The concept is deceptively simple: Every hour, on the hour, runners begin a four-mile loop on an obscure Tennessee farm. The last person standing wins. In 2019 at the Big Dog Backyard Ultra, the last person was Maggie Guterl, running 250 miles in 60 hours, beating everyone who entered, male or female.

Scientists have long predicted that eventually women could surpass men in ultradistance running events, those longer than a 26.2-mile marathon and as much as 200-plus miles. While men still edge out women in the marathon by about a 20-minute margin on average, women are now starting to close the gap at some ultramarathons.

A recent analysis of more than 15,000 ultra events found that once the 195-mile threshold is crossed, female ultrarunners are 0.6 percent faster than men. A look at long-distance results from the past few years shows women are announcing their presence in “ultras”:

●In 2017, 38-year-old Colorado-based Camille Herron beat 120 men and 60 women to win the Tunnel Hill 100-Mile trail race in Illinois.

●Also in 2017, Courtney Dauwalter, 35, finished out the Moab 240 Endurance Run in Utah a full 10 hours ahead of her nearest competitor, besting 98 men and 18 women.

●In August 2019, running as a pair, Emma Mure, 25, and Sara Aranda, 30, set a fastest known time (FKT) of three days and 17 hours for running the 112-mile Wind River High Route in Wyoming; the men’s record had stood at four days and two hours.

“The more women who participate and therefore are able to represent the best talent pool, the more the sex difference will represent the physiological differences (drop to 10 percent) rather than be inflated because of other factors like lack of talent pool in the women,” Sandra Hunter, a professor of exercise science at Marquette University, says in an email.

“Females are closing the performance gap in ultras, and the longer the distance, the smaller the gap,” says Nicholas Tiller, a research fellow in exercise physiology at Harbor-UCLA Medical Center, who is studying the phenomenon. “There are likely a host of reasons why this is the case.”

Women arrived late to the endurance party thanks to rules that kept them out of long events, such as a woman’s marathon, until the early 1970s. They are slowly but surely learning the game. Right now, fewer women participate in ultra running than men, making up about 35 percent of races between the marathon and up to 50 miles. That percentage drops to 25 percent and below at about the 100k and longer mark.

“This is one of the confounding factors when figuring out why females excel at longer distances,” Tiller says. “It’s likely that the type of females who choose these events are super tough and quick to begin with, so it’s self selecting and maybe not a fair comparison of males and females” since so many more men of mixed abilities are competing in these races.

Paul Ronto, one of the researchers on the ultra-events analysis, says he agrees. Ronto says his study looked at averages, which doesn’t equate to the best women being better than best men.

“On average, the fields are about 20 percent female and 80 percent male,” he says. “The women who toe the line are better prepped and professional athletes, where some of the men in the field are less competitive and more experiential in their approach.”

Beat Knechtle, a professor of general medicine at the University of Zurich, in Switzerland, has conducted several studies to try to discern whether the female winners in these recent long races are outliers or indicators of a growing trend. “You need to include all women and men in an analysis to make a conclusion about gender differences,” he says. “Looking only at winners or top athletes can lead to a selection bias.”

His 2015 study looked at the performance differences between men and women in 50-mile to 3,100-mile ultramarathons. His sampling of 128,000 finishers (101,000-plus men and 26,000-plus women) concluded that overall, men were still faster than women by 17 to 20 percent for all those distances. Still, he says, “women can close the gap to men with increasing age and depending on the race distance,” although more studies are needed to show that.

The role of physiology

“There are fundamental physiological differences [with men] that will always put women at a disadvantage,” Hunter says in an email. “On average, males have larger hearts, more muscle mass, less essential fat to carry and greater concentrations of hemoglobin (oxygen carrying capacity by the blood). All these factors will provide a performance advantage of close to 10 percent-12 percent.

“For these reasons, [even if many women can outperform many males], the best male theoretically should be able to outperform the best female and that is what is typically seen in world record performances,’ Hunter says.

There are places where women have advantages for extreme distances, Tiller says. “From a physiological perspective, females have more slow twitch muscle fibers than males, which means they are more fatigue resistant at an endurance event,” he says.

In addition, Tiller adds, females tend to burn more fat than men relative to body mass, which counts in the ultra distance. “We can deplete carbohydrates as an energy source in about two hours,” he explains. “In a marathon, or even in a six- or 12-hour event, the sex differences in fat oxidation doesn’t count for as much. But over a 24-hour event, you might be looking at a 1,000-calorie rate difference, which adds up to a big advantage for females.”

Honing the mental game

Despite the physiological differences, coaches say that the mind-set of a runner can make a huge difference to who hangs on — and wins — an ultra, which can level the playing field between men and women.

When Guterl entered the Backyard Ultra this year, she did so with a dogged determination. “I learned quite a bit racing it last year,” she says, “and through my experience and watching others, I started to believe in myself months in advance of the race.”

Running coach David Roche says that when he saw Guterl a few weeks before the race, she was 100 percent present and ready.

“That might be the most important factor in a race like this,” he says. “Once you check that box, physiologically, they will do what they can with their fitness.”

Roche says that you can’t discount the shot of inspiration women receive from watching their female peers win titles.

“There are so many incredible female performances going on right now,” he says. “These women lift each other up and they are pushing themselves and others to the brink. In the process, they’re passing a lot of men.”

Mure says this is true for her.

“I believe visibility is a crucial element to get more women out there,” she says. “I didn’t even know I wanted to do this until I saw photos of other women running in the mountains. Now I understand there’s so much opportunity.”