If conditions are anaerobic, pyruvic acid is converted into lactic acid. The purpose of this reaction is to convert NADH back into NAD+. This allows glycolysis to continue to produce ATP (and energy) in the absence of oxygen. This occurs under strenuous exercise conditions in the muscles. The vigorous exercise soon strips the cells of available ATP (energy). Oxygen cannot be resupplied to the cells at a fast enough rate, therefore, an oxygen debt or anaerobic conditions result. The reaction is shown on the left