Which movement best describes the unilateral action of the sternocleidomastoid?

• A. Rotates the head and neck to the opposite side
• B. Flexes the lumbar spine
• C. Assists in breathing by elevating the ribs
• D. Abducts the arm

Answer: (A)

Unilateral contraction of the sternocleidomastoid causes the head to rotate toward the opposite side. The muscle’s fibers run diagonally from the sternum and clavicle up to the mastoid process, so when one side pulls, it twists the head to the other side, often with a bit of lateral flexion. This is the movement described by rotating the head and neck to the opposite side.

The other descriptions don’t fit the unilateral action: bending or flexing the lumbar spine isn’t a function of this neck muscle; lifting the ribs to aid breathing is a bilateral action of the muscle during deep inspiration; and abducting the arm involves shoulder muscles, not the sternocleidomastoid.