A tendon is a bundle of tissue that connects muscles to the bones that they move. A tendon reflex occurs when a tendon is stretched and too much stress or pressure is put on it. The brain sends a command that causes the muscle to relax to relieve the stress on the tendon. This helps prevent damage being done to the tendon due to things like lifting loads that are too heavy for the muscles to handle. One classic example is when the doctor strikes a patient's knee with a rubber hammer. The tendon at the knee is quickly stressed, and the tendon reflex causes the muscle to relax, thus the involuntary kick of the lower leg.