How does an antagonist muscle pair work?

• A. They perform identical actions.
• B. They contract together.
• C. They lengthen at the same rate.
• D. They are opposite of one another.

Answer: (D)

An antagonist pair consists of two muscles that have opposite actions around the same joint. They are opposite of one another, so when one contracts to produce a movement, the other lengthens or relaxes to allow it. For example, at the elbow, contracting the biceps brachii bends the elbow while the triceps brachii lengthens to permit the bend. When you straighten the elbow, the triceps contracts and the biceps relaxes. This opposite setup lets you control speed and range of motion and helps stabilize the joint; sometimes both muscles contract together briefly to hold a joint steady.