Most people who get a whiplash-like injury think it is caused by a problem in their muscles. It's easy to see this why this may be the case since muscle pain following car accidents is so common. Deep pain and even spasm can occur after severe trauma resulting in daily pain and even headaches. Since our 10-12 pound head is attached to our necks, by muscles that go into the shoulder region, whiplash can feel like a muscle pull and taking muscle relaxants seems a reasonable approach.
Although tears of muscles fibers do occur in whiplash, these can heal rather quickly due to the rich blood supply. The ligaments such as disks hold the joints of the neck together keeping the nerves from being pressed upon and stretched. These are the structures that are critically injured during whiplash. The muscles that contract to protect the joints from moving too much are generally less of a problem than when the ligaments are injured. A recent study (BMC Musculoskelet Disord 2006;21:103) showed that after whiplash, the strength of the neck ligaments is further reduced. This means that you are more susceptible to getting injured if you previously suffered a trauma.
To detect ligament injuries you can look at MRIs immediately after the trauma. In many cases they can show small tears or the inflammation and swelling that goes with tears of these important structures.
You can also have stress x-rays taken in the positions of forward and backward bending. These types of x-rays can show which ligaments have been traumatized and are allowing the bones of the neck to move too much. When this increased motion is severe, this is called instability. Some newer MRI machines can scan in different positions so that the tears and their motion effects are seen with one test. Some people may find the MRI scanner to be a bit restrictive or claustrophobic. X-rays are usually the most practical and least costly choice.