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After 200 miles and three straight days of running across the Northern Italian Alps, 50-year-old Gregoire Millet wearily walks across the finish line. The brutal 2012 Tor des Géants is the second ultramarathon he has run, and by pushing his body well beyond what most would consider the brink, he takes the silver medal. He has been racing for nearly 79 hours, much of it in the rain, and has slept for only 6. He has crossed a seemingly endless stretch of rocky crags and endured 15 miles of elevation change. He is exhausted.

Yet, according to new research—which Millet himself helped to author—he'd be more exhausted had he run a race half the distance. "Well, after three days you are not completely fresh, you know" he says, "but it seems the exhaustion is not exponential."

Millet's research team, led by Jonas Saugy, a sports physiologist at the University of Lausanne in Switzerland, has tracked the human body's response to the demands of a 200-mile ultramarathon. In a study published today in the journal PLOS ONE, the scientists report that the Tor des Géants finishers suffered far less muscle damage and fatigue than those who had run an equally mountainous 103-mile race across the French, Italian, and Swiss Alps. "It was a surprise it was such a big difference," Saugy says.

Saugy and his team took blood samples and performed a 20-minute series of tests on their study participants before, during, and after the race. The researchers measured muscle strain in the knee and foot in a custom-built chair with a testing gauge, and recorded the electrical activity of the muscles with electrical stimulation. Their findings: Tor des Géants racers finished with roughly 30 percent more strength in their lower leg muscles despite running so much farther.

Saugy argues that the more conservative pace—and the eventual surrender to sleep exhaustion—protects an ultramarathoner's muscles. While racers at the 200-mile Tor des Géants averaged 3.4 mph over the course (a 17.5-minute-mile pace), those who ran only half the distance in the 103-mile race averaged 4.5 mph (13.4-minute miles). Running at a slower pace means running with a softer gait, and, in fact, many of the ultramarathoners spent much of the second half of the race walking. As for sleep, even the top medalists in the Tor des Géants yielded to sleep exhaustion and all the racers slept an average of 9 hours, usually in 20- or 30-minute naps.

"A shorter distance means higher intensity," says Martin Hoffman, an exercise physiologist at the University of California, Davis, who was not involved with the study. "Any time you do a longer event your intensity has to go down," says Hoffman. But what's unexpected is just how much less the longer event demands from your legs and feet.

There's plenty more to be learned about how the body responds to such a grueling completion—though even getting this much data was a chore. The Tor des Géants is so difficult that only two thirds of the runners who enter finish it. To complete this study, Saugy and colleagues had to collect enough participants who'd complete the event and agree to the 20-minute midrace examination (the clock didn't stop while the runners stopped to be tested). While Saugy's team collected 25 subjects, 10 dropped out of the race and an additional six wouldn't stop for the midrace test when the time came.

"And to ask people to do something beyond just running the race is a big deal," Hoffman says, "to get as many as they got is pretty remarkable." Still, he says, the data is robust enough to prove the longer-distance racers had lower levels of muscle damage and fatigue.

Previous studies have shown that sleep deprivation doesn't have a direct effect on the muscle strength of the ultramarathon runners. But Saugy's data shows that even brief sleep breaks were nonetheless an important period of rest for the muscles. "The addition of sleep" once the runners have built up a sleep deficit "was the reason for the muscle conservation," Saugy says.

Ultramarathons, still a growing sport, offer researchers a chance to examine the human body under stresses that would be tremendously difficult—and perhaps even unethical—to produce in a lab. "It's a competition where you can push your body to extreme conditions," he says. "You'll never find conditions like these in other competitions."

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