Lactic acid is not a dead-end, “bad” metabolic waste product, and it does not cause post-exercise soreness. This is a pernicious and seemingly un-killable myth. It originated with “one of the classic mistakes in the history of science,” according to George Brooks, a Berkley physiologist. See Gina Kolata’s clear overview in the New York Times, or a concise professional summary by Robergs in Experimental Phsyiology. For a deeper and geekier, but excellent read, see Dr. Goodwin’s entertaining rant about the prevalence of the lactate myth in the 2012 summer Olympics coverage.