A tendon reflex is an automatic response that occurs when a muscle is stretched, triggering a reaction to prevent injury. This reflex helps maintain muscle tone and posture by quickly contracting the muscle when it senses excessive tension. The most common example is the patellar reflex, where tapping the knee tendon causes the leg to kick. The process involves sensory receptors called muscle spindles that detect changes in muscle length. When the tendon is stretched, signals are sent to the spinal cord, which then sends a signal back to the muscle to contract, ensuring that the muscle does not overstretch or tear.