I have a lot of knee pain if I am not consistent with my cycling. For example if I ride really hard for a few days and then not at all for a few more, I will end up with pain. For me, riding more usually helps it. Ultimately in my case it is some muscles are tighter than others in my legs and this causes my knee joint to be put under strain. Going up stairs can be particularly hard a times but if all is in balance, it is fine.