Runner's knee causes pain in the front of the knee, around the kneecap. The pain can happen with activity or after prolonged sitting with the knees bent. Sometimes the knee will seem to give way. The knee may seem to lock when the rough surfaces of the kneecap and femur catch as they rub over each other.
Sometimes a rubbing or clicking of the kneecap against the femur can be heard when the knee is bent and straightened. The edge of the kneecap may be tender. The knee may contain extra fluid, called an effusion. In some cases, the kneecap tends to slide toward the outside of the knee.
When the pain lasts a long time, the quadriceps muscle in the thigh can atrophy, or become smaller.