Which muscle, when acting together, flexes the neck, and when acting alone, rotates the head?

Which muscle, when acting together, flexes the neck, and when acting alone, rotates the head?

• A. Sternocleidomastoid
• B. Trapezius
• C. External Oblique
• D. Rectus Abdominis

Answer: (A)

The sternocleidomastoid is the muscle that fits this pattern. When both sides contract, it pulls the head and neck forward, flexing the neck. When it works on one side alone, it pulls the head to the same side and, because of its oblique direction, rotates the head to the opposite side. That combination—bilateral neck flexion and unilateral head rotation—is characteristic of the sternocleidomastoid. The trapezius mainly moves the shoulder blade and can assist in neck extension, not the specific neck-flexion/contralateral-rotation pattern described. The external oblique and rectus abdominis are abdominal muscles that flex and rotate the trunk, not the neck, so they don’t produce this neck action.

The sternocleidomastoid is the muscle that fits this pattern. When both sides contract, it pulls the head and neck forward, flexing the neck. When it works on one side alone, it pulls the head to the same side and, because of its oblique direction, rotates the head to the opposite side. That combination—bilateral neck flexion and unilateral head rotation—is characteristic of the sternocleidomastoid.

The trapezius mainly moves the shoulder blade and can assist in neck extension, not the specific neck-flexion/contralateral-rotation pattern described. The external oblique and rectus abdominis are abdominal muscles that flex and rotate the trunk, not the neck, so they don’t produce this neck action.