VO2 max varies with protocol though. XC skiing will tend to give higher VO2 due to more muscle recruitment (arm and upperbody).1) swimming - I die if I fail
2) running - bones and ligaments just break apart.
3a) xc skiing - always has the highest v02 maxes
3b) rowing - right there with skiing
5) cycling - can coast for days.
Of course it depends what kind of boat you're rowing, but if it's a racing shell, you're on a moving seat and use both upper and lower body during the rowing stroke.2) Rowing - Rigorous upper body workout for sure.
Underspecified question. By "hardest" do you mean most difficult to master? Or are you talking about physical exercise? If the latter, then the question makes no sense because you can work as hard or as easy as you want in any of these. Cycling and running engage the legs almost exclusively, so the other three work more muscles but they don't work the cardiovascular system any harder.Rate the level of difficulty of the following endurance sports in order of most difficult to least: running, xc skiing, rowing, cycling, swimming
2. xc skiing