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For those of you minding your health, you have probably heard that lactic acid build-up in your muscles is what causes fatigue and soreness when you workout. Apparently, that’s not true. Dr. George Brooks, from UC Berkeley, researched lactic acid’s effects on muscle for his PhD dissertation in exercise physiology. He found that muscle actually burned lactic acid very quickly. He first published his research that runs counter to the myth in the 1970s. After publishing it, he found it very difficult to get other papers published and obtain grants for his research. It was not until recently that other researchers confirmed his work.

"The evidence has continued to mount," said L. Bruce Gladden, a professor of health and human performance at Auburn University. "It became clear that it is not so simple as to say, Lactic acid is a bad thing and it causes fatigue."

As for the idea that lactic acid causes muscle soreness, Dr. Gladden said, that never made sense.

"Lactic acid will be gone from your muscles within an hour of exercise," he said. "You get sore one to three days later. The time frame is not consistent, and the mechanisms have not been found."

The understanding now is that muscle cells convert glucose or glycogen to lactic acid. The lactic acid is taken up and used as a fuel by mitochondria, the energy factories in muscle cells.

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Through trial and error, coaches learned that athletic performance improved when athletes worked on endurance, running longer and longer distances, for example.

That, it turns out, increased the mass of their muscle mitochondria, letting them burn more lactic acid and allowing the muscles to work harder and longer.

So, coaches and athletes have been able to push the limits of athletic performance despite the belief in a now-debunked myth linking fatigue to lactic acid.