Originally Posted by Phlogiston
Actually, they started bothering me as a result of driving my knees forward into the front of my boots and making a lot of shorter turns across the fall line.
When I kept my weight further back and my legs straighter, made bigger turns and moved faster down the slope, I never had quad problems.
I decided this season to make more turns, and keep my speed down, and the quad problems started.
Are you sure your knees are forward all the time when they are flexed? That is, has someone who knows what to look for observe your turns and body movement. I say that because I thought I was forward too but I really wasn't.
Another thing that may be happening that is related to the above is you may be over flexing your lower body (up/down instead of lateral) when you're turning -- the old school weighing and unweighing.
I am no pro for sure but I can definitely relate to your pain because burning (and cramping in some cases) quad has always been an issue. I know I got weak thighs (experience the same when I mountain bike) but being in a more proper stance and making the correct moves have lessened the pain and lengthened my runs without rest considerably.
As for a remedy, you're certainly on the right track with your strengthening excercises (although I'm too lazy to do them). One that worked well for my pre-teen skier/gymnast daughter when she was going through Osgood-Schlatter and tendonitis (they were quad related) was to lay flat on a mat and have one knee folded while having the entire leg lifted off the ground as much as possible (best if assisted). Kind of like this diagram but with the knee lifted. http://www.exrx.net/Stretches/Quadri...roneLying.html
Hope that helps.