For the better part of a century, athletes and physiologists alike have considered lactic acid a primary cause of fatigue during high-intensity exercise and referred to it as “waste product” of muscle metabolism.

But now this way of thinking has changed, as scientists have learned that this substance we produce in large quantities during exercise, especially highly intense exercise, is not a cause of fatigue and actually helps to prevent it.


The former misrepresentation started with British physiologist and Nobel laureate Archibald V. Hill, who in 1929 flexed frog muscles to fatigue in his lab and noted that lactic acid accumulated when muscular failure occurred. He concluded that the lactic acid caused the fatigue associated with repeated muscle contraction. What he didn’t know is that when the muscle is examined as part of a complete biological system instead of in isolation from the rest of the body, we can see that lactic acid is processed and converted to fuel to help keep the muscles going. It does not cause fatigue.


Nor does lactic acid cause muscle soreness the day after hard exercise. This myth has been around for decades and refuses to go away, despite evidence to the contrary over the past 30 years. Soreness is more likely the result of damaged muscle cells resulting from excessive usage. So if lactic acid is not the villain we’ve made it out to be, what does cause fatigue and the burning sensation in the muscles during short, intense exercise bouts, such as CrosssFit workouts? To get the answer, it’s necessary to understand the pH scale, which tells us how acidic or alkaline (base) the body’s fluids are in range of 1 to 14, as hydrogen ions increase or decrease.

On this scale, hydrogen readings dropping below 7 indicate increasing acidity, while those rising above 7 indicate escalating alkalinity. Examples of acidic fluids are hydrochloric acid (pH = 1) and vinegar (pH = 3), while milk of magnesia (pH = 10.5) and ammonia (pH = 11.7) are alkaline. At rest, the pH of your blood is around 7.4 – slightly alkaline. In terms of your blood, small absolute changes in acid-base balance have major consequences. For example, during a 2- to 3-minute all-out effort, your blood’s pH may drop as low as 6.8 or 7.0. In biochemical terms, this is a huge acidic swing, producing a burning sensation in the working muscles and an inability for them to continue contracting.


Fatigue has set in.


If lactic acid didn’t cause the drop in pH, what did? The answer has to do with our sources of fuel during such short exercise bouts – glycogen and glucose. Both are carbohydrates, but they have slightly different chemical compositions. Glycogen is stored inside the muscle, where it can be quickly broken down to produce energy. Glucose, a form of this carbohydrate-based fuel that is stored in the liver and floats around in the bloodstream, is called on to produce energy for exercise when muscle glycogen stores can no longer keep up with the demand or are running low. As glycogen is broken down to produce energy, it releases one unit of hydrogen. But if glucose must be used for fuel, such as when the intensity of the exercise exceeds glycogen’s ability to keep up, two units of hydrogen are released. This rapid doubling of hydrogen ions in the system lowers the blood’s pH, causing the burning and fatigue associated with acidosis. The same amount of lactic acid is released no matter which fuel is used.

Far from being an evildoer, lactic acid is an ally during intense exercise. It does a great deal to keep the body going when the going gets hard. Besides being converted back into a fuel source, when hydrogen begins to accumulate, lactate transports it out of the working muscle cells and helps to buffer or offset its negative consequences.


After 80 years, lactic acid’s bad boy reputation has been lifted.


Biochemistry of exercise induced metabolic acidosis. America journal of Physiology – Regulatory, Integrative and Comparative Physiology 2004. 
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