By Norma Rubio, Producer, NBC News

While running a marathon used to be the ultimate way to push your body to the limit, there's a growing trend of pushing your body to more extreme levels of fitness -- such as obstacle racing.

These races combine trail running with tough physical and mental challenges such as crawling through mud under barbed wire, climbing over slippery walls, running with heavy objects, and even jumping over fiery hot coals.

The Spartan Race is one of several obstacle competitions around the world that have recently exploded in growth. "People like change and this is primal. It's getting back to our roots of playing in the mud and being a kid again. So, it's different, it's fun," says Mike Morris, vice president of production at Spartan Race.

At a recent race in the desert of Fountain Hills, Ariz., Spartan racers trekked through nearly 5 miles of trails and 18 man-made obstacles. At the end of the race, runners fought one final battle with a Spartan before getting to the finish line.

But if you're thinking you're not cut out for this type of competition...think again. Spartan race organizers proudly pronounce it's an event for all people. "We have a kids' race, we have grandmothers, 70-year-old grandmothers, we have people that this is the first race they've ever done and they've lost 50 pounds and they show up and they can't believe the accomplishment," says Morris.

And then there are professional athletes like 35-year-old Hobie Call from Utah who quit his job to pursue racing full-time and has become arguably the most famous obstacle racer. "No matter how many times you do these races, like I said I've done over 30...and every time it's a unique race. You never run the same race over again."

For Angela Reynolds, a divorced mother of three, each race is an opportunity to overcome challenges. "When I first started I couldn't climb the rope and I had to learn to teach myself and set one up in my garage so I could learn how to do that."

Other extreme challenges include the Tough Mudder, where racers run through live wires or plunge into icy pools.

SealFit mimics elite Navy Seal training, including lifting massive logs.

And if you prefer a fear factor, Run For Your Lives, takes you on a 5K race from zombies -- well, people dressed up as zombies. 

For the artistic-minded, Cirque School offers training in acrobatics, aerial fabric tricks and trapeze. Aloysia Gavre, founder of Cirque School LA, insists this type of workout is for anyone.

To participate in any of these events, however, you'll likely need to sign a waiver indicating the risks. 

But it's clear that these days, many are more than willing to take on those risks and face the challenges that extreme fitness requires.