Reciprocal inhibition involves relaxation of one muscle when its antagonist contracts. Which statement best describes this mechanism?

• A. The agonist contracts and the antagonist relaxes
• B. The antagonist contracts to produce movement
• C. Both the agonist and antagonist contract simultaneously
• D. The agonist relaxes while the antagonist contracts

Answer: (A)

Reciprocal inhibition is the process where, as one muscle contracts to produce movement, its opposite muscle—the antagonist—relaxes to allow that movement to occur. This happens through neural wiring in the spinal cord: when the agonist is activated, inhibitory signals are sent to the antagonist’s motor neurons, reducing its activity so it does not oppose the movement. For example, when you bend the elbow, the biceps (the agonist) contracts and the triceps (the antagonist) relaxes, letting the elbow flex smoothly. If the antagonist were to contract or both muscles contracted together, movement would be hindered or unstable, which is not how this mechanism operates.