The sternocleidomastoid or SCM is a muscle in the front of your neck.  If you feel around in the front for the two big strap muscles that pop out when you bear down, those are the SCM's.  This muscle primarily turns your head and bends it forward.  It's also used to support your head as you get up from lying down.   It's one of the most commonly injured muscles in a rear end accident.  Trigger points in this muscle can cause headaches in the temples and dizziness/imbalance.  This muscle is also closely tied to the TMJ and is often sore in patients with this syndrome.   In patients who develop a forward head posture, this muscle can become overworked.  

How can this muscle get sore and overworked and cause headaches, dizziness, and jaw pain?  The longus colli and deep neck flexors are responsible for helping the SCM lift your head.  However, the longus colli gets injured in rear end whiplash injuries with the same frequency as the SCM.  For reasons not well understood, the longus colli stays weak.  This causes it's helper muscle, the SCM, to get overworked.  Since the strap muscles in the front of the neck also get overworked and these are directly responsible for holding the jaw, people frequently complain of TMJ like symptoms.  Patients alos complain of ear pain because the auricular nerve travels right by the surface of the SCM.  They also complain of headaches because the occipital nerve gets irritated as it sneaks around the SCM.  Finally, the patient can complain of dizziness because the SCM muscle is critical for providing information to the brain regarding balance.

As you can see from the diagram below, the muscle goes from the sternum and collar bone (sternocleido-) to the back of the skull (-mastoid).

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