Which muscle is a prime mover for neck extension?

The splenius capitis muscle originates along the lower half of the nuchal ligament, spinous process of the seventh cervical vertebrae and the upper third or fourth thoracic vertebrae which insert into the mastoid process of the temporal bone. The action of the splenius capitis is to extend the neck posteriorly as well as assisting with lateral rotation and lateral flexion of the cervical spine.