Which of the following is an example of the stretch reflex?

The knee jerk response, commonly demonstrated during a physical exam when a physician taps below the kneecap, is a classic example of the stretch reflex. This reflex is an involuntary response that occurs when a muscle is stretched. Specifically, when the patellar tendon is tapped, it stretches the quadriceps muscle, stimulating sensory receptors known as muscle spindles. This leads to an immediate contraction of the quadriceps in response to the stretch, resulting in the leg kicking outward.

This reflex serves as an important diagnostic tool to assess the integrity of the nervous system, specifically the spinal cord and peripheral pathways involved in reflex actions. The rapid nature of this reflex illustrates the body's ability to respond swiftly to stimuli, aiding in balance and posture control.

In contrast, the other examples revolve around strength training exercises, where movement and muscle contraction are intentionally controlled and do not exemplify the involuntary response characteristic of the stretch reflex.