Trigger Points in the quadriceps muscle can cause knee pain.  One sign that this may be the case is that the pain only happens during certain activities such as running or walking up stairs.  Using a foam roller you can not only locate trigger points but use it to perform self myofascial release.  Place the foam roller between the floor and your quad.  Move back in forth over the quad looking for tender spots.  When a spot is found, hang out there for about two minutes moving the roller back and forth over the spot.  Move on to the rest of the quad looking for other spots.  Remember, trigger points can cause pain far away from where they are located so make sure you check the whole quad all the way up your thigh.