The sternocleidomastoid muscle separates the anterior and posterior triangles of the neck and functions as the head's principal rotator.
The sternocleidomastoid muscle is in charge of rotating the head in an oblique or opposing direction. The neck is flexed as well. Together, they cause the head and neck to expand. The SCM may become weak or paralysed if the spinal accessory nerve is damaged or impaired. This may make it difficult to bend the neck and turn the head because to the affected SCM's atrophy (shrinking).
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