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My shorts are so caked in filth they’re falling off, the elastic waistband sagging with the weight of mud. I’d like to blot my runny nose with my shirt sleeve, but that’s also soaked with mud. Instead, the mucus trails down my upper lip. Shit—is my back bleeding? A few minutes ago, crawling under barbed wire, I’d let my spine arch a half-inch too far and, well, ouch. Would’ve been better to stay low and risk face-planting into the mud. I tug up my shorts for the millionth time as a man and woman in U.S. Marines tank tops charge past me, leaping over four-foot-high hurdles made of logs.

My dad was a marine, and the comparisons to boot camp flashing through my head would probably make him laugh. After all, I’m in Lake Geneva, a ritzy Wisconsin resort town, for the Gladiator Assault Challenge, a race that hundreds of people have paid $60–$90 to do. The demographic here is more yuppy than military. The question is: In what universe do people pay to crawl under barbed wire, or to haul a 20-pound log through a forest, or to slide down a mud-slicked tarp into a mud-filled pond that will, as one male racer put it, freeze your balls off? (It’s March 31 and 37 degrees.)

The same universe that has spawned not only today’s race, but also the Tough Mudder, the Warrior Dash, the Muddy Buddy, the Spartan Race, the Down & Dirty National Mud Run and the dozens of other obstacle races sweeping the United States. Apparently, Americans are eager to get beaten by pugils, those jousting sticks from American Gladiators. And I’m here to find out why.

Obstacle racing has its roots in England, where the Tough Guy Challenge, an eight-mile trail run featuring flooded tunnels, fire pits and commandos firing smoke bombs at participants, was founded in 1987. But the U.S. trend is largely thanks to Chicago. In 2009, a River North company called Red Frog Events held the first Warrior Dash in Joliet for a sell-out crowd of 2,000 racers; today, that 5K is the fastest growing running series nationwide, with a projected 750,000 participants in 48 cities, plus Australia, Ireland, England and Canada, this year. In 2010, the Spartan Race and the Tough Mudder debuted on the East Coast before bringing events to the Midwest. (Because of the terrain needed, the “Chicago” races usually take place in a suburb, Wisconsin or Indiana.)

Last year, roughly 1 million people in the U.S. registered for obstacle racing events. Not surprisingly, races keep piggybacking onto the trend. March’s Gladiator Assault was the brand’s debut; the next round hits Lake Geneva September 29 and 30. The Hero Rush, a firefighter-themed obstacle course, held its first race in Marshall, Michigan, on June 2, with a Chicago stop scheduled for July 28 in Sandwich. I can’t log on to Facebook without seeing an ad for yet another one of these things. The Metro Dash. The Tundra Challenge. The Rugged Maniac 5K Obstacle Course. The Beach Dash. The Barbarian Challenge. The Mudathlon.

Mud is the unifying theme of these races—so much mud you have to throw away your clothes at the end. “Mud in every orifice of my body,” says Lakeview resident Heather Kingery, 37, who ran the Tough Mudder in Attica, Indiana, in November. But the obstacles, and difficulty level, can range wildly from race to race. The Warrior Dash, competitors say, is more about putting together a great costume and getting goofy in the mud; even the “fire pit” obstacle that race organizers tout isn’t much more difficult than jumping over a candle. The Rugged Maniac boasts a course designed by Navy SEALs; the Spartan Race’s biggest draw is the “Gladiator Arena,” where burly men try to knock racers down with pugil sticks. The most extreme is the 12- to 15-mile Tough Mudder, which Kingery describes as “intense from the moment we got there. Everyone you looked at was in shape. There were some guys dressed in loincloths [and wearing] war paint.”

“The last obstacle is this electroshock thing,” says Evanston resident and Tough Mudder finisher Alison Bullock, 24. “There are wires hanging down from poles, 10,000 volts. You have to run through it and they just shock the shit out of you. It’s crazy.”

I can think of a lot stronger words than crazy. But Bullock, who played field hockey at Northwestern, shrugs off the question of why she signed up. “At [field hockey practice], you don’t have a choice—you’re going to run until you puke or fall on your face. At the race, I knew to push myself until I was probably going to throw up. And when you find out what your body is capable of, it’s an awesome feeling.”

Often, it is former high-school or college athletes who rally a group to participate in these races, says Dan Kirschenbaum, director of the Center for Behavioral Medicine and Sport Psychology in River North. “You can’t play college football when you’re not in college anymore,” he says. “Quite a few athletes start looking for something else to challenge themselves physically.” And if they choose one of the mud races that isn’t so intense, it allows them to bring in friends “who aren’t supremely fit,” he adds. Among race participants I interviewed, “My friends were doing it—why not?” was a common sentiment.

For University of Chicago psychology professor Sian Beilock, the question relates to her book, Choke, about succeeding in high-pressure situations. “There’s lots of research showing that people who have multiple self concepts—you’re not just someone at work or just a mother or a father, you’re also an extreme-sport participator—you will be happier psychologically. Even if you fail in one arena you have different ways to prove yourself.… If you walk into a job interview and think about what you succeeded at doing over the weekend, it could give you the confidence you need.”

Andersonville’s Saraheva Monroe, 33, who did the three-mile Mudathlon in Valparaiso, Indiana, last summer, offers a simpler theory: “When my sister and I were growing up, we had lots of fun playing in the mud. She thought [the race] would be up our alley.”

•••

Back on the course, I’m battered and muddy, snotty-nosed and shivering—and, believe it or not, having the time of my life. I’m starting to understand another reason people enjoy these grueling races: camaraderie. The trail feels like a little community, and I get caught up in packs of people, trading jokes and encouragement.

I reach a 14-foot ladder made from planks of wood nailed messily between two trees. There’s a short line; as I wait my turn, I notice the guy climbing is quaking in his mud-drenched sneakers. “I’m afraid of heights!” he yells, sounding panicked. Soon, the entire line is cheering him on, and he triumphantly trembles his way over the top and back to the ground.

The community spirit of these courses inspired nearly all of the racers I spoke with. “[At the Mudathlon], there was a vertical wall that you had to climb up by just holding onto a knotted rope. It was at the end of the race and we were totally slippery with mud…. Everybody was climbing on top of each other and helping each other. It was great. It was cooperative,” Monroe says.

“Let’s say it’s a wall or a net that’s difficult to get over, other teams will stay and help. Just a lot of camaraderie, helping each other out, encouraging people. It was inspiring,” says Lakeview’s Gwen Holtan, 47, who ran a Tough Mudder in April.

“Before you start, there’s a Tough Mudder pledge, and one of the things is ‘leave no Mudder behind,’ ” Kingery says. “For the entire race I was surprised at how much camaraderie there was. It was just so cool to have complete strangers helping you over a hay bale. There definitely wasn’t a spirit of ‘This is a race, I need to get to the finish line.’ ”

I’ve run four marathons solo, focused on nothing but my time. Today, I’m stopping to help people over hurdles, and I love it. Just me and my 1,400 new best friends, all doing something crazy together. “I think I might die!” I scream as I wade through yet another pit of freezing, waist-deep mud.

Two weeks later, I feel awful for that outburst: CBS reports that a man died traversing Fort Worth’s Trinity River while doing an obstacle race called the Original Mud Run. Though that was one instance out of hundreds of races and millions of participants, it sheds light on the risks involved. Tough Mudder participants have to sign a form that acknowledges the possibility of “catastrophic injury”; several people I speak with mention seeing racers break bones on the course.

My scariest moment happens near the end of the race. Running up to a giant pond, I see my boyfriend, Jon, waiting on the other side. I squint, confused. We had come together, but—considering he does CrossFit and I can barely lift a case of beer—I had told him not to wait for me. My brain clears to make out what he’s yelling. “Don’t go in! My head went under!” I can’t swim.

A race volunteer assures me if I stay near the shallower edge, I’ll be fine. Terrified—but also so invested in this damn race—I go for it, with Jon ready to dive in at any moment. Afterward, like Ladder Guy, I feel amazed I faced my fears. In post-race interviews, that sense of accomplishment is the first thing participants talk about.

Monroe, who is prepping for the Beach Dash in July, mentions one more perk: “Half the fun is the pictures after, where you’re just totally covered in mud. Putting those on Facebook and getting the comments, people saying, ‘Where were you? What were you doing?’ ”

Admittedly, a shot of Jon and I drenched in mud has been my profile photo for months.

[video:http://www.youtube.com/watch?v=uVE9kkwa550]