Spinning is a little easier on your legs and shifts some of that work to your lungs and cardio system. Lance had a huge heart so he could pump more O2 to his muscles and he concentrated on aerobic efficiecy quite a bit after cancer. Your cardio (lungs) can also recover quite a bit faster then your legs can. This is why you can see lance dancing up the mountain stages in those videos, he bridges a gap, rests for 20 seconds, then he's off again. I'm sure someone else explained the science and details behind this alot better then me