So what the cell actually does is it undergoes this lactic acid fermentation process in which a special enzyme known as lactate dehydrogenase transforms our Pyrulbate. It reduces the Pyrulbate into lactic acid as well as forming our oxidized version of NADH, our NAD plus. And this NAD plus can then be placed into the glycolysis cycle to produce our continual source of ATP. Now, lactic acid can basically break down into its conjugate base, lactate and this releases H plus ions into the cell and that means it decreases the PH of the cell by increasing the concentration of H plus ions. So the build up of lactic acid in the cell decreases its PH and it's the decrease in PH that basically causes the feeling of fatigue following our vigorous exercise. Now, the lactic acid must eventually be transformed back into Pyruvate and this takes place in the liver.