When the stretch reflex is activated in the quadriceps, what occurs?

When the stretch reflex is activated in the quadriceps, the primary response is contraction of the quadriceps muscle. This reflex mechanism occurs in response to an unexpected stretch of the muscle, which is detected by muscle spindles—sensory receptors located within the muscle.

Upon detecting the stretch, signals are sent to the spinal cord, where a reflex arc leads to an immediate response. Motor neurons are activated, causing the quadriceps to contract to counteract the stretch. This contraction serves to protect the muscle from excessive stretching and potential injury, helping to maintain stability and posture.

In contrast, the other responses—relaxation, lengthening, or stretching of the quadriceps—do not accurately describe what happens during the stretch reflex. The mechanism is designed to enhance muscle performance and prevent overstretching, so contraction is the key outcome in this reflex action.