What action does the splenius capitis perform?

The splenius capitis primarily functions to extend the head and neck, allowing the upward tilt of the head. It also facilitates lateral flexion of the neck, which involves bending the head sideways towards the shoulder of the same side. Additionally, the muscle is responsible for ipsilateral rotation, meaning it rotates the head towards the same side as the muscle that is contracting.

Overall, these actions—extension, lateral flexion, and ipsilateral rotation—combine to enable movements that are essential for various head and neck positions, including those required for looking over the shoulder, tilting the head back, or turning the head to the side. This comprehensive movement capability makes the splenius capitis crucial for activities that involve head and neck mobility.