To test your knee jerk reflex, a doctor or nurse practitioner raps a small hammer on the tendon below your kneecap.
A split second later, when you kick the person with the hammer, it's because the lightning-quick knee jerk reflex occurred within the spinal cord. The kick simply indicates that a section of your spinal cord, and the nerves extending from it, are working properly. Testing a series of different reflexes gives an indication of the health of your whole nervous system.
Now let's follow the path of the knee jerk reflex to see how it works. The tendon below your kneecap connects to the quadriceps muscle on top of your thigh. The hammer's rap on the tendon slightly stretches the muscle. Sensory nerves in the muscle are stimulated by the stretching and send an impulse to the spinal cord. Motor nerves in the spinal cord then conduct the impulse right back to the quadriceps, triggering a muscle contraction that makes you kick.
What's the point of this reflex?
When we stand upright, our muscles constantly stretch and contract slightly, just to keep us balanced. The knee jerk reflex is part of this system. Let's say you're standing, and start to lean back too much. Leaning back stretches your quadriceps and triggers the reflex. However, the muscle contraction won't kick your leg upward this time, because you're standing on it. Instead, the contraction simply brings you back to center, preventing you from falling backward.