Which phenomenon involves reflexive contraction in the agonist muscle and relaxation in the antagonist muscle due to stretch of the muscle spindle?

• A. PIR
• B. Reciprocal inhibition
• C. Autogenic inhibition
• D. MET

Answer: (B)

When a muscle is stretched, the muscle spindle sends a rapid signal that causes the stretched (agonist) muscle to contract to resist the stretch, while the opposing (antagonist) muscle is inhibited so it relaxes. This coordinated response is called reciprocal inhibition. The Ia afferent from the spindle excites the agonist’s motor neurons and, through an inhibitory interneuron, inhibits the antagonist’s motor neurons, allowing smooth, rapid adjustment of limb position.

Other options don’t fit this pattern: PIR involves the same muscle relaxing after an isometric contraction (autogenic inhibition) rather than relaxing the antagonist. Autogenic inhibition specifically reduces tension in the same muscle via the Golgi tendon organ. MET is a manual technique, not a reflex mechanism.