How do we objectively measure "harder"?
Scientifically, we'd need to hook all subjects to the equipment that can measure everything,
from caloric output to mental stress.
And all subjects need to be measured while competing or pushing at the highest levels
possible.
Studies that have done well indicate that soccer and motocross are on top. But those studies
that I'm familiar with are decades old. I don't know of any study that has been done to this
level on MTB'ers. And what type of riding are we testing? We'll get vastly different results
from, say, a pro DH'er and a pro Endurance Racer.
I used to race Enduros competitively on a national level and I've done a fair share of motocross
riding as well. In my opinion, the type of MTB'ing I do now is more physically demanding
than either of those throttle-twisting gigs.
Anyway, Google is your friend - not mtbr. Well, ok...mtbr in an indirect way because I am
giving the answer here:
http://sports.espn.go.com/espn/page2/sportSkills