So what, actually, is lactic acid? For one, it is not a waste product, and it does not make you sore. Soreness is caused by tiny breaks in your muscles. The truth is, lactic acid is a pure, condensed form of fuel - a kind of power bar -- that your body isn't able to process quickly enough to use for the task at hand. As soon as your body catches a break, so to speak, it goes to work feeding this high-octane fuel to your organs and muscles, which gain from its rich ingredients.