Updated June 05, 2014.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
The sternocleidomastoid is a superficially located neck muscle that plays an important role in tilting the head and turning the neck, as well as other things. This muscle is usually quite easy to see. Visually, it appears as a strap like, cylindrical shape that tapers at the ends and runs diagonally from the back of the ear to both the collarbone and the breastbone.
For brevity, the sternocleidomastiod muscle is often referred to as the SCM muscle, or just the SCM.
The SCM makes up part of a group of muscles known as the anteriolateral neck flexors. (The other muscles in the anteriolateral neck flexor group (say that 3 times…) are the scalenes, which are located more deeply in the neck than the SCM.) As synergist muscles to the SCM, the scalenes help tilt and turn the head and neck.
Location of the SCM - Origin and Insertion
The points of attachment for the SCM muscle, (known as origin and the insertion) are relatively simple to follow, unlike, for example, those of the latissimus dorsi muscle. (The latissimus dorsi, also known as lats for short, is a large back muscle that attaches to numerous structures located in the trunk and the arm.) In contrast, to the lats, the SCM only attaches to bones, and to a total of just 4 bones, at that.
One small exception to the simplicity of the SCM attachment pattern is that as the belly of the muscle begins to near the collarbone and breastbone, it branches into 2 “heads”, and thus has 2 origin points instead of the usual 1 that many muscles do.
So from where, exactly, does the SCM muscle originate? One “head” of the SCM attaches on the front (i.e., the anterior surface) of the manubrium. (The manubrium is uppermost section of the breastbone.) The other head attaches on the top part (called superior aspect) of the collarbone, near the midline of the body.
Above that, the SCM inserts on the mastoid process, which is a little projection of bone emanating from the temporal bone. As the name suggests, the temporal bone is the part of the skull (one on each side) that forms the temples. The mastoid process is located at the bottom of the temporal bone, behind your ear. You can actually identify the mastoid process by touching that area behind your ear. Then, if your walk your fingers down a bit you may be able to feel the soft tissue that is the SCM muscle. Some fibers of the SCM muscle also insert on the bottom of the occipital bone, which is located right next to the mastoid process, away from the ears.
What the Sternocleidomastoid Muscle Does for You:
In part, the neck movements produced by the sterocleiomastoid vary depending on whether one or both of the SCM muscles are working.
When just one SCM muscle contracts, it tilts your head to the same side (called the ipsilateral side) to which the muscle is located. For example, the SCM on the right side of your neck tilts your head to your right.
The SCM also turns (rotates) your head to the opposite side (called the contralateral 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 (called neck extension).
When both SCM muscles contract (called bilateral contraction; bilateral means 2 sides) the result is a neck extension movement that takes place at your first intervertebral joint. The first intervertebral joint is the top most spinal joint; it is the place where your head sits on your neck. This extension movement brings the head backward. But a contraction of both SCM muscles also flexes your neck, which brings your chin down in the direction of your breastbone. And bilateral SCM muscles contraction thrusts the chin forward - as long as you keep your head level, that is.
When both SCM muscles contract simultaneously, it helps the breathing process, too. With your neck held stationary, the bilateral contraction of the SCM muscles results in a lifting up of the manubrium as well as the ends of the collarbones that are nearest to the midline of the body.
The SCM plays a role in torticollis, a condition in which the head is persistently turned to one side. With torticollis, and also forward head posture the SCM may be in a perpetually shortened state.
The SCM as an Anatomical Landmark
The SCM muscle is of interest to anatomists because the unique position it occupies in the neck makes it key to understanding the layout of cervical muscles. The SCM diagonally divides the neck musculature into anterior (front) and posterior (back) triangles on either side, making the cervical spine easy to study.
First, there is the anterior triangle. The anterior, or front facing part of the SCM is considered to be the lateral (side) border of the anterior triangle of the neck. The anterior triangle is bordered on top by the jaw bone, on the bottom by the sternum bone (breastbone) on medially by the imaginary midline, or line of gravity, that divides the body into left and right halves. The anterior triangle contains several other sub-triangles, if you will.
The front part of the posterior (back) triangle is bordered by the posterior or back facing part of the SCM. On top, the posterior triangle has an apex or point where the SCM and the trapezius muscles meet on the bottom of the occipital (back skull) bone. The bottom of the posterior triangle is the middle third of your collar bones.
Related:Levator Scapula Muscle
Sources:
K apandji, I.A., "The Physiology of the Joints". Fifth Edition. Churchill Livingstone. English Edition 1987. 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, 1983
Moore, Keith, L., Dalley, Arthur, F. Clinically Oriented Anatomy. 5th Edition. 2006 Lippincott, Williams and Wilkins.