For the process of transformation of glucose into pyruvate, NAD⁺ is converted into NADH. These same NADH molecules are then reutilised on the pyruvate to lactic acid reaction and are therefore transformed back into NAD⁺ molecules that will again be used for another glycolysis reaction.
From the first rectangle, on the left, to the last rectangle, on the right, it comes NAD⁺ - NADH - NADH - NAD⁺.
