An example of muscles working in pairs.

• A. Quadriceps & Hamstrings
• B. Deltoid & Pectoralis
• C. Biceps & Triceps
• D. Heart & Lungs

Answer: (C)

Muscles work in opposing pairs to move a joint. When one muscle contracts (the mover) the opposite muscle (the antagonist) relaxes to allow the movement, and they swap roles to reverse the motion. The biceps brachii is the mover for bending the elbow, while the triceps relaxes; to straighten the elbow, the triceps contracts and the biceps relaxes. This paired action enables smooth, controllable movement of the arm. While other pairs like quadriceps and hamstrings also illustrate this idea at the knee, the biceps and triceps are the most classic example used to show how opposing muscles create and reverse joint motion. The heart and lungs aren’t a paired muscular system, and the deltoid and pectoralis major aren’t the straightforward antagonist pair for a single joint in the same way.