The knee reflex, also known as the patellar reflex, is an involuntary response that occurs when the knee is tapped just below the kneecap. This action stretches the quadriceps muscle, which sends a signal through the spinal cord to the nerves that control the muscle. As a result, the quadriceps contract, causing the lower leg to kick forward. This reflex is a simple example of how the nervous system works to protect the body. It helps maintain posture and balance by allowing quick reactions to stimuli. The knee reflex is often tested by doctors during physical examinations to assess the function of the nervous system.