Even as recently as three years ago, at the dawn of obstacle racing, nobody really trained for Spartan RacesTough Mudders, and Warrior Dashes. Most just showed up with a few buddies and a zeal to do something different on a Saturday afternoon. You didn't have to train to compete; you just needed a willingness to persevere through the challenge of running anywhere from three to 13 miles over muddy trails and dirt roads while hurdling various obstacles – fire, barbed fences, rivers of ice-cold water, and fields of live wires.

RELATED: 'The 10-Week Obstacle Race Training Plan'

This all changed in early 2011, when Spartan co-founder Joe De Sena made an offer that a handful of top endurance athletes couldn't refuse: $100,000 to anyone who could win all 16 Spartan races that year. Spartan is similar to other obstacle races, with one big difference. Most circuits cater their courses and activities to increase the fun factor for participants: Tough Mudder, the biggest obstacle race, with a half million participants annually, doesn't use race clocks or declare winners, and Warrior Dash has short, easy courses lined with beer stations and live music. But Spartan aims to make obstacle racing a legitimate sport, with skill-based obstacles like balance beams, monkey bars, javelin throws, and wall climbing. The circuit has four standard distances – a three-mile Spartan Sprint, an eight-mile Super Spartan, a 12-mile Spartan Beast, and a 26-mile Spartan Ultra Beast – all with race clocks, prize money, and official winners. "Everybody else is focused on a party – we equate ourselves more with the Olympics," says De Sena. Which explains the $100,000 bonus purse: De Sena wanted to attract elite-level athletes to his events, not only to improve the caliber of competition but also to show the endurance-racing world just how much athletic skill his races require.

The kind of athlete De Sena's offer did attract was Hobie Call, a 35-year-old construction worker, father of five, and world-class marathoner who had qualified for the 2008 Olympic Trials with a blistering 2:16. After he missed making the team, he needed a new challenge – and Spartan was it. He'd kept in shape by finding unusual activities, setting an unofficial world record in the "lunge mile" by doing a mile's worth of leg lunges in less than 25 minutes. He then repeated the lunge mile in 34 minutes wearing a 40-pound vest. Next, he ran a 4:40 mile in the same vest. Meanwhile, he developed a brutal upper-body circuit of 150 push-ups, 50 strict pull-ups, and 80 reps each of 10 different dumbbell exercises. Before he'd heard of obstacle racing, Call had inadvertently transformed himself into the ultimate Spartan competitor. "I read about Spartan, and I thought, I bet I'd be pretty good at that," he says.

In his first race in 2011, Call destroyed the Spartan field, beating all 1,600 other participants by more than seven minutes, proving he had the right combination of speed, endurance, strength, and stamina. After he won his second, third, fourth, fifth, and sixth Spartan races in 2011 – becoming a major sensation on the Spartan Facebook page, with fans offering to board Call in their own homes for races across the country – it seemed certain that De Sena would be writing a very big check. The race organizer countered by offering $20,000 to anyone who could beat Call before the year ended. It was a brilliant publicity stunt. The 'Wall Street Journal' splashed the story across its front page, and other media coverage quickly followed. Spartan employees began scouting contenders at other obstacle races, looking for someone to beat Call and approaching guys like Chris Baynes, a 42-year-old banker and triathlete who had never heard of Spartan when he won a small obstacle race near Boston.

In the end, De Sena kept his money, but not because Baynes triumphed. Call failed to finish the Spartan Death Race, a two-day, 90-mile obstacle nightmare. But the $100,000 stunt didn't matter. What it did was hook Call, Baynes, and another dozen top athletes on the sport, inspiring them to create new ways to train specifically for obstacle racing that have since trickled down to regular competitors. "All of a sudden, in just the past six or eight months, people are saying, 'Hey, I want to train for this,' " says Call, who recently released his own DVD, 'Hobie Call's How to Train for Obstacle-Course Racing.' "I'm seeing that switch happen quick."

In addition to Call's DVD, the first books on training for the sport – Brett Stewart's 'Ultimate Obstacle Race Training' and James Villepigue's 'Obstacle Race Training Bible' – came out at the end of last year. Spartan's daily training email, with a "workout of the day," now boasts 300,000 subscribers. The enthusiasm to train for Spartan has spread to other obstacle circuits, too:

Andrew "Mustache Man" Thom, a veteran of 14 Tough Mudders, says participation in his Tough Mudder boot camps in Pennsylvania has grown fourfold in less than a year.

How do you go about training for a sport that includes so many disparate variables? Running is considered paramount, with most plans calling for three five- to 15-mile weekly runs. You also need strength and agility, ideally from doing low-weight, high-rep upper-body moves (like push-ups) with short rests to raise your heart rate. Thom asks participants to run through cold water, carry tires, run barefoot, crawl up hills, and "anything else I can concoct that causes you pain and discomfort," he says. "At the end, I want that feeling of 'Holy shit, we just went to a war!' "

It also helps to have actual obstacles for practice, which is why Baynes built a climbing wall and ropes course in his Massachusetts barn. But De Sena insists that kind of dedication is not unusual. "We've got thousands of people who are literally building spear-throw targets in their backyards," he claims. "You can visualize a kids' playground? People are basically building adult playgrounds."

Perhaps the biggest testament to the sport's cultural shift is in its numbers. In the past three years, Tough Mudder has gone from three events in 2010 to 54 in 2013, across six countries. Call, who remains dominant in Spartan racing with 26 wins to date and only three losses, credits the sport's boost in popularity to the pure fun of the experience. "People do it, and the next time they bring five friends with them," he says. "I tell you, it's going to end up being a professional sport someday because, as an athlete, it's uniquely challenging. You have to have all-body fitness, and that's what people want. They don't want to be a gym buff who can't run, and they don't want to be one of those skinny runners." Amen to that.