Lactic Acid

Lactic acid is a byproduct of glucose and glycogen metabolism during intense anaerobic muscle activity. A small accumulation causes muscular fatigue and pain, and retards contraction. Lactic acid is carried by blood to the liver, where it is reconverted to glucose and returned as blood glucose to the muscles. It is this elevation of blood lactic acid in sustained strenuous exercise, such as in marathon running, which results in muscle fatigue and pain. Recovery is enabled when sufficient oxygen returns to the muscle.