Can You Push Yourself Too Hard?

By Alice Park

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

For Rusch, this is the sweet spot of the ordeal, the day she falls into the rhythm of the race and no longer feels as if she is playing catch-up with her teammates. "As the race gets longer, women tend to do better and better," she says. "I think that has a lot to do with body fat, since women, even athletic and fit women, have more stores to pull from. Guys have about 6% body fat, while women will carry about 12%." The calculus of human exertion, however, is working against all the contestants. Adventure racers typically burn as much as 500 calories an hour, but the body running full steam can process only about 300 calories an hour. Contrary to what you might think, at this level of exertion the last thing racers want to do is eat. "We try to force ourselves to eat small amounts every single hour the whole entire race, aiming for 300 calories," says Rusch, "but it doesn't always happen."

DAY 3

This is Judgment Day, the one, says Rusch, "that separates the men from the boys." With very little sleep--probably only a few hours in the past 48--the racers are beginning to feel the physical and mental toll of their almost constant racing. Drinking enough water to fuel the body's internal needs is critical. After a person is up for two days straight, the body's metabolic systems start to overheat like an aging car. Without water and time to rest, muscles begin to falter and the kidneys start to sputter. The heart becomes less efficient as a pump, and in general the body is less able to tolerate extremes in temperature.

DAY 4

The body is nearly shut down by this point, coasting, as racers say, on fumes. The energy deficit that the contestants have been running since the first day is now catching up with them, and their bodies have become catabolic (preferentially feeding off their own muscle and fat stores to fuel critical metabolic and cellular processes). The damage from repetitive impact on bones and muscles leaves its mark, especially in the legs and feet as capillaries break down. Rusch carries a second pair of shoes to accommodate feet that she knows from experience will balloon in size. In fact, after three years of repeated assaults on her feet, Rusch has gone up a shoe size.

DAY 5

The lack of sleep, the dehydration, the energy deficit and the physical battering that her body is enduring are starting to affect Rusch's mind. "You start hallucinating and falling asleep while on the bike," she says. "I've had vitamins in my hand and had them all turn into squirming bugs. In New Zealand one time, we were walking through a marsh in the middle of the night, and I saw a Vietnamese woman selling fruit at a little stand. I asked a teammate for some money to buy mangoes as I started to change course and walk toward the stand. My teammate just grabbed my arm and pulled me back. There was no Vietnamese woman and no fruit stand."

BY THE END OF THE RACE, blood levels of nitrogen and protein waste products--and those of potassium, sodium and other substances not normally found there--are rising. Rusch knows too well what that feels like. "You're just in this state of slow deterioration, and you're doing everything you can to buy time," she says. "You can't recover until you stop." Even then, say exercise physiologists, the body doesn't always bounce back completely. Ultra-athletes may be more susceptible to developing arthritis and fractures when they are older, and their muscles may not recover as quickly from tears and bruises. Still, says Rusch, "a journey like that is an amazing thing." And for her, well worth the physical toll of pushing her body to its limits. o