The main purpose of the examination of the sternocleidomastoid and trapezius muscles is to assess for dysfunction of the spinal accessory (eleventh) nerve.

Testing of the sternocleidomastoid and trapezius muscles does not need to be performed routinely, but they should be tested in the rare situation when a unilateral lesion of the spinal accessory nerve is suspected. It is also appropriate to test the function of these proximal muscles when assessing for a myopathy or other diffuse neuromuscular process.

The sternocleidomastoid and trapezius muscles are innervated by the spinal accessory nerve (the spinal portion of the eleventh cranial nerve); this is an unusual cranial nerve because its nuclei originate in the high cervical cord rather than the brainstem. The left sternocleidomastoid muscle turns the head to the right, and the right muscle turns the head to the left; the trapezius muscles shrug the shoulders upward.