When running downhill, I usually try to avoid hard heel strikes which indicates that you are leaning back too much and overstriding with your knees too straight or locked and a lot of shock transmitted to your legs, knees, and hips. I always try to land midfoot or forefoot with my knees flexed. A small gentle heel strike followed by a quick roll onto the forefoot is okay if forcing a midfoot landing is not comfortable. Gravity also provides the power when going downhill. The main thing to do to prevent injury and hurting your quad muscles when running hills is to minimize the pounding of downhill running by leaning forward and avoiding hard heel strikes. The arms are used to maintan the balance and rhythm (speed). I don't run fast when going downhill, the faster you go the more painful your quad muscles will get later on. My thoughts!