Originally Posted by Bode Klammer
Running helps maintain bone density, and it develops the back muscles of the leg, which helps prevent ACL sprains. Cyclsts can lose bone density, and develop weak upper bodies and stiff backs. Running is also less time consuming, and running injuries are far less serious than cycling injuries. I do both, but if I could only do one it would be running.
Trying to reproduce the movement patterns of skiing is a fool's mission. If you could train every muscle in your body to be as strong as Bode Miller's or Lindsay Vonn's, you still couldn't ski like them. Ski fitness is about avoiding injuries, not about skiing better.
Recreational skiing does not require unusual levels of strength or fitness. Unless you compete at a very high level (in which case you need a coach), all you need to do is correct all the damage done by sitting at your desk all day. The best exercise program is one you enjoy and stay with.
Long distance runners have good hamstring ratios, but that's in part because the rest of them is so f'in atrophied. You are correct about bone density, but not about upper body strength as it relates to MTB. The loss of bone density for recreational cyclists who do other things as well hasn't been shown to be an issue, much less for mountain bikers who if they do keep an emphasis on riding, not on long aerobic epics, will bear far more forces than roadies. Trial or scree running certainly have the benefits of weight bearing exercise that you mention, but also allow you to better develop you lower body, and work movement patterns more than "regular" running.
I agree with your point that recreational skiing does not require unusual strength or fitness, and also that anything that will keep you active is way better than something that you do for 6 weeks and give up. I know people who own and maintain horses who basically just stay fit riding and shoveling, and moving hay, which actually is fairly varied when you think about it.
Where I'd disagree is with the assertion that you can't benefit from motion sports that are somewhat related to skiing. If someone goes from not sliding or rolling on anything to trying to ski for 10 or so days a year, that's a pretty big shift. A good analogy is roller skiing for nordic skating. While they are two separate activities and the technique varies subtly, if you can roller ski, you can do ok nordic skating the first day. If you either sit at a desk or just run to stay in shape, and then try to nordic skate a few days a year, you'll perpetually be at the beginning end of the curve. I just got back from a MTB ride, where I used edging, fore-aft weight transfer, the bike version of angulation, terrain absorption, and a variety of other skills that carry over.
The best carryover is imo actually from using specialized skates designed to mimick skiing, and next from inlining, but as many people don't do these and find MTB more accessible, it's good to flag MTB for its benefits.