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. Source: PTI
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