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Lactic acid has been blamed for soreness, injury to muscles, cramps, oxygen debt, etc. Lactic acid really is not so bad. In fact, it may actually be nice in a way, as far as metabolites go. Lactic acid does not cause soreness. A 2004 study found that lactic acid not only does not cause soreness, it may actually counteract fatigue. The study found that the loss of potassium is more likely the cause of fatigue.

We have known for at least 15 years that lactic acid has nothing to do with delayed-onset muscle soreness, the kind you feel 24–72 hours after a hard workout session. That soreness is due to a mechanical tearing of myofibrils during eccentric or lengthening contractions. In fact, if during your workout you had someone let the weight down for you on every negative or eccentric contraction so that you do just the positive or concentric reps, you would never get sore. Although concentric contractions do not produce the damage that eccentric contractions do, concentric contractions produce more lactic acid.

Lactic acid is the breakdown product of glucose and glycogen produced during a process called glycolysis. The prefix “glyco” refers to the sugar glucose (which when strung together and stored in muscle makes glycogen); “lysis” means splitting or breakdown. In essence, lactic acid is a glucose molecule cut in half. Glycolysis can proceed so quickly (as when you weight train) that the formation of pyruvate far exceeds the capacity of the mitochondria to accept pyruvate into the Krebs cycle (which ultimately results in the aerobic formation of ATP, the primary form of cell energy). This excess pyruvate is converted to lactic acid. The terms lactic acid and lactate are often used interchangeably, even though they are not the same compound. The lactic acid formed through glycolysis quickly releases a hydrogen ion and forms lactate.

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