The Anatomy of the Sternocleidomastoid Muscle

The SCM muscle helps turn and bend your neck

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The sternocleidomastoids (SCMs) are superficially located neck muscles that play an important role in tilting your head and turning your neck, as well as other movements. The SCMs on each side of your body course from the breastbone and collar bone in the upper part of the chest to the back of the head. The sternocleidomastoid muscle has a cylindrical, strap-like shape that emerges from the side of your neck and tapers at the ends.

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Anatomy of the SCM

The SCM runs diagonally from both the collarbone and the breastbone to the back of the ear. On either side, the SCM diagonally divides the neck musculature into anterior (front) and posterior (back) triangles. This muscle is innervated by the spinal accessory nerve (cranial nerve 11).

The SCM has two heads. One originates from the collarbone/clavicle and the other from the breastbone/manubrium.

Origins of the SCM:

  • One head attaches to the front (i.e., the anterior surface) of the manubrium. The manubrium is the uppermost section of the breastbone.
  • The other head attaches to the top part (called the superior aspect) of the collarbone, near the midline of the body.

SCM insertions:

  • The SCM inserts behind the ear at the mastoid process, a projection of the temporal bone of the skull.
  • The SCM continues on to attach to the mastoid bone.
  • A few fibers of the SCM insert at the bottom of the occipital bone, which is located right next to the mastoid process at the bottom of the skull.

Sternocleidomastoid Function

The SCN can produce several different neck movements. One side can contract, or both sides can contract.

When just one SCM muscle contracts:

  • It tilts your head to the ipsilateral (same) side to which the muscle is located. For example, the SCM on the right side of your neck tilts your head to your right.
  • One SCM can also turn, or rotate, your head to the opposite side. For example, when you rotate or turn your head to the right, your left SCM is contracting. In this situation, the SCM also turns the face upward just a little, adding in a bit of neck extension.

When both SCM muscles contract, the movements can include:

  • A neck extension movement that takes place at your first intervertebral joint, which is the topmost spinal joint; it is the place where your head sits on your neck. This extension movement brings the head backward.
  • A contraction of both SCM muscles can flex your neck, which brings your chin down in the direction of your breastbone.
  • Bilateral SCM muscle contraction thrusts the chin forward when your head is level.

Simultaneous bilateral SCM muscle contraction also plays a role in the breathing process. In this case, it lifts up the breastbone and the area of the collarbones that are nearest to the midline of the body.

The SCM is part of a group of muscles known as the anterolateral neck flexors. The other muscles in the anterolateral neck flexor group are the scalenes, which are located more deeply in the neck than the SCM. The scalenes are synergist (helper) muscles to the SCM. They assist the SCM in turning and tilting both the head and neck.

Associated Conditions

The SCM becomes chronically shortened in cases of torticollis. Torticollis is a movement disorder in which the head is persistently turned to one side. Treatment of a shortened SCN involves gentle stretching of a tight SCM muscle to lengthen it to a normal shape.

An impairment or injury to the spinal accessory nerve can cause weakness or paralysis to the SCM. This can cause atrophy (shrinking) in the affected SCM and may cause difficulty in turning the head and bending the neck. Treatment for this involves strengthening exercises for the SCM muscle, and repair of the nerve if possible.

Rehabilitation

Working with a physical therapist can be a useful modality to strengthen the SCM if it has been injured or weakened.

Exercises for the SCM may include:

By working to keep good posture and gently stretch and strengthen the SCM and other muscles around your neck, you can experience improvement of symptoms associated with the SCN.

Frequently Asked Questions

  • What is the function of the sternocleidomastoid muscles?

    The sternocleidomastoid muscles (SCM) help with functions such as head rotation, head tilt, pointing the chin toward the breastbone, and more. Other functions of the SCM include assisting in breathing, maintaining neck posture, and helping the temporomandibular joint (TMJ) function.

  • What are synergist muscles?

    Synergist muscles help other muscles perform one or more functions. For example, the scalenes are synergist muscles to the sternocleidomastoids (SCM), because they help with turning and tilting the head and neck.

  • What is innervation?

    Innervation is when an organ or body part is supplied with nerves. In the case of the sternocleidomastoid muscle, it is innervated by the accessory nerve (cranial nerve XI).

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2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. MedlinePlus, U.S. National Library of Medicine. Torticollis.

  2. Bordoni B, Varacallo M. Anatomy, head and neck, sternocleidomastoid muscle. StatPearls.

Additional Reading
  • Kapandji, I.A., "The Physiology of the Joints". Fifth Edition. Churchill Livingstone. English Edition. New York.

  • Kendall, Florence Peterson, McCreary, Elizabeth Kendall, and Provance, Patricia Geise. Muscles Testing and Function with Posture and Pain. 3rd. Baltimore, Maryland: Williams & Wilkins

  • Moore, Keith, L., Dalley, Arthur, F. Clinically Oriented Anatomy. 5th Edition. Lippincott, Williams and Wilkins.