Why do my knees hurt walking downhill but not uphill?

More stress downhill: It is not uncommon for someone who may have some early arthritic changes underneath their kneecaps to feel more pain descending stairs or going downhill. There is more stress place on the articulation between the kneecap and the end of the thigh bone going downhill. You may want to think about using hiking poles if you are going to walk uneven routes, you will get an upper body workout as a bonus.

Patellofemoral: Often kneecap pain will cause more pain going down hill.