Final answer:
Reciprocal inhibition is the term that refers to the simultaneous contraction of one muscle and the relaxation of its antagonist. It allows for smooth and efficient movement.
Step-by-step explanation:
The term that refers to the simultaneous contraction of one muscle and the relaxation of its antagonist is reciprocal inhibition. Reciprocal inhibition occurs when the agonist muscle (the muscle that is contracting) sends signals to inhibit the antagonist muscle (the muscle that is relaxing) through the nervous system. This coordinated action allows for smooth and efficient movement.
For example, when you flex your elbow to lift a book, the biceps brachii muscle contracts (agonist) and the triceps brachii muscle relaxes (antagonist). This reciprocal inhibition allows for the efficient movement of your arm.