Why are so many Americans suddenly spending their weekends jumping over flaming hay bales and crawling under barbed wire? That's been the burning question for those on the sidelines watching obstacle racing go from a fringe pursuit for wannabe jarheads to the event every cubicle jockey wants to brag about. Though sports psychologists and other academics are only beginning to study the phenomenon, two rationales suggest we were primed to embrace the mud.

THEORY #1: OBSTACLE RACING IS THERAPY
Some psychologists say the sport is a natural outgrowth of a society deeply affected by the protracted wars in Afghanistan and Iraq. We've already adopted soldier fashion (see: field jackets, anoraks), consumed all manner of military-focused media (TV's Stars Earn Stripes being the most ridiculous yet), and played countless hours of kill-or-be-killed video games (the Call of Duty series has sold 100 million copies since 2003). And with so many veterans returning home with severe trauma, we're looking to "symbolically take on their pain in order to empathize with them," says Roger Stahl, a communications professor at the University of Georgia and author of Militainment, Inc.: War, Media, and Popular Culture. Enter obstacle races, which are infused with battleground energy and paraphernalia—tanks have popped up at some Tough Mudder events. "We're also asking a lot of questions about these conflicts," says Stahl. "Participating in these rituals is part of that."

THEORY #2: OBSTACLE RACING IS REBELLION
There's an inherent coolness in being part of renegade sport. Like skateboarding in the '70s of snowboarding in the early '90s, there are no real rules for mud runs, and winning doesn't mean much for most entrants. (Tough Mudder events aren't timed, and you can skip obstacles.) "In organized sports, there's no place for people who just want to go out and have fun," says Jay Coakley, a Colorado sports sociologist. "This is a resistant response to that overly structured part of our culture. When we grow up, we don't play anymore. Obstacle racers are saying, 'Dammit, I want to keep playing!'" And unlike with other endurance sports, most obstacle courses don't demand months of training or specific athletic skills or gear. "It's like a playground for adults," says Hobie Call, a former elite marathoner who made a name for himself by winning 11 Spartan Races in 2011. "After I finished the first one, I felt like a kid again for the first time in years."