The sternocleidomastoid (SCM) is a muscle of the neck so-named because it originates on the sternum (sterno) and the clavicle (cleido) and inserts on the mastoid process (mastoid) which is an easily located bony prominence behind the ear (The mastoid process also serves as an attachment for the posterior belly of the digastric, splenius capitis, and longissimus capitis muscles.) The muscles pass diagonally across the front and side of the neck beginning at the top of the sternum and ending behind the ear. This two-sided muscle is large and ropy, making it the most prominent muscle visible at the front of the neck.
The muscle is made up of two branches: The medial or sternal branch, which is directed superiorly, laterally, and posteriorly and the lateral or clavicular branch which is directed almost vertically upward.
The right side rotates the head to the left and flexes it to the right. The left side rotates the head to the right and flexes it to the left (When a muscle causes lateral flexion to the side it is on this is referred to as ‘ipsilateral flexion’ when a muscle rotates a segment to the opposite side this is called ‘contralateral rotation.) Both sides together flex the neck and head forward. The SCM is also an important accessory muscle of inspiration (respiratory inhalation) and is highly active during costal (high chest) breathing especially during rapid breathing. 1Costa D, et al. “Participation of the sternocleidomastoid muscle on… [Electromyogr Clin Neurophysiol. 1994 Jul-Aug] – PubMed result.” Web. 17 Oct. 2010.,2Kendall, Florence P., Et Al. Muscles Testing and Function with Posture and Pain. Baltimore, MD: Lippincott Williams & Wilkins, 2005. Print. With habitually faulty forward head posture the SCM, along with the scalenes, may develop shortness. This will in turn place an overload strain on the muscles that help control forward neck flexion.
Sternocleidomastoid Origin, Insertion, and Action
Origin: Manubrium of the sternum and medial clavicle.
Insertion: Mastoid process, occipital prominence behind the ear.
Action: Both sides flex the head and neck forward. The left side causes rotation of the head to the right and lateral flexion to the left. The right side causes rotation of the head to the left and lateral flexion to the right.
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|1.||↲||Costa D, et al. “Participation of the sternocleidomastoid muscle on… [Electromyogr Clin Neurophysiol. 1994 Jul-Aug] – PubMed result.” Web. 17 Oct. 2010.|
|2.||↲||Kendall, Florence P., Et Al. Muscles Testing and Function with Posture and Pain. Baltimore, MD: Lippincott Williams & Wilkins, 2005. Print.|