Many people struggle to run a mile; others can knock out a 10K on a whim. A few brave souls take on “ultramarathons” — races longer than 26.2 miles (42.2 kilometers). But how far can a human run before they just have to stop?
To answer that question, first we have to define what it actually means to “stop.” Dean Karnazes holds the unofficial record for the longest run without sleeping, at 350 miles (563 km), which he ran over three and a half days in 2005. In 2023, ultrarunner Harvey Lewis set a new record in a type of long-distance race called a backyard ultra. In this type of competition, runners complete a 4.17-mile (6.7 km) loop every hour, on the hour, until there’s only one runner left standing. Lewis ran 108 of those loops in as many hours (equal to 4.5 days), totaling 450 miles (724 km), with just a few minutes at the end of each hour to rest before setting off again.
Because ultramarathon runners often do take short breaks to walk, eat, tie their shoes, go to the bathroom or — depending on the type and length of the race — sleep, there’s no official record for the longest run with no stops. But if there were, nature’s call would likely be the biggest constraint.
“I think peeing is going to be the limiting factor there,” said Jenny Hoffman, a physicist at Harvard University and an ultrarunner. Hoffman holds the world record for the fastest crossing of America on foot by a woman. (She completed the feat in 47 days, 12 hours and 35 minutes.)
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Short breaks for biological needs aside, humans have a number of traits that allow us to perform well at endurance running, said Guillaume Millet, an exercise physiologist at Jean Monnet University in Saint-Etienne, France. Humans have comparatively large glute muscles to help with forward propulsion, the ability to store elastic energy in our tendons and muscles, and strong neck ligaments to keep our brains stable while running.
Humans are also well adapted to running in the heat because we can regulate our body temperature through sweating. “Even if the external temperature is quite high, we’re able to maintain our core temperature relatively low, and this is a huge advantage compared to most species,” Millet told Live Science.
Despite these adaptations, humans never specifically evolved to run such extreme distances. “For most of our existence, until recently, people had to work very hard to survive,” Lieberman said. “If you run properly and don’t get injured and fuel properly, it’s amazing what the body can do, but that’s not what we evolved to do. Those are taking normal adaptations to their extreme.”
A range of physical factors, such as injury, muscle fatigue or sleep deprivation, can force a runner to stop and recover. But mental fortitude also plays a role in endurance running. To keep moving for days at a time, ultra runners must be able to power through pain and exhaustion.
“We evolved an extraordinary capacity to force ourselves to do all kinds of extraordinary things. You have to want to do it,” said Daniel Lieberman, an evolutionary biologist at Harvard University. “So I think the most important thing about humans that limits endurance is mental.”
Those who do push themselves to such extremes need extensive training to avoid injury. Before her transcontinental run, Hoffman worked up to running 200 miles (322 km) per week to make sure she had both the aerobic fitness to exercise for long periods and the bone strength to handle the repetitive pounding against the pavement.
Still, more people attempt ultramarathons every year, with participation skyrocketing 1,676% between 1996 and 2020. As the sport rises in popularity, new runners will challenge — and perhaps break — old records.
“I think that that limit is going to keep getting pushed,” Hoffman said.
