Well, there goes about five years of reading on exercise physiology back in the 1980's. From the NY Times:
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.
The notion that lactic acid was the devil was gospel back when I was in college. Seeing this is like reading that water is no longer wet.
Here's the full article link:
NY Times--reg required.