Question

multiple choice question

what is the most common unilateral action of the sternocleidomastoid?

contralateral head rotation
drawing the head straight forward and down
swallowing
lateral excursion of the mandible

Explanation:

Analyze the sternocleidomastoid muscle actions

Using the Sternocleidomastoid Muscle knowledge point

The sternocleidomastoid (SCM) muscle is a prominent, paired muscle in the superficial layer of the anterior neck. It has two heads of origin: the sternal head (manubrium of the sternum) and the clavicular head (medial third of the clavicle). It inserts onto the mastoid process of the temporal bone and the superior nuchal line of the occipital bone.

Determine unilateral action

Using the Sternocleidomastoid Muscle knowledge point

When only one SCM muscle contracts (unilateral contraction), it pulls the mastoid process of that side anteroinferiorly toward the sternum and clavicle. This mechanical pull causes:

• Ipsilateral lateral flexion (tilting the head toward the same shoulder).
• Contralateral rotation (turning the face/head to the opposite side).

Therefore, the most common unilateral action listed is contralateral head rotation.

Evaluate the other options

Using the Sternocleidomastoid Muscle knowledge point

• "Drawing the head straight forward and down" (flexion of the cervical vertebrae) is a bilateral action of the SCM muscles, not a unilateral action.
• "Swallowing" is primarily performed by the suprahyoid, infrahyoid, and pharyngeal constrictor muscles.
• "Lateral excursion of the mandible" is performed by the pterygoid muscles.

Answer:

• Contralateral head rotation (Correct answer)
• Drawing the head straight forward and down
• Swallowing
• Lateral excursion of the mandible