Why must skeletal muscles work in pairs to create movement?

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Illustration of the human muscular system from a side view, showing detailed muscles throughout the body with labels identifying major muscles in the head, torso, arms, and legs. — human muscle system Lateral view of the human muscular system.

Skeletal muscles must work in antagonistic pairs to create movement because muscles can only contract, not expand. When one muscle in a pair contracts to move a limb in one direction, its opposing muscle must relax to allow that movement. Then, the opposing muscle contracts to move the limb back. This coordinated action of contracting and relaxing muscles allows for controlled and precise movements. For example, the biceps muscle in the upper arm contracts to bend the elbow, while the triceps muscle relaxes. To straighten the arm, the triceps contracts, and the biceps relaxes.12