Sternocleidomastoid

Origin = Superior aspect of the sternum and the inner third of the clavicle, by two heads.

Insertion = Mastoid process of the temporal bone.

Action =

One side - Lateral flexion of the cervical spine; rotation of the atlantoaxial joint and cervical spine to the opposite side.

Both Sides - Flexes the atlanto-occipital joint; if flexion continues, flexes the cervical spine.