When faced with an auto accident claim, what is the proper use of an accident reconstructionist, a biomechanist, and a medical doctor?

An accident reconstructionist looks at vehicle dynamics.  That is where their training and experience lies (scene and vehicle analysis that leads to an accurate description of the vehicle to vehicle interaction). Reconstructionists analyze police reports, photographs, witness statements, tire skid mark paths and lengths, and vehicle damages.  They may create a scene diagram to allow them to calculate the angles between the vehicles involved. They research the weights and capabilities of the vehicles involved from standard published industry sources, including gross vehicle weights, body dimensions, bumper heights, centers of gravity, and stiffness coefficients.  They may download information from the CDR "black boxes".  They may make calculations of speeds based upon data drawn from all of these sources, in accordance with the laws of physics.  Often they will translate the speeds involved into "g-forces", or the equivalent force of gravity that we all experience in our day to day lives.   For very low speed accidents, there is a large body of research that they may draw upon to express an opinion about the probability of injury.  The "g-forces" involved in a very low speed accident may be less than those experienced during a sneeze, stepping off a curb, or sitting down onto a chair.

Medical doctors often express injury opinions but usually based on history - the patient could walk before the event and not after the event, so the event caused the symptoms.  However, the medical profession (defined loosely to include chiropractors) is focused on diagnosis and treatment based entirely on symptoms - "my arm hurts" results in a diagnosis of muscle strain, broken bone or infection.  The goal and focus of medical doctors is forward to the restoration of health, so they concentrate on treatment time, a cast or antibiotics. There is no expertise in identification of the cause.
 
The biomechanist lies between these two and provides the linking utility between the vehicle dynamics and the explanation of the medical diagnosis.  Biomechanics is the scientific basis for stating  the probability for injury from this particular vehicle accident.
 
Simply put, biomechanics is the science concerned with the action of internal and external forces on the human body (the mechanical cause of the injury or death).  It is not the same as a medical diagnosis, which is the identification of the cause of patient symptoms for the purpose of rendering medical treatment, or an accident reconstruction, which determines the vehicle dynamics.
 
A biomechanical analysis requires a review of three essential elements; the patient’s medical history including ongoing treatments, a detailed description of the accident (height of fall, speed of impact, vehicle dynamics, etc.), and the past and current condition of the patient.  The medical history is important as it may reveal a previous injury or condition that would make the patient more susceptible to injury, i.e. osteoporosis, a spinal fusion, or a recent surgery as well as provide competing etiology.

As an example, a victim of an auto accident died of a torn vein leading to his heart. Based upon data from both the coroner and the accident reconstructionist, the biomechanist was able to show that it was not the trauma of the vehicle accident that tore the vein.  The forces experienced in the crash were not strong enough to do that much damage.  The victim's vein was torn during a botched medical procedure administered in the hospital ER.
 
In automotive accidents, the biomechanical analysis couples the forces the body would experience (based on the collision forces identified by an accident reconstruction) with the medically diagnosed injury (provided by the medical doctor or other treatment provider) to address the probability of a relationship existing between the event and the injury claim.