Lactic Acid: The False Science Behind the Myth
Warning: do not read if you are sympathetic to amphibians or fond of the color green:
Early research studies in the 1920's showed that lactic acid accumulated when severed frog legs were forced to twitch to exhaustion using electrical stimulation. The scientists involved assumed that the lactic acid build up caused muscle failure.
Researchers in the 1970's believed that lactic acid buildup caused acidosis (acid build up in the muscles) which resulted in muscle fatigue and caused soreness. This myth is still prevalent today.
Now we know these beliefs to be false. In a living system lactic acid is formed when glucose is burned anaerobically for fuel. The body uses lactic acid as a superfuel to help bridge the gap between anaerobic and aerobic respiration. Muscles can use lactic acid directly for energy. Any unused lactic acid is shuttled to other areas such as the heart or liver to be used as fuel (oxidized) or converted into glucose. In a severed frogs leg, lactic acid built up because it had nowhere to go. In a dynamic living system, muscles, livers, heart or brain can use lactic acid.
Not only doesn't lactic acid cause muscle soreness, but in a surprising twist lactic acid actually prevents muscle fatigue!
Four reasons lactic acid prevents muscle fatigue:
- Lactic acid serves as a powerful fuel for muscles reducing fatigue (Bergman et al. 2000, Brooks 2002, Gladden 2008, Cruz et al. 2012) .
- Lactic acid actually prevents acidosis in the muscle. Acidosis occurs when protons (hydrogen ions) build up in the muscle tissues reducing pH. This can cause muscle fatigue. The formation of lactic acid consumes two free protons, which reduces muscle acidity.
- Lactic acid prevents depolarization of muscle cells. Muscle cells contract when potassium (K+) inside the cell switches places with sodium outside the cell. Muscle cell contractions are strongest when there is a large difference in polarization or charge between the inside and outside of the cell. The inside of the cell needs to have a greater positive charge than the outside of the cell. After the muscle contracts, the cell reabsorbs the potassium to reuse. High intensity exercise results in potassium being released from the cell faster than it can be reabsorbed. This causes the cell to have weaker contractions and eventually results in muscle fatigue. Lactic acid can partially restore muscle cell function in a depolarized state; thus staving off fatigue. Lactic acid does this by increasing muscle excitability and improving its ability to contract (Nielsen et al. 2001, de Paoli et al. 2010).
- Lactic acid may act as a cell signaling molecule to promote beneficial fitness adaptations during intense exercise (Hashimoto and Brooks 2010). This can help your body become fitter faster and reduce fatigue.
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- Connor H, Woods HF. Quantitative aspects of L(+)-lactate metabolism in human beings. In: Porter R, Lawrenson G, editors. Metabolic Acidosis (Ciba Foundation Symposium 87). London (UK): Pitman Books Ltd. 1982;214–34. Scribd.
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- Nielsen OB, de Paoli F, Overgaard K. Protective effects of lactic acid on force production in rat skeletal muscle. J Physiol. 2001;536:161–166. Pubmed. Full text.