There’s nothing easy about running a marathon or biking up a mountain or swimming for miles on end. Let’s be clear about that. But, at a certain point during each of these activities, most athletes hit their stride. They fall into a rhythm, a kind of auto-pilot that makes more bearable that which would otherwise be unfathomably difficult. For the truly trained, muscle memory kicks in and assists in accomplishment.

Those who opt for obstacle courses don’t always have that luxury because their pursuit is far less uniform in its challenges. Successful extreme obstacle course athletes, constantly confronted with new physical hurdles from mud pits to flaming hay bales to electroshock traps, must adapt to new, threatening environments all the time. For them, muscle memory is a myth.

Sound appealing? Here are five of the most insane obstacle courses we could find. Check ‘em out and decide if you want to give one a go.

Tough Mudder

The Tough Mudder folks say their obstacle courses are “probably the toughest [events] on the planet,” and having witnessed some of them first hand, we know they’re not blowing smoke. Designed by the British Special Forces, these 10-12 mile hellish journeys held in locations all over the world at different times of year push entrants to the physical and mental brink. Classic Tough Mudder obstacle elements include mud, fire, ice water and 10,000 volts of electricity — and that’s just to start things off. As taxing as these races are, the Tough Mudder philosophy stresses teamwork and overcoming fear as its central tenets. Competitors are more concerned with crossing the finish line than their rankings and events are usually accompanied by a fair amount of post-race partying. That’s assuming you’re in the mood to have fun after avoiding eyebrow scorching by such a narrow margin.

Men’s Health Urbanathalon

Three intense cities. Three very intense races. Held each year in New York, Chicago and San Francisco, the Men’s Health Urbanathalon is the only event on our list that takes place on rough and tumble city streets. Challenges include monkey parallel bars, stadium stair climbs, marine hurdles and…taxi cabs. Yes, taxi cabs. Don’t be fooled by the lack of mud. Urbanathalon courses are the real deal. Entries are open to both individuals and relay competitors. Challenges usually start before dawn and span the course of 10+ miles of gritty city block. If you’re thinking about entering, Men’s Health offers training tips for those who hope to finish the day in one piece. Similar to Tough Mudder, athletes who make it out of the concrete jungles alive will have an opportunity to enjoy a post-course festival with music and plenty of swag to take home.

Race The Reaper

Now spreading across the western and midwestern US with yearly events held in Oklahoma, Oregon, Texas and Oklahoma, Race the Reaper is a 6-mile, 20+ obstacle course that takes competitors through acres of extreme terrain delineated by territory monikers like “Death Trap” and “Tunnel of Doom”. As is true for many of the other events on our list, racers can expect to meet with all the usual perks of a grueling military-grade course: ropes, nets, walls and lots and lots of mud. Athletes are also chased by an actual reaper…or a guy in a reaper costume, we can’t be sure. Those who finish will be rewarded with free beer and a finisher’s medal. With a name like Race the Reaper, it’s tough to believe, but most of the organization’s events are held to benefit local charities. So think about it this way: if you don’t beat the reaper, you’ll probably end in up heaven, you good-deed-doer you.

Spartan Beast

Started in 2005 in the UK by seven “ultra athletes” and a Royal Marine, Spartan Races claims its Spartan Beast course is one of the most challenging in the world, tougher even than the clearly tough, Tough Mudder. Like Tough Mudder, Spartan offers a variety of different events, all named different maddening things (Spartan Sprint, Super Spartan, Spartan Beast, Death Race, etc.), and all replete with challenges designed to exhaust the mind and body.  Over 12 miles long and packed with at least 25 obstacles, Spartan bills this event as the “toughest race on the planet”. Most competitors don’t finish.

Death Race

Co-Creator of Death Race, Andy Weinberg says: “The main goal of this race is to make you quit. ‘You can’t do it. You’re wasting your time.’” This race is the Spartan organization’s most grueling challenge and arguably the most intense of any on our list. Held annually in Pittsfield, VT, Death Race is not for anyone but the most well-conditioned athletes who also happen to enjoy severe physical and psychological torment. The 40-mile-long course takes racers through woods and up mountains and has been known to last for almost two days straight. Challenges may include competitors chopping wood for hours on end and then running with one of the 30 lbs stumps above their head and then lifting 30 pounds rocks before crawling through mud and under barbed wire. When pushed to their physical breaking points, athletes are given mental tasks (sometimes puzzles and brain teasers, sometimes memorization) on top of endurance feats. Racers have no idea what they’ll be asked to do next or when the torture will end.

Cover Photo Credit: The 621st Contingency Response Wing – flickr.com