I notice that a lot of times while hiking my knees start to hurt more while walking downhill than uphill. While hiking uphill is pretty rough on the leg muscles, the knee doesn't get as much strain. However, with downhill, all of your weight is placed on the poor joints of your knees. Your feet are bent forward, and you are relying on your knees to prevent your body from tipping forward. It can actually be pretty tough to walk downhill if you have knee problems!