Competing in the Tour de France, a bike race over the equivalent of three Mount Everests, is like running a marathon a day for 20 straight days. It is the epitome of prolonged physical agony.
But for American cyclist Floyd Landis, the tour is just a slice of his daily pain. On a bike, Landis can climb the Alps with the best of them. But on his feet, he can barely climb stairs.
"He has a shot of winning," said Daniel Coyle, author of "Lance Armstrong's War." "Amazing, considering he can't cross his legs or walk across a parking lot."
Landis has osteonecrosis, which means that not enough blood gets to his right hip, so the ball of that joint has shriveled like a hunk of rotting driftwood. Those who have the disease describe the hurt as "bone crunching bone" mixed with "electric shocks."
Medical science can't explain how cyclists manage the anguish. They may feel pain differently than the rest of us do -- or they simply redefine it.
Landis is just the latest example of a cyclist turning suffering into speed. Two years after a hunting accident, Greg LeMond won the Tour with 37 shotgun pellets still lodged in his body.
And the icon of the sport, Lance Armstrong, didn't win his seven Tours until after an excruciating battle with cancer -- a trial that rebuilt his body and mind.
"Cycling is so hard, the suffering is so intense that it's absolutely cleansing," Armstrong wrote in his autobiography. "I didn't [ride] for pleasure. I did it for pain."
Coyle said the race is a true test of endurance and strength. "What we see in the Tour de France," he said, "is a grand experiment to see how far human willpower can go."
ABC News' Bill Weir reported this story for "Good Morning America."