The stretch reflex is a simple, automatic response that occurs when a muscle is stretched. This reflex helps maintain posture and balance by quickly contracting the muscle to counteract the stretch. For example, when the knee is tapped during a physical examination, the leg kicks out due to this reflex. The stretch reflex involves sensory receptors called muscle spindles, which detect changes in muscle length. When a muscle is stretched, these spindles send signals to the spinal cord, which then activates motor neurons to contract the muscle. This process is crucial for activities like walking and running, ensuring smooth and coordinated movements.