There are sports that require physical pounding (football),
psychological toughness (tennis), and mind-blowing hand-eye
coordination (baseball), but the sports that demand the very
highest physiological output, by my estimation, are cycling and
cross-country skiing. Both have some of the highest caloric
requirements – 6,500 a day for your average Tour de France rider
and more than 1,000 an hour for a competitive cross-country
skier. They also boast the top two VO2 maxes (a measurement of
how much oxygen a body uses during exercise) ever recorded, and
share bragging rights for Fittest Person on the Planet. The
current record holder for VO2 max is pro cyclist Oskar Svendsen,
and I'm not surprised his sport tops the list: I find fever-
pitch bike racing to be the hardest imaginable sport, in which
you are constantly pressed to push the furthest limits of your
body.