There are plenty of other organizations just as willing to get their hands dirty making computer programmers and accounts-payable department members into military-style trainees for a day. The same year Warrior Dash took off, a Harvard Business School student from England named Will Dean proposed a mud and obstacle race for the school's annual Business Plan Contest. As a former high school rugby player and member of the British Foreign and Commonwealth Office's counterterrorism policy department, Dean figured some hard-hitting special-forces-inspired team-building activity followed by a whole lot of drunken revelry would draw at least 500 paying customers. The judges thought that was a nice little dream that had no target audience.

Unbowed, Dean and a partner sank $8,000 into a Facebook-based marketing campaign for their first Tough Mudder event in Allentown, Pa., in 2010. The event was a success and, as word spread through social media, the pair planned two more events that year and drew a combined 50,000 participants. That number grew to 14 events and 140,000 runners last year and has ballooned to 36 events in the U.S., U.K., Canada and Australia this year.

Tough Mudder organizers expect nearly 400,000 to wade through mud bogs, crawl under barbed wire, splash through cold water and run through a ring of fire to the free beer at the end. Why? Partially because it sounds a lot sexier than a 10K runner or marathoner's description of running in a straight line.

"Telling someone you finished an event in 50 minutes or that you did 26.2 miles and they were all the same isn't as popular or shareworthy as saying you went through electric shock, you went over a 10-foot wall, crawled on your belly beneath barbed wire or ran through fire," says Alex Patterson, Tough Mudder's chief marketing officer. "We tend to see the experience as making you feel good -- instead of the car you drive, the watch you're wearing or the cellphone in your pocket."

That leads to the other, more primal reason behind the event's success. This is an event that costs $90 to $155 to attend, depending on when a participant registers and how many folks they bring along, and has a 16-exercise training program just to get people through their first event. Yet the numbers climb because of something that occurred to Patterson as he and the event's organizers walked through an airport dragging wheeled luggage that people thought nothing of carrying just a few decades ago: With each heated apartment, spigot of hot running water, elevator ride and dishwasher scrubbing last night's tomato sauce off the plate, "Human beings are constantly innovating themselves out of essentially being human beings."

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