Mud crawling. Rope climbing. Tire swinging. Chain pulling. Spear tossing. Fire jumping. Forget your standard marathon: Over the past few years, macho obstacle course races have taken over the competitive fitness scene, becoming the fastest growing sport in U.S. history. The three most popular races— Tough Mudder , Spartan Race , and Warrior Dash —have gained such a following that not only have their attendance rates far surpassed marathon and triathlon numbers, niche categories have been established as well. From superhero scrambles to zombie races to mommy runs , interest is growing at an exponential rate, particularly among women.

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"It's definitely become a huge trend in the industry, and women who've never done competitive racing in their lives want to conquer this," explains Kayley Hoffman, a trainer at As One Fitness in New York City, a studio that specifically caters to obstacle race athletes. "You would think, with the mud and getting dirty, women wouldn't want to do it, but it's actually the opposite. They want to tap into their competitive side and find the fun elements of working out."

Races on the Rise

When you hear "obstacle course," you might imagine something like a summer camp color war, but make no mistake—these races aren't for kids. Endurance events like Tough Mudder, which was founded in 2010, are meant to test both mental and physical strengths. Spread out across a dozen miles of muck-engulfed terrain, Tough Mudder obstacles involve harrowing elements. For a $130 entrance fee, racers get to swing on monkey bars elevated above tubs of ice, slither across a quarter pipe sponged in grease, stomach-duck under electrical wires—all, of course, while treading through thick sludge.

"There's something so primal about getting dirty," Tough Mudder's Ben Johnson posits. "There's the idea that you're completely outside acceptable daily behaviors. The obstacles allow people to laugh at themselves while they are crawling in the mud and not take themselves too seriously. They can be social while doing activities they wouldn't normally enjoy."

Tough Mudder has soiled over 1.5 million participants since its inception four years ago, with some 15,000 people participating in each event; more than 4,000 of them even have Tough Mudder tattoos. In the beginning, the ratio of male to female participants was one to four. Since then, female participation has grown, Johnson says, with over 600,000 female participants to date. Johnson adds that Tough Mudder is eager for more women, who now make up 30% of entrants, to get involved and is working on marketing to ensure "there isn't anyone who feels unwelcome at Tough Mudder."

Spartan Race, which also held its first event in 2010, is similarly looking to target the demographic; co-founder Joe De Sena says his competition is currently 37 percent female. While the company makes significant investments in women's marketing, they are less interested in the inclusiveness of the sport and are unapologetic about how difficult the competition is.

"We're big believers that it's healthy to be uncomfortable, and we're not into sending a different message just because it has commercial appeal," De Sena says. "Our message applies to both men and women, but obviously the balance is mostly men. Women in general have less upper-body strength, so the rope climbing and sandbag obstacles are going to be harder for them. But its real life—we won't sugar coat it and make it easier for women."

THE FEMALE FACTOR

Spartan Race's female participants might be small in number, but they're certainly dedicated. They even have their own community—self-dubbed Spartan Chicked—that has over 16,000 active members who train together and blog about their experience. Still, the disinterest in competing against gruff dudes in mud is what's helped kickstart dozens of women-specific races. Last year, three Tough Mudder enthusiasts founded Mudderella, an obstacle race for women that's on courses about half the size of Tough Mudder ones. The creation of Mudderella came on the heels of several other women-only obstacle races with cheeky names like Diva Dash, Dirty Girl, Gritty Goddess,Pretty Muddy, Muddy Mamas, Kiss Me Dirty, and Lozilu.

"It's not that women can't and shouldn't do Tough Mudder, but some women may feel more comfortable with their girlfriends than with men."

Some see these lady events as a cop-out (De Sena calls them "soften versions for women who can't play at our level" and noted that "when you go to a birthday party, you don't want to tell friends you did a Spartan Race-type thing, you want to say you did a Spartan Race"), but the number of participants certainly proves there's real interest. Mudderella held just one event in 2013, followed by eight more this year, and next year are looking to expand to upwards of 20 races. So far they've had 70,000 participants.

"We saw that there was a gap in the market. It's really powerful for women to come together to accomplish a new challenge," says Mudderella co-founder Antonia Clark. "Mudderella is an athletic challenge that inspires women to step outside their comfort zone. It's not that women can't and shouldn't do Tough Mudder, but some may feel more comfortable with their girlfriends than with men."

Clark wouldn't say if Mudderella is easier than its co-ed counterpart, but offered that the two are indeed different. Tough Mudder is focused on testing upper body strength with obstacles like monkey bars and wall climbing, while Mudderella targets core body strength with balance and weight-shifting challenges.

As for what's drawing so many women to these obstacle races, Lara Eurdolian, a New York City blogger who ran Mudderella last year, cites the psychological aspect of being able to take on such an ambitious challenge.

"It's not so much about being an athlete. It's a race that's testing yourself, and that's what's appealing," Eurdolian explains. "The women's races are less intimidating. Some girls don't want men to see them working out. They are self-conscious about that, which is why there are women's gyms. With these races, they feel like they can handle the load. It's challenging, but then you look at Tough Mudder and its crazy obstacles and you see all the men who look like they've been training for years doing it, and there's the concern that you might not make that."

Eurdolian says she found Mudderella to be harder than Tough Mudder because fewer obstacles meant more running: "I wish it was marketed differently. I wish it wasn't, 'Tough Mudder is for strong men, Mudderella is for women.' It's really doable and fun, even just for someone who's athletic but not a crazy marathoner. It's kind of like a food fight: It's a ridiculous experience, and it's fun to see what it's all about."

Margaret Schlachter, the first female professional obstacle racer, says women love the races because they spice up regular fitness routines, getting you out of the gym and also enhancing the social aspect of working out. Schlachter quit her job a few years ago to go pro and is paid by sponsors to participate in obstacle races; she's also written a book about herexperiences and runs the site Dirt in Your Skirt. She doesn't look down on women-specific races because she sees them as a stepping stone into more competitive territory.

"It's a very empowering tool for women. It encourages them to take fitness back," she says. "The women-only events are great for people who are a little scared or intimidated because there's no competition factor. They feel the camaraderie of the sport, of the people working together, and don't want the judgment."

EXTREME ECONOMICS

While obstacle race participation is increasing at an insane rate, competition founders say the economics are less than stellar. Many of these events have sponsors like Reebok, Under Armour, and Athleta, but according to Spartan Race's De Sena, sponsorship money "feeds dogs while we need to feed elephants." A Spartan Race costs roughly $450,000 to put on, and as of now, the company has yet to see ample returns.

A Spartan Race costs roughly $450,000 to put on, and as of now, the company has yet to see ample returns.

"This is the worst genre of sports to get into," De Sena continues. "It's not economically efficient at all. The collecting-per-person revenue outweighs the cost five to one. It's a killer, between trying to build an obstacle race on a mountain, dig the mud pits, and then clean up the land to follow EPA regulations. There isn't a day that goes by where I don't question my sanity and think, 'God, wouldn't it be easier to do a different style race?' But I love this, I love pushing the limits, and I've created a monster that needs to get fed every day."

Last year, Superhero Scramble co-founder Sean "Ace" O'Connor laid out the full economics of his competition in Obstacle Racer Magazine. Explaining that site fees can cost up to $50,000, insurance up to $25,000, and obstacles up to $200,000—plus the expense of fire and rescue, police and safety, advertising, entertainment, and freebies—individual races can run companies more than $420,000. O'Connor said he hoped revealing these numbers would make "entrepreneurs who want to jump on the bandwagon and start your own race think twice."

In addition to event costs, both Spartan Race and Tough Mudder pump tons of cash into marketing campaigns in an effort to enter the sport into the mainstream. One of Tough Mudder's PR objectives is to eliminate the race's dangerous reputation, making it known that scary obstacles, like those that involve fire and electricity, can easily be avoided by just walking around them. De Sena says Spartan Race currently spends well into the seven-figures to raise awareness and hopes to see profit in four years time.

Josh Kravetz, who runs the women-only Diva Dash has similar sentiments about his business's economics. A smaller-scale obstacle race with some 7,000 participants per event, Diva Dash races cost about $100,000 to run, with most of the expenses coming from setup, permits, and transportation.

One of Tough Mudder's PR objectives is to eliminate the race's dangerous reputation.

Kravetz says his company is careful not to become too big too fast and notes that the saturated market caused several niche obstacle race companies to go under over the last year. Dirty Girl, 5K Foam Fest, Hero Rush, and Ruckus Run are no longer around, not just because of increased competition and rising costs, but also because many companies expanded too rapidly.

Dirty Girl was Diva Dash's main competition, growing from a market of three cities to 25, a move that put them on the road to bankruptcy and has the Better Business Bureau investigating its practices.

Another economic challenge obstacle races face is their dwindling return rate. Die-hard fans that have Tough Mudder tattoos and professionals like Schlachter are in the minority; most participants see the obstacle races as one-and-done activities.

"This is an industry that has a pretty low return rate," Kravetz says. "The thing is to market to new participants because people don't feel they have a reason to come back."

Tough Mudder told Racked they have a 20 percent return rate, acknowledging this is a hurdle they're looking to overcome. Part of their strategy involves the creation of Legionnaires, a Tough Mudder group for returning participants who get access to exclusive new obstacles. The Legionnaires are marketed as elite competitors, and their color-ranking system of apparel gives racers levels to aspire to.

Despite its challenges, it's believed the obstacle race industry will only continue to grow. Recently there's been such acute enthusiasm that competitions like Spartan Race, America's Next Ninja War Hero, and Bad Ass Dash are even broadcast on TV.

"It's not accepted by everyone, but we are going to reach the masses until they believe it's a real sport," says De Sena. "It's a very human thing to do, to climb, crawl, jump, and get wet. The human body is not meant to sit all day in front of a computer or sit in traffic. The body is like a Ferrari, and that would be like keeping a Ferrari in a garage or scratching it up. These races are like taking your Ferrari out on the track for a nice spin."

Editor: Julia Rubin