Can You Push Yourself Too Hard?

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If you want to experience the extremes of human fitness, just try keeping up with Rebecca Rusch, 36, one of the world's top expedition adventure racers. Several times a year, Rusch runs, bikes and paddles practically nonstop for five or six days, with only a few hours' sleep, through several hundred miles of jungles, mountains, lakes and raging rapids.

Marathoners work their bodies hard. So do triathletes and those who make their living playing sports. But nobody pushes the envelope of human endurance quite as far as extreme racers. Burning calories and shedding electrolytes faster than their bodies can replace them, athletes like Rusch will lose up to 10 lbs. of water, fat and muscle in the course of a five-day race. By the time Rusch crosses the finish line, her organs are faltering, her muscles are deteriorating, and she's hallucinating wildly. "I don't understand why I do it," says Rusch, who has racked up a dozen top-five finishes over the past eight years. "But I keep coming back for more."

Fitness at such a level takes a significant toll on the body. Physical activity, after all, is a form of stress, and extreme, unrelenting physical stress for days on end can cause permanent damage. That may include structural damage to joints, bones and muscles, as well as less visible but more insidious changes to critical body functions. "It's not a physiologically healthy sport," says trainer and former adventure racer Terri Schneider.

The ultimate in adventure racing is the Raid Gauloises World Championship in September. The culmination of a series of shorter qualifying races throughout the year in places such as Australia, Sweden and the U.S., the contest this year will lure more than 200 competitors--including Team Montrail, the six-person American team Rusch captains--to the French and Swiss Alps for a 372-mile trek up and down the mountains.

To give TIME a sense of what it's like for these superathletes, Rusch took us through a typical grueling week of racing.

DAY 1

This is the toughest part of the race for Rusch. Men tend to start out strong and finish weak; female racers, by contrast, gain momentum. "The first day is survival for me because everyone is so amped and they go out fast," says Rusch of the five-man, one-woman team she leads. "I'm just hanging on so I don't fall back." After just two hours, she has used up most of her glycogen--a form of energy derived from sugar and stored in muscles and certain organs--and her body starts running on fat and whatever calories she gets from the food she eats while racing. Most adventure racers put on a few pounds during prerace training. Even a lean athlete, who typically carries only 4% to 6% body fat, can access about 40,000 calories of energy--enough, says Ian Adamson, a champion adventure-racer captain, "to run coast to coast across the U.S."

DAY 2