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Biomechanical Engineer Testimony in a Low-Impact Collision Case

BPS is here to serve our clients during this COVID-19 crisis. Pursuant to Governor Lamont’s Executive Order, legal services are essential services. Whether or not we are in our offices, Brown Paindiris & Scott, LLP Lawyers are available by email, phone and video conference. Read More.

September 14, 2009

The defense approach in the trial of a low-impact collision case is to retain accident reconstruction and biomechanical engineering experts to support the contention that the low-impact collision could not have caused the injuries claimed by the plaintiff.[1] Such defense expert testimony can have a strong impact on the jury, but plaintiff’s counsel can, with the proper preparation, diffuse this impact.

The Expert’s Investigative Process

To attack the defense expert’s opinion, it is first necessary to have some basic understanding of the process that accident reconstruction/biomechanical engineers employ. For years, engineers and scientists have studied the relationship between force, acceleration, deceleration, and the change in acceleration and their impact on the human body. These researchers have used cadavers, live animals, anthropomorphic dummies, and live human subjects to measure and understand the injury mechanisms in motor vehicle collisions. Many studies have examined how much force must be applied to a structure in the human body to cause an injury. In most low-impact cases, injury is not caused by the speed of the impact, but by the change in velocity that occurs ( deltaV).

Accident reconstruction/biomechanical engineers ascertain whether the forces resulting from the motor vehicle collision were sufficient to cause injury. To make that determination, it is necessary to analyze the dynamics of the occupants in that motor vehicle, and then to compare the forces known to well-accepted human tolerance levels for injury. The first step in determining whether a motor vehicle collision was of sufficient severity to cause an injury is to calculate the deltaV sustained by the plaintiff’s vehicle in the accident. Change in velocity as it pertains here can be defined as the change in the speed of the vehicle during the time of the impact.

It is necessary for the engineer to calculate the deltaV in a specific accident, usually through the use of a computer program such as CRASH III. These programs require that the engineer input information such as vehicle data, accident site data, and vehicle damage (crush) data. The computer program then calculates a deltaV. Once the engineer ascertains the deltaV, he can determine the g forces that were applied to the vehicle in the collision.

Once this number is established, it is then necessary to ascertain what effect the deltaV or g forces had on a particular plaintiff. This is known as the human dynamics analysis. Under this approach, the engineer inputs the g forces or deltaV along with specific personal characteristic data about the individual plaintiff into a second computer program to ascertain whether the collision was of sufficient force and velocity to cause the injuries claims.

The Expert

Properly evaluating whether a particular collision was sufficient to cause injury to a vehicle occupant requires a great deal of expertise. It requires engineering expertise in accident reconstruction so that the engineer can quantify the deltaV and the applicable g forces, and it requires knowledge of the biomechanics of the human body. Therefore, it is necessary that the expert have the qualifications to make these determinations. All too often, the experts employed by the defense lack sufficient expertise. While sometimes the expert lacks expertise in both areas, more often than not he lacks expertise in either the accident reconstruction aspect (vehicle dynamics analysis), or in the biomechanical aspect (human dynamics analysis). It is imperative that, on receiving the expert’s report, the attorney obtain the expert’s curriculum vitae and take the expert’s deposition to evaluate whether the engineer has the necessary expertise and qualifications to render the opinion expressed in the report.

Deflating the Defendant’s Experts

The first area of attack is on the specific expert’s qualifications. Frequently retired police officers, looking to supplement their pensions, will enter the forensic arena asserting their qualifications as being the investigation officer of thousands of motor vehicle collisions. Being a police officer in and of itself does not establish qualifications. It is therefore necessary to depose this expert to determine whether or not real qualifications exist.

In certain circumstances, the engineering expert is highly qualified in the field of accident reconstruction but lacks expertise in the second half of the equation, human dynamics analysis. Here the expert will properly calculate the deltaV and g forces involved. He then compares the g forces to that in the scientific studies, despite the fact that he has no expertise in the field of biomechanical engineering, or specialization in human anatomy and the biomechanics of injury.

Defense Misuses of Data

One approach used by the defense engineer employs a sleight-of-hand tactic. It must be understood that in a collision, the amount of force applied by the bullet vehicle (defendant) to the target vehicle (plaintiff), when plotted, is on some type of curve, rather than linear. That is, at some point during the collision there is peak moment when the force is greatest. For example, as the two cars first touch, the force is small, but as the collision progresses, the force becomes greater and decreases again as the cars come apart. In this deceptive approach, the expert uses an average g force, which is usually about half the peak g force. The engineer then compares the average g force of a particular collision with the scientific studies, which use a peak g force. It is improper to compare an average g force with a peak g force.

Even when the experts are qualified and the calculations accurate, one still must be cautious of the experts’ use of scientific and experimental test data. It is important to determine whether the factual circumstances of the case are identical, similar, or dissimilar to the test methodology used in the studies. Changes in the occupant’s physical characteristics and body position, as well as the absence, presence, and/or location of a headrest, can greatly affect the results. This is why some people in a crash sustain significant and serious injuries while other occupants in the vehicle walk away without a scratch.


In the low-impact collision case, as in any case, it is important in pretrial discovery to obtain the qualifications and basis for an expert’s opinion. After receiving the expert’s report, it is imperative that plaintiff’s counsel request forthwith a copy of the expert’s curriculum vitae, a list of all documents reviewed and relied upon, copies of all the raw data (which are the rough calculations made and used by the expert), and a list of all studies and papers relied upon in formulating the opinion. It is recommended that plaintiff’s counsel retain his own accident reconstruction / biomechanical engineer to, at the very least, assist in preparing a strong cross-examination, even if counsel does not intend to use the expert at trial. Small changes in numbers or factors inputted into the process can result in significant changes in the outcomes. Without the assistance of an engineer, plaintiff’s counsel will be hard-pressed to recognize the defense expert’s subtle miscalculations.

[1] See, e.g., Hisenaj v. Kuehner, 942 A.2d 759 (N.J. 2008). Plaintiff claimed permanent injuries in the nature of herniated discs in her cervical and lumbar spine as a result of the accident, which involved the defendant rear-ending plaintiff’s vehicle at a very low impact level. The Court noted that the narrow issue on appeal was whether the Appellate Division overstepped it bounds when it reviewed the trial court’s admission of expert testimony from defendant’s biomechanical engineer by concluding that it did not rest on a scientifically reliable foundation. Defendant’s biomechanical engineer had testified that, given the circumstances of the low-impact collision, no biomechanical mechanism existed that would have caused a chronic injury to result from the impact. The court concluded, based on the record and arguments presented to the trial court, and applying the abuse-of-discretion standard that the trial court’s evidential ruling on the admissibility of the scientific evidence was within the range of sustainable trial determinations.