate threshold during training can help you increase your fitness level and performance over time.
The clear example is in high-intensity interval training (HIIT). During HIIT, you alternate between higher and lower levels of intensity. Typically, the higher levels of intensity take you to your lactate threshold, and you can only maintain the intensity for a short period of time. “By training at a high intensity (lactate threshold training), the body creates additional proteins that help absorb and convert lactic acid to energy, ultimately making the body more efficient,” explains Zaslow.
Also, during HIIT, the short, recovery periods give your body time for oxygen supply to catch up with demand, allowing you to return to another high-intensity round. Over time, this type of training can help you improve your cardiovascular fitness and efficiency, making it possible to provide sufficient oxygen at higher-intensity levels for longer periods of time.
This type of training improves your fitness in a way that requires longer, more intense effort to reach that lactate threshold. “With appropriate and gradual progression of training, you’ll notice you can exercise for longer periods prior to fatigue,” says Dy.
The lactate threshold is there for a reason—it’s protective. When your muscles really start to protest, slowing down and giving them a break isn’t a sign of weakness, but an acknowledgment of your current limits. Working safely within your limits while still pushing yourself can help you improve your fitness over time while preventing the likelihood of injury or other detrimental exercise outcomes, like overtraining.
Ultimately, Dy points out that research is evolving, and while the muscle burning so often associated with lactate and other metabolites isn’t pleasant, there’s good news: “Lactic acid build up is not something to be feared.” Instead, appreciate what it’s telling you and use it wisely to help you gradually increase your fitness and reach your long-term goals.
Laura Williams, M.S., ACSM EP-C holds a master's degree in exercise and sport science and is a certified exercise physiologist through the American College of Sports Medicine. She also holds sports nutritionist, youth fitness, sports conditioning, and behavioral change specialist certifications through the American Council on Exercise. She has been writing on health, fitness, and wellness for 12 years, with bylines appearing online and in print for Men's Health, Healthline, Verywell Fit, The Healthy, Giddy, Thrillist, Men's Journal, Reader's Digest, and Runner's World. After losing her first husband to cancer in 2018, she moved to Costa Rica to use surfing, beach running, and horseback riding as part of her healing process. There, she met her current husband, had her son, and now splits time between Texas and Costa Rica.
