AcroPhile

I'm not saying that stance width is necessarily causing your problem but it is a good place to start. A lot of different aches and pains can be caused by poor adjustment of any of your gear. Boots too tight, straps of your bindings in the wrong position or too tight, high back in the wrong position (especially when it's too far forward), or stance width/angle can all cause all sorts of weird problems. Since you said your having problems with your upper legs boots or bindings are most likely not the problem but rather a stance issue is the most likely culprit. Of course, it can be that you aren't used to working those particular muscles and it is actually a matter of being out of shape.

HoboMaster

Even if your in decent shape, snowboarding uses specific muscles, and if you don't snowboard often those muscles don't get used that much. Bicycling helps because it targets those thighs, especially if you do stand-up hill climbs. It depends on the person too, but along with reducing the stance width, try a less angular stance. Having a 16' duck both ways kinda pushes both feet out. Try something like 15/-12 or 15/-9 or 12/-9, and see how that feels.

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