Tour de France riders move oxygen around their bodies really
fast! According to this article, Are Tour de France Riders Healthy, over on bicycling.com, Tour de France riders are very fit. In fact
about twice as fit as the average non-Tour rider of the same age range and who’s in good shape.
That’s because they have a higher VO2max than you or I. VO2 max is a respected measure of fitness and it basically means breathing as hard as you can. According to active.com there are two factors which affect our VO2 :
“One is a strong oxygen transport system, which includes a powerful heart, hemoglobin-packed blood, high blood volume, high capillary density in the muscles, and high mitochondrial density within the muscle cells.The other is speed, or the capacity to contract a large number of muscle fibers simultaneously, as the more muscle tissue is active at any given moment, the more oxygen the muscles demand.”
So these riders not only have strong heart and respiratory systems, they also have really effective, responsive muscles.