Athletes almost always have to test their limit of endurance. Some competitions are designed to assess just this – from the Tour de France to the Ironman triathlon.
However, a study of how humans expend energy during long and grueling sports events suggests that endurance limit contests are unnecessary. Regardless of the activity, everyone has the same metabolic limit.
In other words, we all can sustain the same maximum possible level of exertion in the long term.
As far as long term physical activities go, the researchers noted that humans could only burn 2.5 times their resting metabolic rate. No one has ever surpassed this limit, not even the world’s fastest marathoners.
According to associate professor of evolutionary anthropology at Duke University and co-author of the study Herman Pontzer:
“This defines the realm of what’s possible for humans.”
Hypothetically, if a person does manage to go beyond the “realm of what’s possible,” the body breaks down its tissue to make up for the caloric deficit. Yes, it’s every bit as horrific as it sounds.
Why does the body impose this limit, you ask?
According to the researchers, it could be due to the digestive tract’s ability to break down food.
Our guts can effectively absorb a limited amount of calories per day. As such, your resting metabolic rate does not improve, even when you eat more food.
How Our Resting Metabolic Rate Affects the Limit of Endurance
For the study, the researchers measured the daily calories burned by a group of athletes. The participants ran six marathons every week for five months as part of the 2015 Race Across the USA.
Alongside the marathon data, the researchers also analyzed other feats of human endurance. These include pregnancy and punishing 100-mile trail races.
Upon plotting the data on a graph, an L-shaped curve became apparent.
Although the athletes started with high energy expenditure, it inevitably plunges. Eventually, the energy expenditure hits rock-bottom at 2.5 times their basal metabolic rate and it maintains this rate until the event ends.
During the first and final legs of the Race Across USA, co-author Caitlin Thurber collected and analyzed urine samples of the marathoners. She noted that after 20 weeks of running, the athletes burned 600 fewer calories a day than expected.
This finding suggested that the body reduces its metabolism to keep a sustainable level. Thurber noted:
“It’s a great example of constrained energy expenditure, where the body is limited in its ability to maintain extremely high levels of energy expenditure for an extended period.”
While the researchers believe no one has ever sustained levels beyond this limit, they hoped someone would. According to Pontzer, elite endurance athletes should take on the challenge and aspire to break the limit of endurance.
The associate professor added:
“Science works when you’re proven wrong. Maybe someone will break through that ceiling some day and show us what we’re missing.”
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