I tend to agree with RealRyder. Sure, I tend to use spin classes as interval workouts, because it's easier for me to push myself in intervals inside than on the road. Even so, and while I attempt to get a spin bike to feel as close to my road bike as possible with the position, I still find that spin classes overload my quads. Standing is more quad intensive than remaining seated on any bike, but spinning in general (I do not stand much even indoors) really pumps up my quads. It just seems harder to engage the hams, glutes, and even calves the same way as on a road bike. Maybe some of that has to do with the enormously wide spin saddles or maybe it's a larger biomechanical issue with spin bikes...I'm not sure.