A sprinter's legs burn at the finish — lactate floods the muscles. In intense exercise, oxygen can't keep up — pyruvate converts to lactate (anaerobic glycolysis), regenerating NAD+ so glycolysis continues making 2 ATP per glucose. The burn is acidity, not damage; the liver later recycles lactate to glucose (Cori cycle). Trained athletes clear lactate faster — the difference between gasping and gliding is metabolic, and lactate threshold marks it.