Sternal Head: Manubrium of sternum (superior and anterior aspect)
Clavicular Head: Medial quarter of clavicle’s superior surface
|Insertion||Mastoid process of temporal bone & anterior portion of superior nuchal line|
Unilateral contraction: Ipsilateral flexion and contralateral rotation
Bilateral contraction: Head/neck extension, head/neck flexion, & assists in inspiration by elevating clavicle and manubrium of sternum
|Nerve||Accessory nerve (CN XI)
Branches of cervical plexus (C2-C3)
Superior thyroid artery
Location & Overview
The sternocleidomastoid muscle (often abbreviated to SCM) is a neck muscle which is large, palpable and has an easily recognisable shape. The sternocleidomastoid muscle has two heads, a clavicular head and a sternal head. However, some sources break this division down further into four portions, the sterno-mastoid, sterno-occipital, cleido-mastoid, and cleido-occipital  .
The sternocleidomastoid muscle is usually stronger and thicker in men than in women . Approximately 65% of the sternocleidomastoid’s muscle fibers are comprised of fast twitch muscle fibers. The remaining fibers are slow twitch. This means the sternocleidomastoid is best at expressing a large amount of strength quite quickly and less adapt at endurance-based activities  .
Origin & Insertion
The sternocleidomastoid muscle originates from the superior edge of the manubrium and from the medial quarter of the superior surface of the clavicle. The two heads of this muscle (sternal and clavicular) merge to form a single muscle belly which continues up the neck in a superior and lateral direction. The sternocleidomastoid then inserts onto the mastoid process of the temporal bone and on the anterior portion of the superior nuchal line (located on the occipital bone of the skull)   .
Unilateral contraction the sternocleidomastoid muscle can assist in rotation of the head. This rotation causes the anterior (face side) of the head to turn away from the side of the muscle contracting (contralateral rotation). A unilateral contraction can also cause the neck to flex towards the side of the muscle which is contracting (ipsilateral flexion)  .
A bilateral contraction of this muscle, with the cervical spine in a rigid position (held in place by other muscles) can cause flexion of the cervical spine, causing the head to lean forward. If the cervical spine is not in a fixed position, the result of bilateral contraction will be extension of the head and bending of the of the cervical spine  .
The posterior edge of the sternocleidomastoid muscle is where the cutaneous branches of the cervical plexus emerge. The nerve endings assist the muscle in its proprioceptive functions. Moreover, the cranial nerve or XI passes into the posterior triangle to innervate the trapezius and the sternocleidomastoid muscle.
The sternocleidomastoid is also innervated by branches of the cervical plexus. The cervical plexus lies deep to the sternocleidomastoid muscle on the anterolateral aspect of the neck.
Blood is supplied to the sternocleidomastoid via the occipital artery and superior thyroid artery which are branches of the external carotid artery. Inferior to the sternocleidomastoid, is the jugular vein, which is where it drains venous blood  .
The middle third of the sternocleidomastoid muscle receives around 42% of its blood from the superior thyroid artery, 23% from the external carotid artery, and 27% from branches of both. In the majority of cases, the lower third of the sternocleidomastoid receives blood from a branch arising from the suprascapular artery (>80%).
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|↑1, ↑5, ↑6, ↑9, ↑11, ↑13, ↑15, ↑18||Bordoni B, Varacallo M. Anatomy, Head and Neck, Sternocleidomastoid Muscle. [Updated 2022 Apr 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532881/|
|↑2, ↑3, ↑7||Kennedy E, Albert M, Nicholson H. The fascicular anatomy and peak force capabilities of the sternocleidomastoid muscle. Surg Radiol Anat. 2017 Jun;39(6):629-645. doi: 10.1007/s00276-016-1768-9. Epub 2016 Nov 2. PMID: 27807639.|
|↑4||Meznaric M, Eržen I, Karen P, Cvetko E. Effect of ageing on the myosin heavy chain composition of the human sternocleidomastoid muscle. Ann Anat. 2018 Mar;216:95-99. doi: 10.1016/j.aanat.2017.12.001. Epub 2017 Dec 28. PMID: 29289708.|
|↑8, ↑10, ↑12, ↑16||Moore KL, Dalley AF, Agur AM. Clinically oriented anatomy. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2014.|
|↑14, ↑17||Kohan EJ, Wirth GA. Anatomy of the neck. Clin Plast Surg. 2014 Jan;41(1):1-6. doi: 10.1016/j.cps.2013.09.016. PMID: 24295343.|
|↑19||Kierner AC, Aigner M, Zelenka I, Riedl G, Burian M. The blood supply of the sternocleidomastoid muscle and its clinical implications. Arch Surg. 1999 Feb;134(2):144-7. doi: 10.1001/archsurg.134.2.144. PMID: 10025452.|