Feel the burn, bro? Have you ever pushed through a grueling workout only to feel that intense burning in your muscles? No pain, no gain, am I right? Kinda, but there is more nuance (more on that below). For years, many believed this discomfort came from lactic acid buildup. This idea was that the muscles got fatigued and 'ran out of oxygen' and lactic acid was produced which increased the PH in the muscle. This acidic environment caused the muscle to 'burn'. It seemed logical at first, but recent studies challenge this traditional view. Let’s dive into the realities of lactic acid and uncover what actually happens in our muscles when we push ourselves.
Lactic Acid vs. Lactate
What is the difference between lactic acid and lactate?
Lactate is the ionized form of lactic acid. During intense exercise, lactic acid might be produced momentarily, but the main substance that accumulates in your muscles is lactate. This distinction is important because lactate is the deprotonated (lost a proton) form of lactic acid, and understanding this difference clarifies why lactate itself isn’t the direct cause of the well-known “burn.” Although the terms are often used interchangeably, they refer to different chemical forms. Interestingly, lactate levels can rise considerably during intense workouts; for instance, they can increase up to 20 times in trained athletes compared to their resting state. Despite this increase, lactate itself does not directly lead to muscle fatigue.
Debunking the Myth of Lactic Acid
Many gym-goers, including myself, have heard numerous trainers claim that lactic acid is the main reason for muscle fatigue. However, studies, including one from the Journal of Applied Physiology, show that lactate production alone doesn’t hinder performance. In reality, other factors, such as muscle acidity, contribute to the burning sensation we experience during strenuous exercise. Researchers noticed that athletes who have high levels of lactate could still produce a lot of power. Also, lactate seemed to appear even when oxygen was available, and in some cases, lactate was used as an energy source by other tissues (like the heart and brain). This flew in the face of the old narrative that lactate was just a toxic waste product.
Let's Talk about Hydrogen Ions
If lactate isn’t the primary culprit, then what causes the burning sensation? According to a study published in the American Journal of Physiology, the answer lies in hydrogen ions. During intense exercise, as lactate forms, so do hydrogen ions. This simultaneous increase leads to a drop in muscle pH, making the environment more acidic.
As a result, it’s this acidity, rather than the lactate itself, that creates that sharp burning feeling. The rise of hydrogen ions can lead to fatigue; for example, studies suggest that when pH levels dip below 6.8, overall muscle function is impaired.
Your muscles contain sensory nerves that can detect changes in the chemical environment. As hydrogen ions build up, the fluid surrounding your muscle fibers and nerve endings becomes more acidic. This drop in pH triggers pain receptors, sending signals to your brain that say, “Hey, something’s going on here, and it’s not comfortable!” The result is the burning sensation you feel.
When this environment remains highly acidic, your muscles might also have a harder time forming the cross-bridges between actin and myosin filaments—the very mechanism that contracts your muscles. If these cross-bridges can’t form effectively, your muscle’s performance plummets and you feel fatigued.
In other words, fatigue sets in once your muscle cells struggle to perform the basic mechanical actions needed for movement. Although lactate’s presence is part of the high-intensity metabolic puzzle, the direct mechanical hindrance stems more from these acidic conditions (hydrogen ions) than from lactate itself.
Rethinking Muscle Fatigue
Understanding that lactate isn’t the main villain opens the door for new training and recovery strategies. If muscle fatigue is primarily caused by hydrogen ions, it prompts a re-evaluation of how we approach our workouts.
Athletes might benefit from techniques that buffer acidity in their muscles, such as considering bicarbonate supplements (baking soda, yes that stuff you make cookies with)....but be careful, there is a fast dose response curve and if you take too much you'll be running to the bathroom. This strategy has been shown to counteract the acids produced during intense activity and allows you to train longer. But seriously be careful!
NOTE: The old saying “no pain, no gain” does hold some truth—research suggests that aiming for a perceived effort level of at least 5 on a 1–10 scale can lead to meaningful fitness improvements. Striking the right balance ensures you’re challenging your muscles enough to promote growth and endurance without pushing yourself to the brink of injury.
Why Should I Care?
Knowing how your body actually responds during tough exercise can give you a mental boost. The burn often feels like a “stop sign,” but understanding it’s primarily about hydrogen ions—and not a toxic meltdown in your muscles—can give you more confidence to push through those final reps or those last seconds of an interval. It’s not that your muscles are “damaged” by lactic acid; instead, you’re facing a rapid change in pH that’s meant to protect you from completely overdoing it.
This knowledge doesn’t mean you should ignore the signals your body sends. Pain is still an important alert system that helps you avoid injury. But by recognizing the difference between “harmful” pain (like joint pain or sharp, stabbing sensations) and “productive” discomfort (like a deep muscle burn in your quads at the end of a sprint), you can make more informed training decisions.
What Do I Do With This Information?
Understanding that hydrogen ions (and not strictly “lactic acid”) cause the burn can be liberating. It reminds us that our body is a complex, adaptive system. We no longer have to demonize lactate; rather, we can acknowledge the real signs of muscle fatigue and work on strategies to push that threshold a bit further.
Train at the Right Intensities: If you’re doing interval training, the name of the game is short, intense work intervals separated by rest or lower-intensity intervals. This approach pushes your anaerobic energy systems just enough to stimulate adaptation without completely burying you in fatigue.
Pace Yourself: In workouts lasting more than a few minutes, pacing is crucial. Going out too hard might lead to a faster rise in hydrogen ions and an earlier fatigue point. Gradually ramping up can help manage that burn for a bit longer.
Include Aerobic Base Work: A stronger aerobic base improves your body’s ability to deliver oxygen, clear metabolic by-products, and even use lactate as fuel. Mix in lower-intensity, steady-state workouts with your intense sessions to maximize overall fitness.
Mind Your Nutrition: Proper fueling helps ensure that your body has adequate resources for ATP production. Carbohydrates, for instance, are critical for anaerobic glycolysis and lactate production (and subsequent recycling). If your carbohydrate stores are low, your ability to generate power in high-intensity work will decrease.
Optimize Recovery: If you regularly push your muscles to the limit, make sure your recovery strategy is on point. Get enough rest and good sleep. Do some light mobility work, get a massage, and integrate active recovery sessions - these techniques can help maintain good blood flow, ease muscle soreness, and support lactate clearance.
Use this knowledge to train better. If you love pushing your limits, realize that the burn is simply a signal that your muscles are hitting peak intensity—an indication that you’re stimulating the adaptations you came for. If you’re more of a moderate-intensity trainer, keep in mind that mixing in short, intense intervals can help improve your lactate threshold and overall metabolic efficiency.
Now, go out there, embrace the burn and keep training smarter. Your muscles—and your future PRs—will thank you for it.
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