When you step up-down stairs, everybody thinks that only knee joints move. In fact, not only the knee joint but also the other joints. The picture showing joints move when step up stairs , the pelvic bone a little bit tilt during hip joint and knee joint bend up together and ankle joint flexed place down to the stair. That showing every joint in your body are link together . Therefore, if some joints cannot move well that can effect to other joints movement then make you feel painful.
Based on that information , it is possible the reason that the knee pain may come from the muscles. The tendons that connect to the knee joint that can cause an imbalance. This could be due to the muscle being too tight or weaker than the other side, or it could be in the directly joints moving is not good. In order to ,it must be examined by individual symptoms.