This story was originally published by Inside Science News Service.

(ISNS) -- Cross-country skiers often collapse due to exhaustion at each race's end, only to come back a day or two later and compete again at seemingly superhuman levels. Speed skaters racing on the traditional large oval maintain a painful crouch at speeds greater than 30 mph in race after race throughout the games. How do they do it? And why don't elite runners run in as many distance events at the Summer Olympics?

Sports scientists say that the endurance sports of the winter games are more like bicycling or swimming than running. There just isn't as much pounding on the joints or the muscles, for example. Therefore, athletes can recover faster, and compete in more and longer races in the Winter Olympics than in the summer games.

Winter athletes have the benefit of gliding on their skis and skates. Track athletes may run more preliminary heats in some of their events, and with each stride, a runner's foot strikes the track with great force.

"When they land there's a tremendous amount of pounding," said Robert Chapman, an exercise physiologist at Indiana University, in Bloomington. "Cross-country skiing is going to be different, speed skating is going to be different, because there's not as much pounding on the muscles. In theory they would be able to recover…a little bit quicker to be able to come back and do the multiple events."

Not only that, but skiers seem more versatile than competitors in many of the summer sports.

"There have been skiers who have won both the sprint events and the 50-kilometer. Cross-country is kind of weird, because that would never happen in running," said Stephen Seiler, an exercise physiologist at the University of Agder, in Kristiansand, Norway. "If you've got great [skiing] technique for 1,500 meters, you've got great technique for 50 kilometers. So the difference in speed between those two is much smaller than you would think it would be."

Speed skaters can be similarly versatile. One reason for this is that the length of the races is much less varied than in running. While world-class men sprint the 100-meter dash in fewer than 10 seconds, and the 200-meter in fewer than 20, the shortest speed-skating race, the 500-meter, takes substantially longer. The winning times in Sochi were more than 34 seconds for the men, and more than 37 seconds for the women. The longest speed-skating races are 5,000 meters for the women, and 10,000 meters for the men, which take about seven and 13 minutes, respectively.

The energy systems used to power athletes' muscles over those distances and durations will be roughly the same, said Chapman. "In running it's going to be a lot different."

In the 1980 Winter Olympics, in Lake Placid, N.Y., for example, American Eric Heiden swept all five gold medals in speed skating: the 500-, 1,000-, 1,500-, 5,000- and 10,000-meter races.

"That would be like somebody being good at the 200 [meter sprint] or 400 [meter sprint] and also being good at the 5,000 meters or 10,000 meters in distance running, which you just never see," said Michael Joyner, a physician-researcher at the Mayo Clinic, in Rochester, Minn., with a special interest in the physiology of endurance performance. "That's because [speed skating is] so aerodynamic and so technique driven. But you do see that in a sport like swimming."

At the Olympics, the shortest cross-country ski races are called sprints, despite their length of greater than half a mile, and duration of longer than two minutes. All other individual events are longer than six miles and take nearly a half hour or longer. There are numerous events for both men and women, at multiple distances and in different styles of skiing. But the physiology of top performers might explain why so many of them are able to win medals in multiple events.

"Once you get to about 10 minutes of performance, the physiological determinants of who goes faster are pretty similar," said Joyner.

One of the enduring mysteries of cross-country skiing, though, is that skiers carry faster paces over long distances than might be expected based on the physiological demands of the sport, Seiler said. He added that he does expect that as the sport evolves, research may lead to the creation of specialized sprint ski teams that train differently than distance racers.

The cross-country ski events at the Winter Olympics culminate with the team sprint events on February 19 and on the final weekend, the women's 30-kilometer and the men's 50-kilometer. The speed skating competition also concludes on the final weekend with the finals in the team pursuit.

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