motocross like racing? its tough, you need to keep control of your bike, but not like skiing COULD be
skiing COULD be (like others said) hiking or skinning up a huge, steep face, in the high altitude mind you, and the skiing down. while skiing down, you use your abs to your toes to control those skis. And unlike motocross, when you hit a cliff or a jump, you don't have shocks, you have knees.
if you've ever tried downhill racing, it also takes a lot out of you. not only do you have to control skis while going 80mph, but you stay in a tuck and your thighs burn to the point of tears