"Everyone who has even thought about exercising has heard the warnings about lactic acid. It builds up in your muscles. It is what makes your muscles burn. Its buildup is what makes your muscles tire and give out... But that, it turns out, is all wrong. Lactic acid is actually a fuel, not a caustic waste product. Muscles make it deliberately, producing it from glucose, and they burn it to obtain energy. The reason trained athletes can perform so hard and so long is because their intense training causes their muscles to adapt so they more readily and efficiently absorb lactic acid." - Shannon Jiménez

I was tutoring a student in AP Biology today whose teacher is still perpetuating the "lactic acid makes your muscles sore" myth, which insprired me to look up this old article. If you can't trust your biology teacher, who can you trust? - Shannon Jiménez

It's the ability to metabolize lactic acid that is the culprit. Not all muscle uses it effectively and that's when you have lactic acid buildup. - FFing Enigma