Hmmm - I just might have to buy a mountain bike.
Anyway, it looks like you could potentially atrophy your lower body muscle with steady, intense biking. However, if you're just trying to lose the fat off the top of the muscle (like I'm trying to do), you're probably better off sticking with HIIT cardio - either sprints or HIIT on the bike/elliptical, building overall muscle to increase your resting metabolism, and keeping a clean diet.
The question you need to answer is do you really want to atrophy your lower body muscle or do you just want to lose the fat off the top of the muscle?