Action of Splenius capitis.

• A. Bilaterally extends head/neck; unilateral rotation and lateral flexion to the same side
• B. Extends trunk; lateral flexion to opposite side
• C. Elevates ribs
• D. Abducts scapula

Answer: (A)

Splenius capitis acts on the head and neck. When both sides contract, it extends the head and neck. When it contracts on one side, it rotates the head to the same side and laterally flexes the head/neck to that same side. This pattern fits the described action precisely: bilateral extension with unilateral rotation and lateral flexion to the same side.

Anatomically, it originates from the lower nuchal ligament and spinous processes of the upper thoracic vertebrae and inserts on the mastoid process of the temporal bone and the occipital bone, with innervation from the dorsal rami of cervical nerves. This muscle’s actions are distinct from those of other muscles listed: extending the trunk is primarily due to the erector spinae, elevating ribs relates to respiratory muscles, and abducting the scapula involves serratus anterior (and others), not the splenius capitis.