Can You Push Yourself Too Hard?

You bet! Just ask the athletes who run the world's most brutal races

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,...

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