What is the action of the splenius capitis?

• A. Extends the head and neck; rotates head to the same side.
• B. Flexes the head forward and rotates opposite side.
• C. Adducts the head toward midline.
• D. Elevates the scapula.

Answer: (A)

The splenius capitis primarily acts to extend the head and neck when both sides contract, and to rotate the head to the same side when it contracts unilaterally. This combination—bilateral extension and unilateral ipsilateral rotation—fits the muscle’s orientation, which runs from the neck back to the skull to pull the head backward or twist it toward the same side. The action described as extending the head and neck and rotating the head to the same side is the best match.

Context: the splenius capitis originates from the lower nuchal ligament and the spinous processes of the upper thoracic vertebrae, inserting on the mastoid process and the superior nuchal line. Its fibers are aligned to produce extension when both sides work and ipsilateral rotation when one side works. By contrast, flexing the head forward or rotating to the opposite side are actions more characteristic of other muscles (such as the sternocleidomastoid for flexion and opposite rotation, or muscles that elevate the scapula).