♪music♪ LESTER HOLT, anchor: The United States hasn’t won an Olympic medal in cross-country skiing since 1976, but in 2010, several skiers hope to change that. If they’re successful, you can be certain it’s due to their incredible endurance. As Olympic athletes, trainers, and a sports scientist and chemist funded by the National Science Foundation explain, cross-country skiers are among the fittest athletes in the world. ♪music♪ Olympic cross-country skiing is a sport known for pushing its athletes to the limits of human endurance. ANDY NEWELL (U.S. Cross-Country Ski Team): You have to be a little nuts to want to push your body so hard and just really push yourself to the point where you collapse at the finish line. HOLT: Whether the event is a nearly-one-mile sprint, or a more than 30-mile race, these Olympians are considered to be among the most aerobically fit athletes in the world. Dr. DEBORAH KING (Ithaca College): Cross-country skiing involves using both your legs and your arms, so almost all the muscle groups are being used to ski. And that means you have to supply energy to all the muscles in your body as you ski. HOLT: What supplies energy to muscles? During intense exercise, an athlete sucks oxygen into the lungs, which is then pumped, by the heart, throughout the body in the blood stream. When oxygen reaches the muscles, it combines with an enzyme called ATP, the carrier of chemical energy within the cell to produce muscle contractions which allows movement. Olympic cross-country skiers can, with practice, improve this biochemical process. Dr. JOSEPH FRANCISCO (President, American Chemical Society): What they’ve essentially done is trained their body in how to be more efficient in taking up oxygen and actually transferring that oxygen to be used at a cellular level in the whole ATP to energy conversion. HOLT: But even the fittest of the fit have limits to how much oxygen their bodies can process, a peak measured by what’s known as “VO2 Max.” TROY FLANAGAN (U.S. Ski and Snowboard Association): VO2 Max stands for the volume of oxygen that an athlete can consume when they are exercising maximally. So the maximum volume of oxygen that the lungs can actually grab, consume and send off to the muscles. HOLT: At the U.S. Ski and Snowboard Association’s Center for Excellence, in Park City, Utah, athletes undergo VO2 Max testing up to three times a year to measure their maximum aerobic capacity. Troy Flanagan is the center’s sports science director. FLANAGAN: We want to know if their training is actually effective or not, whether it’s increasing the size of their aerobic capacity. HOLT: The test is done on a treadmill, with the athlete hooked to special equipment that measures volume of air taken in, and amount of oxygen consumed, and other physiological factors. It’s a quick but painful test, because it pushes athletes to the absolute limit of their ability to consume and process oxygen. FLANAGAN: We can pretty much max an endurance athlete out in about 6-8 minutes, which sounds amazing when these athletes normally train for 6-8 hours in the woods on snow. So you can imagine how hard this test is. HOLT: Liz Stephen, a member of the U.S. Cross-Country Team who did a partial VO2 Max test for our cameras, explains how she copes with the pain. LIZ STEPHEN (U.S. Cross Country Ski Team): It’s a mental game for me, you know? Your body can do a certain amount and from there it’s up here. HOLT: During the test, the treadmill slowly but steadily rises, until Stephen hits her max. FLANAGAN: Where the yellow line plateaus out, that means that no matter how much air you’re breathing in and out, your lungs cannot consume any more oxygen. HOLT: To ensure a “true max” has been reached, the test continues until the athlete no longer can. STEPHEN: I don’t usually remember too much about the last minute, you’re just hanging on for dear life and all of a sudden you’re done. HOLT: Compared to athletes from other endurance sports, cross-country skiers have on average the highest VO2 Maxes. For Stephen, the test is important personal data about her aerobic training. STEPHEN: I really believe it’s just a test, and some people, you know, test really well, and some people don’t, and I think for me, I use it to see improvement for myself. HOLT: On race day, aerobic capacity is just one of many factors determining which athlete will be able to push to the upper limits of human endurance, and perhaps become an Olympic champion.