I suffer pain in my knees coming down hill. The problem is the force on my heel transferring its way up my leg. This is called heel strike. It's a problem on the way down because I'm moving faster. I have the same mass but greater speed means more momentum. There is a bigger change in momentum in approximately the same time as I put my foot down. Bigger rate of change of momentum means bigger force which hurts. The moral of the story is to slow down!
