A stretch reflex is a quick, automatic response of a muscle to being stretched. When a muscle is stretched, sensory receptors called muscle spindles detect the change in length and send signals to the spinal cord. This triggers a reflex action that causes the muscle to contract, helping to maintain posture and balance. One common example of a stretch reflex is the knee-jerk reflex. When a doctor taps the patellar tendon just below the kneecap, it stretches the quadriceps muscle. This stretch activates the muscle spindles, resulting in a rapid contraction of the quadriceps, causing the leg to kick forward.