What do pregnancy, the Tour de France and polar trekking have in common? They all push participants close to the same limit of human endurance, according to a new study.

Key points: Study compared extreme endurance events to find maximum sustainable energy output

Study compared extreme endurance events to find maximum sustainable energy output Limit seems to be 2.5 times the energy a person's body uses at rest, regardless of event

Limit seems to be 2.5 times the energy a person's body uses at rest, regardless of event Insights provide clues about our evolution and could help design better athlete training regimens

The report, published today in Science Advances, measured the daily energy expenditure of participants in a 20-week endurance race across the USA and plotted it against the expenditure of people in other endurance events, such as Ironman triathlons, the Tour de France, polar trekking — and carrying a baby.

The aim was to find the maximum amount of energy output a human body could sustain for a prolonged period of time, without having to break down its own tissues to make up the deficit.

And while common sense tells us intense activities can be sustained for shorter periods of time, and less intense activities for longer, researchers have now managed to put a number on it, said study co-author Herman Pontzer, an evolutionary anthropologist at Duke University in the US.

According to their analysis, the maximum energy expenditure a person could sustain without having to dip into their body's energy reserves during endurance events lasting between half a day and more than 250 days was about 2.5 times their basal metabolic rate (BMR) — the amount of energy their body would burn at rest.

In each event, the athletes' energy expenditure started high, but eventually declined and plateaued at that same point of around 2.5 BMR, regardless of the event. This is only slightly higher than the metabolic rate a woman's body sustains during pregnancy.

Pregnancy hovers just below this metabolic ceiling, Professor Pontzer explained, partly because it's longer and because, unlike endurance sport, it requires weight gain (not just maintenance).

"Pregnancy is the longest duration, highest energy expenditure thing that humans can do. Mothers probably aren't surprised by this."

The energy expenditure limit was different for events lasting less than half a day, such as marathons, the authors noted.

The limits of human endurance

Professor Pontzer said the shared threshold comes from how much energy our bodies are able to take in, rather than how much they can expend.

This challenges the idea proposed by previous research that it was humans' ability to regulate our body temperature that curtailed our endurance.

"The limits to how much you can do seems to be how quickly you can get calories in," he said.

"All of the limits of our endurance, whether we're talking about cycling or running or trekking across to the South Pole or, you know, building a kid — all of those really high effort endeavours are all governed and all constrained by the same machinery."

The Tour de France is a closely studied exercise in human endurance. ( AP: Laurent Cipriani/file photo )

The fact that all these endurance activities exist along the same continuum gives us clues about how humans evolved, and why our energy-intensive pregnancies can only last so long, Professor Pontzer added.

"What evolution really cares about when it shapes organisms is how many kids they have."

"The fact that pregnancy seems to be in the same mix as the other activities is a clue that the machinery has all evolved to enable the really expensive pregnancies that humans have, and it's had knock-on effects to this machinery that allows us to do these big endurance events."

Alternatively, the opposite could be true: that humans evolved their endurance ability first, and that changed the way we reproduced.

"That's also a possibility. It's kind of fun to think about."

Designing smarter training regimens

As well as illuminating our history as a species, the study has applications for athletes, said exercise scientist Frank Marino from Charles Sturt University.

"In the sports science world, it gives us an idea about how much energy can be expended in a particular kind of event," said Professor Marino, who was not involved in the study.

"The information that they're using can also be applied in other areas to understand how our health is intimately tied to how we expend and use energy."

But while the Professor Marino acknowledged the research was a useful contribution to the field, he cautioned that it was a theoretical conclusion drawn in part using data from other studies.

Professor Pontzer also hoped his work would give insights to athletes designing training regimens.

"Most people aren't going to compete in Tour de France, most people aren't going to trek across the South Pole but there's a lot of people who are serious athletes and they plan out months-long training regimens that they plan to keep up," he said.

"This research is really relevant to that sort of time scale."

And just because we can put a number on the limit of our endurance doesn't mean racing's going to become boring.

"You look at, say, the 100-metre sprint in the Olympics or the marathon runners. The world records of those races haven't changed at all, hardly, in decades," Professor Pontzer said.

"The people are still running 9.8-something to win a 100-metre sprint in the Olympics, and that's been the case for decades.

"But it's still exciting to watch because you don't know who, on that day, on that track, is going to be the one that pushes it closest to the wall."