Hmmm, veddy interesting! Here's my unprofessional, unexpert opinion -- it's probably something about your bike fit. Maybe the fore/aft placement of your seat is in relation to your pedals?

I typically ride my mountain bike for about an hour after work most days of the week, and usually ride my road bike for a couple of hours on the weekend. Sometimes the road bike gets skipped and I'll ride the mountain bike on the weekend. When I do this, I notice that my ... ahem ... butt is more sore after riding the road bike. I think it has to do with sitting different on the different bikes. It's like it has to get used to riding in the saddle all over again.

-- Melissa

PS I'm not a doctor, and I don't even play one on TV, but I think you have your quads/hamstrings opposite. Hams are the back, quads are the front. Then again, I could be wrong. I should probably leave this one to the good folks on this forum who took more than basic bio and chem. (That's probably pretty much everyone!)