Heard a story on KCRW this morning. Apparently lactic acid might not be responsible for muscle fatigue after all (as my high school biology class taught). The connection was made because lactic acid was found in fatigued muscles, but recent research suggests that it may prevent muscle fatigue. It turns out that if lactic acid is removed from overworked muscle fibers, they actually stop working.
Of course, it is probably responsible for the burn you get while exerting.
(It’s kind of like when I learned that a fever was actually an immune system technique to fight infection, and not something the infection did itself.)