Tapping on the lower knee ligament results in a knee-jerk reflex, which is an involuntary popping up of the lower leg. The approximate location of the infrapatellar ligament is below the knee, in a slightly concave fossa. When the knee is half-bent and the calf is naturally hanging down, a normal person will complete a “knee-jerk reflex” due to sound nerve reflexes, i.e., the calf will involuntarily spring up to a small extent, and then naturally fall back down. When tapping on the ligament below the knee, the muscle shuttles in the muscle sense a certain traction and upload this signal to the center of the knee-jerk reflex, which then innervates the effector, i.e., the quadriceps muscle, to contract, thus generating an upward movement of the lower leg. The knee-jerk reflex is a normal reflex phenomenon of the human body, if there is any abnormality, we should consult a doctor in order to clarify the cause.