Final answer:
Muscles act in pairs, with one contracting and the other relaxing to enable movement; thus, the correct answer is D. Contracting and relaxing.
Step-by-step explanation:
Muscles work in pairs to produce movement by contracting and relaxing. When one muscle of the pair contracts, the other relaxes, allowing joints to move. For instance, when you bend your elbow, your biceps contract (shorten), while your triceps relax (lengthen). The correct answer to the question is D. Contracting and relaxing. When muscles work in pairs, the contracting muscle, known as the agonist, shortens to execute a movement.
Conversely, the opposite muscle, known as the antagonist, relaxes and lengthens to allow the movement to occur. This is essential in different types of isotonic contractions: concentric contractions involve muscle shortening, whereas eccentric contractions occur when the muscle lengthens during relaxation. For example, when the biceps muscle contracts, it causes the arm to bend at the elbow joint, and when the triceps muscle contracts, it causes the arm to straighten. So, the correct answer is D. Contracting and relaxing.