On Jun 20, 8:53 am, Larry wrote:
Back when I jogged (before discovering roller skiing), I generally
fared better in terms of endurance, although I never really exerted
myself too much. I also do some strength training (sit-ups, push-ups,
pull-ups) a couple of times a week. My resting heart rate is about
60-65 per min. I think what's different with skiing is that the upper
body is much more actively involved, even at lower speed. When
running, I generally stay with breathing in and out on 4 steps, or 3
steps when accelerating. When skiing, I try to exhale when I pole to
maximize the push-off. By definition, my breathing cycle accelerates.

A major part of it is that you are now using both arms and legs to
propel yourself. The more muscles involved the more oxygen is
required. But I'm betting an even bigger reason for a reasonably fit
person is that you just can't relax on rollerskis and possibly
technique inefficiencies. You didn't say how accomplished a skater
you are but When I first began I couldn't skate for more than 100 yds
with dying of exhaustion. After a few months of practice and lessons
I could ski 10k non-stop with no improvement in fitness. Muscle
tension is a great waster of energy. You need to relax and have
efficient technique. I see this with a lot of my students in the
instructional club I run. They can not relax enough and their balance
is not good enough to allow them to use the aerobic fitness they
have. Work on balance and relaxation. That is harder on rollerskis
than on snow however