The reason lactate does not exist as it's protonated acid form in the body is because the pKa of lactate is a low 3.9. This means is that in a solution with a pH higher than 3.9, the compound will exist as the deprotonated carboxylate, lactate. As shown below, in Table I from Cairns (2006), the lowest intramuscular pH values during exercise have been measured between 6.3 and 6.5 (also note the duration and type of exercises that produced these low pH values). Therefore, the majority of lactate/lactic acid found in the muscle and blood exists as the non-protonated version (lactate). Conversely, if intramuscular pH, or plasma pH were to drop under 3.9, lactate would accept a proton to become lactic acid – but as Table I shows, drops in pH of this magnitude do not occur during exercise.