The average person can burn up to 4,000 calories — a limit a group of scientists consider the peak of human performance — before depleting the body’s energy stores. And while extreme distance runners reach maximum performance during high-intensity races, expectant mothers often hit the same mark at a lower intensity over a longer period of time, typically without completing a triathlon.
“You can do really intense amounts of work for a day or so,” Herman Pontzer, a Duke University researcher who co-led the study, told CNN. “But if you have to last a week or so, you have to maintain less intensity.”
Longer pushes require lower intensities, but over a short period of time, the human body can successfully exert 4,000 calories on average before hitting the wall. That’s 2.5 times the basal metabolic rate, or amount of calories a body needs to operate while at rest.
The average person won’t reach those limits in a typical workout (except maybe Crossfit, Pontzer noted), but pregnant women and extreme athletes cut it close. Weeklong races and nine-month pregnancies similarly push the body to its limits, often burning calories at a rate the body can’t keep up with.