That Burning Feeling Isn't Caused by Lactic Acid

If you've ever pushed yourself through a hard workout, you've probably "felt the burn." It's that familiar sensation of pain and exhaustion in the muscles, where they almost feel like they're on fire.

This feeling has long been said to be caused by a build-up of lactic acid in the muscles. In fact, this remains a common explanation from recreational exercisers to elite athletes .

But this belief is wrong - and lactic acid actually has nothing to do with the burn we feel when we exercise.

The origins of the lactic acid myth go back to the early 1900s. Scientists discovered that when they electrically stimulated the muscles of amphibians , lactic acid accumulated. This happened both when the muscles lacked oxygen and when they were stimulated until they fatigued.

Based on this, and the understanding of biochemistry at the time, it was concluded that when muscles work hard and they aren't getting enough oxygen, lactic acid accumulates and muscle fatigue develops.

However, this early work never explicitly said that lactic acid caused muscle fatigue. Rather, it said that muscle fatigue is accompanied by increases in lactic acid. Just because two things happen at the same time does not mean one thing causes the other. But the misapplication of cause-and-effect led to the birth of the lactic acid theory of muscle fatigue .

Lactic acid accumulation was thought to cause muscle pain and fatigue in two main ways: by impairing some of the processes involved in muscle contraction , and by reducing the muscles' ability to generate energy . Both effects were thought to be due to lactic acid making our muscles more acidic, which could also cause sensations of pain and burning.

But as our knowledge of biochemistry improved, by the end of the 20th century we discovered some things that cast doubt on the lactic acid theory.

First, the normal pH of our body tissues means that lactic acid actually cannot exist in our bodies ; nearly all of it is broken down into lactate and hydrogen. These chemicals are produced in our bodies all the time, but in much larger amounts when we do hard exercise.

Lactate is produced within our muscles through the process of glycolysis , which converts glucose (sugar) into energy. Hydrogen is also produced within our muscles when we're more active and exercise intensely.

Second, while lactic acid and lactate sound similar, there's a big difference. If lactic acid were to exist in our body, it would make our muscles more acidic - but lactate does the opposite.

Lactate production actually makes our muscles less acidic by consuming hydrogen. This is important because it's hydrogen, not lactate, that increases muscle acidity.

So in short, lactate doesn't increase acidity in our muscles when we exercise. It actually reduces acidity, which helps us exercise harder, for longer.

Lactate production also isn't proof that our muscles have run out of oxygen, as is sometimes believed. We produce and use lactate when oxygen is abundant - including at rest.

Lactate can also be used as fuel for our muscles, heart and brain - both when we're exercising and resting. New research is also shedding light on a much broader range of benefits that lactate brings, such as immune system regulation, reducing inflammation and wound healing.

But as can be expected with something as complex as the human body, lactate's role isn't always straightforward.

Lactate appears to play a small part in the complex process of muscle fatigue in combination with other substances that are produced by the body during hard exercise. But lactate alone has very little detrimental effect on exercise performance.

What's actually going on?

The question of what actually causes our muscles to burn during intense exercise is something that's still being studied. But it appears to come down to what triggers our nerves.

Specialised nerves in and around our muscles detect substances produced when we exercise. These nerves connect to areas of our brain responsible for sensations of muscle pain, as well as the emotional responses associated with these sensations. The harder we exercise, the more of these substances we produce and the faster the nerves fire.

So the burning sensation in our muscles during hard exercise is due to the increased stimulation of these nerves. The nerves likely respond to a combination of different substances the body produces during exercise, rather than any single one.

Muscles soreness in the hours and days after a workout is also not caused by "lactic acid" - nor is it caused by lactate . Lactate returns to resting levels within about 30-90 minutes after intensive exercise. That's long before the typical peak in muscle soreness that occurs one to three days following a hard or new workout. The causes of post-workout soreness are not fully understood, but likely relate to increased sensitivity of nerves to contraction, stretching, and compression of our muscles.

As persistent as the lactic acid myth is, it's just that: a myth. It's an example of how easily a single, misunderstood view can be passed on and become an unchallenged "fact".

So the next time you find your muscles burning during a workout, just remember: it's not lactic acid.

Shaun Phillips does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.