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- The sternocleidomastoid attaches from the sternum and medial clavicle to the mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occiput.
- The sternocleidomastoid flexes the lower neck, extends the head and upper neck, and laterally flexes and contralaterally rotates the head and neck at the spinal joints. It also elevates the sternum and clavicle.
- The SCM is unusual in that it crosses the cervical spinal joints anteriorly, so it flexes the neck; but crosses the atlanto-occipital joint posteriorly so it extends the head.
- The sternal head is more involved with contralateral rotation; the clavicular head is more involved with lateral flexion.