- Cyclists who compete in the Tour de France are among the fittest athletes on Earth.
- Experts are still debating how to define fitness.
- Instead of VO2 max, measures of the ability to sustain high amounts of effort might be the best measure of fitness.
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.