The sternocleidomastoid aka SCM, muscle originates on the Sternum and attaches to the Clavical and the Mastoid process of the temporal bone on the back of the skull. When acting alone it rotates to the opposite side and slightly (laterally) flexes to the same side. It also works in with the posterior Levator Scapula on the opposite side of the contracting SCM. If turning the head to the right the Left SCM pushes the head to the right where as the Right Levator Scapula pulls the head to the right. When the SCM is fully inflamed, it will become stiff and also might involve the Levator Scapula making the neck completely stiff and painful to turn the head.
When Inflamed: Pain will travel from the neck where it will be tender to the touch and refer pain over and around the eyes and back of the skull on the occipital bone.
RX: The SCM is difficult to ice because for the carotid artery and nerves, so take anti-inflammatories like Advil or Aleve and try to rest the neck. If injured in a whiplash accident you must wear a neck brace to support the head and neck taking tension off the SCM.