The most common long-term running injury is runners knee, pain behind the knee cap during running. You probably have runner's knee if your knee cap hurts when you walk or run, particularly when you walk down stairs; and it hurts a lot when you push the kneecap against the bone behind it. It usually does not hurt to pedal a bicycle. The back of the kneecap is shaped like a triangle with the point fitting in a grove in the lower part of the bone behind it. During running, the knee cap is supposed to move up and down and not side to side. If it moves from side to side, the back of the kneecap will rub against the front bottom of the femur, the long bone of the upper leg, causing pain.