Exploration
Ultimate limit of human endurance has finally been found
US scientists have redefined the limits of human endurance in a groundbreaking new study.
Is there a point of maximum exertion during endurance? It’s a question that has seen athletes push themselves to the physical edge in search of the answer. From Ross Edgley’s 157-day swim around the British Isles to Jim Walmsley’s recent 50-mile world record, humans have been completing superhuman feats since the dawn of time.
Jim Walmsley recently ran 50 miles in 4 hours, 50 minutes and 8 seconds
© HOKA
But what is the absolute limit of human endurance? Scientists have conducted multiple studies in an attempt to accurately calculate the point at which the body uses up its energy reserves during multiple-day endurance challenges.
And that definitive answer was never found… until this month, that is, after a team of scientists from Duke University revealed the results of their study into just how far you can push your body. And their findings reveal that there is a limit to human endurance, and it’s due to the fact that humans can only burn calories up to a maximum of 2.5 times their resting metabolic rate when doing prolonged exercise over a number of weeks.
It’s all about your resting metabolic rate
The resting metabolic rate (RMR) is the rate at which your body burns energy (calories) when it’s resting. Put even more simply, it’s the minimum energy your body requires to do the simple tasks to keep you alive, such as breathing, digesting food and pumping blood around your body.
Kiko Matthews overcame brain cancer to row the Atlantic solo in 49 days
© Kiko Matthews
There are online calculators that can estimate your RMR online, such as this one, but when comparing yours to others beware that every person’s is different and is dependent on many factors, in particular your height and weight. As a rough guide, a man’s average is around 1,600, and a woman’s is approximately 1,400.
The scientists at Duke University calculated this new maximum-endurance figure – published in Science Advances – by using athletes running across the US as guinea pigs. They collected data from competitors in the Race Across The USA, which comprises running 3,080 miles from Los Angeles to Washington DC in 140 days (a marathon a day for six days a week for 20 weeks).
They then recorded how many calories the athletes burned every day and plotted that data over time, comparing it to data from other endurance events. In doing so, they discovered that whatever challenge was being undertaken – be it Geraint Thomas’s historic Tour de France victory or Jasmin Paris smashing the 268-mile Montane Spine Race record – energy expenditure levelled off after about 20 days at 2.5 times an athlete’s RMR (which equates to roughly 4,000 calories for an average person). That’s the point at which the body begins to burn calories more quickly than it can absorb food, so it will begin raiding its own fat stores instead, resulting in a dip in performance.
If you’re doing intense exercise for a shorter amount of time, such as a marathon, you can use up to 20 times more than your RMR. This figure then begins to fall as the distance and time increases, so on a 25-hour ultramarathon it would be up to nine times, and during the 23-day Tour de France it’s around five, eventually flattening out at 2.5. Once this point is reached, no one can surpass it, whether you are a pro endurance runner or Tony from accounts.
New horizons
Ross Edgley swam for six hours, twice a day, on the Great British Swim
© Harvey Gibson/Red Bull Content Pool
“This defines the realm of what’s possible for humans,” said Dr Herman Pontzer, who worked on the research. The researchers are still not sure exactly why this is, but one theory is that it’s related to your digestion, and when your body reaches the maximum point at which you can break down food and absorb the calories you need to keep you going. “There’s just a limit to how many calories our guts can effectively absorb per day,” said Dr Pontzer.
An interesting outcome of the experiment also revealed that pregnant women have a similar maximum sustainable energy expenditure to endurance athletes – with their energy peaking at 2.2 times their resting metabolic rate (or 4,000 calories a day for the average person). “To think about pregnancy in the same terms that we think about Tour de France cyclists and triathletes makes you realise how incredibly demanding pregnancy is on the body,” said Dr Pontzer.
Despite the results, Dr Pontzer hopes that an athlete will challenge the outcome of his study. “Science works when you’re proven wrong. Maybe someone will break through that ceiling some day and show us what we’re missing."
