For most people, biking 125 miles over rugged mountain passes would be the challenge of a lifetime. But cyclists competing in this year's Tour de France are tackling such physical feats almost every day for three weeks -- covering more than 2,000 miles and powering through nine mountain stages, including one next week that will finish on a 2,645-meter (8,677-foot) summit.
Clearly, Tour riders are among the fittest athletes around. But are they the most fit?
That may depend on how you define fitness, say experts, who continue to debate its definition. Complicating the question, professional athletes tend to avoid revealing details about their own performance measurements. Meanwhile, few to no studies have accurately compared different kinds of athletes at their peaks.
Overall, cross-country skiers edge out cyclists on many categories. On the other hand, cross-country ski races rarely -- if ever -- cover multiple mountain passes while racing for weeks.
"It's clear to me that Tour riders are among the fittest because they have to do so many days of very hard effort," said Christopher Minson, an exercise physiologist at the University of Oregon, Eugene.
"Every rider has different abilities, but all have to be able to ride long miles hard and fast, and even hard-core sprinters have to be able to get over thousands of meters of passes," he said. "Every rider out there has to have a high ability to suffer long hours in the saddle and maintain high speeds."
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Traditionally, coaches have focused on one measurement to gauge of their athletes' fitness: VO2 max. Essentially a symbol of cardiovascular fitness, VO2 max refers specifically to the maximum amount of oxygen a person can bring in to the body for use during exercise. The more oxygen the muscles can take in, the more work they can do.
Measured in units of milliliters of oxygen per kilogram of body weight per minute of time spent being active, VO2 max is consistently highest among cross-country skiers. Retired Norwegian skier Bjorn Daehlie holds the record for highest VO2 max ever documented, said Phil Gallagher, director of the Applied Physiology Laboratory at the University of Kansas, in Lawrence.
Daehlie's record was 96. By comparison, fit college students in Gallagher's studies regularly earn VO2 max measurements in the high 40s or low 50s. He guessed that Tour riders, especially mountain-climbers, have VO2 max levels in the mid- to high-80s.
While VO2 max has been long used as the ultimate gold standard of fitness, a growing number of experts now think that lactate threshold -- or anaerobic threshold -- is more important. That's a measure of the percentage of an athlete's VO2 max that he can sustain before his body switches to using something other than oxygen as fuel. At that point, the body becomes far less efficient.
No matter what the sport, endurance athletes have the highest anaerobic thresholds. And training can make a big difference. Doing intervals with the goal of hovering just above and below the threshold level increases an athlete's ability to resist accumulating lactate in the blood.
"Stage racers that are used to doing mountain-climbing definitely have really high anaerobic thresholds," Gallagher said. "They are able to sustain a lot of power over longer periods of time."
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As for power, Tour cyclists can maintain outputs of between 400 and 450 watts without conking out. Those levels are a good 100 watts higher than what decent competitive cyclists can sustain before hitting their anaerobic thresholds during races. It's hard to know how those levels compare to other kinds of sports, where power isn't so easily measured.
Tour riders have larger hearts than average athletes as a result of training. They are probably also at the top of the list on measures of mental toughness.
For title of Best Athletes in the world, though, cyclists have some close competition. Decathletes, for example, compete in an unbelievable variety of sports at just under elite Olympic level.
Gymnasts are superior when it comes to agility, body control and strength-to-weight ratios. Elite runners rank high on measures of endurance, VO2 max and lactate threshold. Soccer players are good, all-around athletes. And with plenty of both strength and endurance, members of the U.S. Special Forces have Gallagher's vote.
"Everyone has their pet favorite sports that they think are going to produce the greatest, most fit athletes in the world," Minson said. "Until everyone can come to a consensus about what fitness is, then we're never going to be able to answer that question."