I've always heard if it hurts in the front of your leg/knee, your saddle is too low. If it hurts in the back of your leg/knee, the saddle is too high. It's a generalization, but at least it's a starting point. Here's a couple of questions to think about: When your foot is at the bottom of your pedal stroke, is your leg so fully extended that your knee is almost locked out? When you pedal at a high cadence, do you notice your hips rocking back and forth at all? If either of those things, your saddle is definitely too high. Even if not, I would try lowering the saddle by a small amount. You don't ever want to make a big change. Maybe go 1/4-1/2 inch, but no more and see if that helps. And as Veronica said, don't mash those pedals. Spin at a higher cadence and use all those nice gears on your bike. Good luck!

annie