It has been quipped that revolutions in science occur, not when a new idea wins over converts from the existing ranks, but when the old scientists die off and take their antiquated notions with them.
Lactic acid has been, for a century, the agent responsible for muscle fatigue. You exercise. Your body produces lactic acid which makes your muscles tired. You stop exercising. The lactic acid drains away and your are good to go again.
Apparently, according to research done by George A Brooks, of the department of integrative biology at the University of California, Berkeley (as reported in this New York Times article, this understanding is completely wrong. Lactic acid is fuel. Used by the muscle to boost performance.
Brooks' ideas were rejected, challenged, ridiculed, left unfunded. He soldiered on, and now it sounds like the evidence has become solid enough to challenge the scientific orthodoxy. Still it may take a while longer for sufficient scientists to retire or expire, and for the next version of pop sci and textbooks to get released, before "lactic acid = good" becomes the new collective wisdom.