Lactic Acid = Fuel

Lactic acid, the stuff that makes your muscles burn is a good thing. It is not a waste byproduct that makes your muscles sore and ache. It is a good thing that your muscles produce through glycolysis to generate more energy. It comes down to how your muscles utilize that energy to determine how you will perform. Lactic Acid Is Not Muscles' Foe, It's Fuel details the continued findings of George A. Brooks, Ph.D.

Lactic acid is produced from the glycolytic pathway through the action lactate dehydrogenase converting pyruvate to lactate under anaerobic conditions, which oxidizes NADH to NAD+ and H+. The generation of NAD+ allows glycolysis to continue, because it is a substrate for glyceraldehyde-3-phosphate dehydrogenase. For a review of glycolysis and lactate metabolism visit: THE Medical Biochemistry Page (It's a great site for all of Biochem)

What does that all mean? It means that through the lactic acid shunt, muscles can continue to burn glucose. More than that, lactate is burned in the mitochondria of your skeletal muscle cells to generate more ATP(ie. ENERGY!!). For more information read the following site: Muscles burn lactic acid as well as carbos

Further reading:
Lactate doesn't necessarily cause fatigue

Endurance training increased gluconeogenesis during rest and exercise in men

Endurance training has little effect on active muscle free fatty acid, lipoprotein cholesterol or triglyceride net balances