The longus colli is a commonly injured muscle in rear end whiplash injuries.   It can be found by putting your finger between the SCM (big strap muscle in the front of the neck) and the trachea (hard windpipe in the middle) and pressing deep toward the front of the spine.  Be careful, in many patients who have had a whiplash, this hurts! 

The muscle frequently gets weak and can cause the strap muscles in the front of the neck to overwork.  This can cause significant problems in the jaw by pulling on the TMJ (since they all attach there).

As you can see from the drawing, this muscle sits in the front of the spine.  It's thought that spasm in this muscle causes many whiplash patients to loose the normal curvature in the neck.