How lactic acid really works: The science of fatigue and lactate

science of lactic acidThere’s a lot of fuss in the running world about lactate. Or, depending on who you ask, lactic acid.

It’s been blamed for fatigue, soreness, overtraining, and probably more, but until recently, the phenomenon of lactate accumulation during intense exercise was poorly understood.

This week’s article will dispel some of the myths about lactate, lactic acid, and how they relate to exercise and fatigue.

How lactic acid got a bad reputation

The association of lactic acid and its negatively-charged ionic form, lactate, with fatigue during exercise has a long history.

During the course of a prolonged and intense effort, muscles lose power. The growing fatigue with exercise can be resisted for a while through great concentration and mental effort, but eventually everyone succumbs to fatigue.

Early physiologists studied the origins of muscular fatigue using electric impulses sent to muscles from dissected frogs. Even these dismembered muscles fatigue after a while, proving that there is a chemical component to fatigue. When these muscle fibers are analyzed, they show a high concentration of lactate and acid (hydrogen) ions. Therefore, physiologists concluded, the reason for muscular fatigue during exercise is accumulation of a compound called lactic acid.

This theory remained more or less unchallenged for much of the twentieth century. It was only after the body’s energy supply systems were subjected to rigorous biochemical accounting that some discrepancies turned up. For one thing, the body doesn’t actually produce lactic acid, just the negatively-charged ion, lactate. “Acid” (hydrogen ions) is indeed produced, but not from the exact same biochemical step.

Furthermore, the ratio of lactate to hydrogen ions produced during exercise isn’t 1:1, as you would expect if lactic acid was being produced. These ambiguities led to a reexamination and eventual overhaul of the “lactate paradigm” in the early 2000s, spearheaded by Roger Robergs.

The real science behind lactate

Robergs, an accomplished biochemist, took a hard look at each step in the metabolic process that turns sugars (glucose in the blood and glycogen in the muscles) into energy when you exercise.

Most runners have heard the following story about energy pathways: Aerobic respiration turns sugars into fuel using oxygen, and doesn’t have any harmful byproducts. Anaerobic respiration, which doesn’t kick in until you’re operating past your aerobic limit, can generate energy from sugar without using oxygen, but results in waste products—lactate and acid.

Robergs and others showed that this common understanding has some flaws. It turns out that anaerobic respiration functions all the time, turning sugar into a compound called pyruvate, releasing some hydrogen ions at the same time. Aerobic respiration works to clean up the pyruvate, using oxygen to burn the pyruvate into carbon dioxide and water, which can be exhaled. The aerobic process also consumes acid (hydrogen ions), which retards the buildup of acid in the muscles.

The generation of lactate is actually a side reaction: when excess pyruvate and acid start to accumulate (when the rate of anaerobic respiration overtakes the aerobic system’s ability to remove the waste), the body uses a pyruvate molecule and a hydrogen ion to create lactate, another way in which it can slow down the buildup of acid. The lactate can also be shuttled out of the muscles, into the blood, and burned in other areas of the body for more energy.

Practical implications of our new understanding of fatigue

All of this biochemistry is jolly interesting to a physiologist, but are there any practical applications of all this? We can take a few lessons from this right off the bat:

  • a better understanding of the biology of fatigue only reinforces the concept that your aerobic strength is a huge factor in your performance. While your body has various mechanisms to buffer the acid produced during high-intensity efforts, all of these are limited. Only increasing your aerobic fitness will allow you to substantially increase how far and how fast you can run.
  • Additionally, recognizing that lactate has a greater role than simply causing fatigue allows you to better understand the place of high-intensity workouts at or faster than the “lactate threshold.” These workouts aren’t just running hard for the sake of running hard—they train your body to produce, process, and burn lactate (as a fuel!) at a greater rate. This can improve your stamina over short and medium-duration races like the 5k and 10k.
  • Finally, there is still the inescapable fatigue that comes with acid overload. There really is no getting around this in shorter races. You can run hard interval workouts and races to improve your ability to buffer the acid produced when running at very fast speeds, but everyone is ultimately limited by the acidity in their muscles and blood.

So, is there such a thing as “lactic acid production” during exercise? Not really. Your body certainly produces acid during exercise, and it produces lactate as well. But it’s the former, not the latter, that’s the main culprit for fatigue.

Regardless, it will likely still be a long time before we stop hearing about lactic acid buildup and so on. The fuss about terminology might be overblown, but understanding the real mechanisms at work when we run hard and get tired can help understand the purpose and importance of the various workouts you use in training.


1. Robergs, R. A., Biochemistry of exercise-induced metabolic acidosis. AJP: Regulatory, Integrative and Comparative Physiology 2004, 287 (3), R502-R516.
2. Brooks, G., The lactate shuttle during exercise and recovery. Medicine & Science in Sports & Exercise 1986, 18 (3), 360-368.
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3 Responses on “How lactic acid really works: The science of fatigue and lactate

    • The answer to that question is not a simple yes or no. Lactate can be used as an energy source and is reconverted back into energy through the Cori cycle. However, if there is too much lactate produced the resulting excess hydrogen ion will impair muscle performance and slow you down.

  1. The terminology used for aerobic- anaerobic respiration isn’t as sharp as we would like to see. Most of the workouts, for instance a 1500m. runner, can be based on a large amount of high quality aerobic training. Once you got that level, you would need less ‘anaerobic fatique’ training and a few races to be on your PB-level.

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