No matter what the sport, top athletes are always looking to push themselves to their limits. This drive has led to the creation of competitive events that test human physical limits. In this slideshow, explore some of the most grueling endurance contests on Earth.
We begin with a race across an entire continent -- The Race Across America, a 3,000-mile bike race from the Pacific coast of the United States to the Atlantic. Participants may race solo or in teams of two-, four- or eight-person members alternating turns riding.
All participants have 12 days to finish the race. Solo riders can expect to sleep as little as 90 minutes per day and spend more than 20 hours each day on their bikes. Hallucinations from sleep-deprivation are a serious enough concern that support crews monitor the riders.
Runners in the Marathon des Sables, which Discovery contributor David DeFranza listed as the world's toughest footrace, brave approximately 150 miles of Moroccan desert while carrying their food and supplies on their backs.
The race is broken into several stages with time limits for each one. Organizers have scattered mandatory checkpoints with water along the way to ensure runners stay hydrated.
Even with the safety measures in place, runners still run the risk of getting lost. In 1994, Mauro Prosperi lost his way in a sandstorm and wandered alone for about 125 miles. He survived by eating bats and drinking his own urine before rescuers found him nine days later and more than 30 pounds lighter.
Contenders in the Iditarod Trail Invitational run, bike, sled, or ski their way across 1,000 miles of Alaskan snow in February. They have a maximum time limit of 30 days to finish and must carry their own supplies.
Temperatures can drop to minus 50 degree Fahrenheit. To cope with the extreme cold, participants need to consume extra calories and take extra precautions, such as covering their entire bodies when they sleep to avoid frostbite.
If they manage to reach the race's finish line, contestants can look forward to having a hearty meal at the Scneiderheinzes' home in Nome.
The Ironman World Championship is an annual triathlon held in Hawaii. Qualifying for the race is hard enough. But even staying in the race is a feat. To stay competitive, participants have to swim 2.4 miles within two hours and 20 minutes, bike 112 miles in under 10 hours and 30 minutes, and then finish running a 26.2-mile footrace by the 17th hour of the event.
Amazingly, past winners often have managed to finish in slightly more than eight hours. The race itself can attract top athletes, with Lance Armstrong (seen in this photo nearing the finish line) recently placing second to Olympic silver medalist and triathlete Bevan Docherty.
In the late 1970s, Martin Luther King Jr.'s assassin escaped from prison and managed to run a measly 8 miles in 55 hours. Gary Cantrell, a local ultrarunner, figured he could have run at least 100 in that time, and so began the Barkley Marathons, an annual race in Tennessee's Frozen Head State Park.
Participants have 60 hours to run 100 miles of the steep, thorny, and muddy trail. In other words, the run tests endurance and pain tolerance. To prove they followed the course, runners rip a page from books scattered along the trail.
Every 20 miles, the trail loops back to Cantrell, the event's host, and a campsite of food, sleeping bags, and beer. There, runners figure out whether to endure another 20 miles or stay. Almost no one finishes.
The most famous bicycle race on Earth, the Tour de France is a three-week, contest spanning approximately 2,100 miles and ending in Paris.
This year, the race consists of a prologue, nine stages of level terrain, four medium mountain stages, five mountain stages, and two stages of time-trials. Each stage take a day. The event is highly competitive with racers in 2011 getting 8,000 euros (more than $10,000) for every stage won in addition to other prize money for specific achievements, such being the best climber.
This photo shows the 2011 winner, Australian Cadel Evans, with the pack during one stage of the Tour.
The Badwater Ultramarathon is a 135-mile, summer race from Death Valley (280 feet below sea level) to the trailhead of Mount Whitney (almost 8,300 feet above sea level).
Temperatures reach around 130 degrees Fahrenheit, while asphalt bakes to nearly 200, hot enough to force runners to race on the road's white lines or risk their shoes melting.
Every 15 minutes, support crews douse the racers with ice water to prevent overheating. At the age of 67, Badwater legend Arthur Webb finished his tenth consecutive race in 2009.
An annual event that takes runners across three countries, France, Italy and Switzerland, the Ultra-Trail du Mont-Blanc packs a scenic, high-altitude, 103-mile tour of the Alps into 46 hours or less. Most top finishers get through the race in about 20 hours.
The race is popular among ultramarathon runners. With only 2,300 spots available for race participants, thousands of potential contestants are frequently turned away. There are three other races, however, held in conjunction with the Ultra-Trail du Mont-Blanc, offering variations on the trail.