Phrases such as “their legs must be swimming in a sea of lactic acid!” or “you can see their stride shortening as the lactic acid builds up” are frequently used by commentators to describe athletes' visible exhaustion.
However, Loughborough University’s Dr Mark Burnley, an expert in endurance physiology, says the idea that lactic acid causes fatigue is a myth.
In a new video, he explains what lactic acid is, why it doesn’t cause fatigue, and how the misconception is linked to stags from the 1800s…
Watch the video on YouTube or read what he had to say below.
What is lactic acid?
“Every time there's a major game on or a sporting event where fatigue gets mentioned, the only explanation given is lactic acid. And it's quite a long way removed from the truth. Fatigue is extremely complicated”, said Dr Burnley.
“Lactic acid is a molecule which is found in all sorts of situations, both biological and chemical – it was first found in sour milk, which is where it gets its name from.
“Essentially, lactic acid is a partially broken-down carbohydrate, but in our bodies, we should refer to it as ‘lactate’ rather than ‘lactic acid’. From a physiological standpoint, under normal conditions lactic acid loses a hydrogen ion and becomes lactate.
"Lactate is relatively easy to measure in the body with a blood sample. Even at rest, blood lactate concentration is not zero, indicating continuous turnover in the body. It’s a very useful chemical that is used for lots of different processes.”
Does lactate cause fatigue?
“As far as we can tell, no, it doesn't”, said Dr Burnley, “Experiments have been done both in humans and in animals and they have shown pretty convincingly that fatigue is not caused by lactate or lactic acid.
“What fatigue is, is a reduction in muscle force generating capacity.
“Researchers have bathed rat muscle in a medium containing the amount of lactate that you would produce at the end of a 400m run and found it had little or no influence on muscle force generation.
“So, if you bathe muscle in lactate and force doesn't drop, that's pretty good evidence that lactic acid isn't causing fatigue.”
He continued: “Other experiments have bathed muscle in potassium, which we know causes fatigue. This reduced muscle force by about 75%.
“What they then did was add lactate to that medium, again the same amount that you would produce after a 400m run.
“The researchers found the addition of lactate didn't cause force generating capacity to go down any further.
“Actually, what happened was force went back up to where it was to start with. So, it completely reversed the effect of fatigue in that situation.
“In that experiment, lactate actually had a protective effect rather than a fatiguing effect.”
What chemicals cause fatigue?
“There are lots of other things that cause fatigue, and a lot of those things are intensity dependent”, said Dr Burnley.
“So, if you're doing a marathon, for example, the lactate concentration in your blood or muscle won't rise very much above the resting level.
“So, it can't possibly be that lactate causes fatigue. In that situation, fatigue is likely caused by running down muscle glycogen stores, the main source of carbohydrate energy in your muscles.
“If you do really, really high intensity exercise, like an 800m run or a mile run, then it is metabolite accumulation that's probably causing most of the muscle fatigue.”
He continued: “However, though complicated, if we had to attribute fatigue to one molecule, we should consider inorganic phosphate.
“When you're trying to produce energy in the muscle, you break down a thing called ATP and you need to make that ATP up again because there's not a lot of it in the muscle.
“So, to do that, we break down a thing called phosphocreatine into creatine and inorganic phosphate.
“When you're working really hard, you do that a lot and therefore you accumulate a lot of inorganic phosphate.
“We know that high concentrations of inorganic phosphate can result in a loss of muscle force. And that is the dictionary definition of fatigue.”
Where does the lactate-fatigue misconception come from?
“There are probably three reasons for the misconception that lactate or lactic acid causes fatigue”, said Dr Burnley.
“The first is an historical reason. Lactate was discovered in the late 1800s in the muscles of stags that had been hunted to exhaustion.
“They were found to have very high concentrations of lactate in the muscle. So that was the immediate link: lactate causes fatigue.
“The second reason is that other molecules – including inorganic phosphate – and the way they work in muscles weren’t studied for another 120 years.
“So, humans had a very long time to be talking about lactate before anything else came along that was more accurate in terms of what was happening in the muscle.”
Dr Burnley said: “The third reason is because there is a correlation between high intensity exercise and the accumulation of lactate because when you do high intensity exercise, you break down carbohydrates through a process called glycolysis, which produces lactate.
“So, if you're doing high intensity exercise, lactate will always accumulate. And it's not difficult to then just make that jump that: lactate has accumulated, exercise feels hard, therefore lactate must have caused exercise to feel hard and caused fatigue.
“That idea then entered physical education textbooks and coaching manuals. So, it's not surprising that this notion gets handed down, even to athletes and coaches.”
Take away message for commentators and those watching the Games in Paris?
Dr Burnley said: “My takeaway message for commentators and members of the public is if they ever hear the phrase “swimming in a sea of lactic acid”, just remember it's a bit more complicated and a lot more interesting than that.
“But also swimming in a sea of lactate is not a problem because we think it might be protective.
“Actually, lactate is one of the good guys. It's like a sheep in wolf's clothing, it's got all the bad press and it's not really deserved.”
