Daniel G. Kagan, trial lawyer and partner at Berman & Simmons

Defense Biomechanical Expert


The family doctor and the neurosurgeon both say the accident caused the injury. So you make your demand to the insurance company. The company says no...

Particularly in cases in which the stakes warrant the expense, insurance companies are turning to "biomechanical experts" who will say that an impact that fails to cause visible property damage cannot cause physical injury.

I. Background

A client comes into your office reporting an injury following an automobile accident.
She was stopped in traffic and was rear-ended. X-rays ruled out fracture. She was diagnosed in the emergency room with cervical strain and discharged with instructions for conservative care.

A visit to her family doctor resulted in a referral for physical therapy, which provided only transient relief. Developing numbness and tingling in the upper extremities led to a neurosurgical referral. An MRI reveals a C6-7 moderate bulge with possible cord compression.

The police report indicates that no property damage was sustained in the accident. Photographs from the insurance company show a pristine-appearing rear bumper on your client's car. The family doctor and the neurosurgeon both say the accident caused the injury.

So you make your demand to the insurance company. The company says no, citing the report of its biomechanical expert who, having viewed the property damage photographs, has declared that this accident could not have caused injury.

Has this happened to you (yet)?

Particularly in cases in which the stakes warrant the expense, insurance companies are turning to "biomechanical experts" who will say that an impact that fails to cause visible property damage cannot cause physical injury. 

From an expert of this type you can expect to hear such opinions as "the forces in this accident were the equivalent of flopping down in a chair" or "stepping down from a short step." This testimony is effective and dangerous if left unchallenged. It might cause you to consider accepting a low-ball offer rather than face the expert at trial.

If an insurance company retains such an expert in your case, forget settling for anything approaching fair value. If you like the client and the case, get ready for trial. Assuming the other aspects of your case are solid, with an appealing, credible plaintiff, supportive doctors, and a clean pre-accident medical history, you can beat the defense biomechanical expert.

Overcoming this expert, however, takes solid preparation and an aggressive approach. There are two stages at which you can attack the defense biomechanical engineer.

First, you can get the expert excluded under Daubert and Kumho Tire.

Second, you can use readily available material to cross-examine the expert effectively.

The foundation for both the motion in limine and for cross-examination is obtained through document production and deposition.

II. Striking the Biomechanical Expert Through Motion in Limine
A. A Brief Primer on Daubert and Kumho

In 1923, the U.S. Court of Appeals for the District of Columbia Circuit held in Frye v. United States that the standard for admitting scientific expert witness testimony is whether the technique used by the expert is "generally accepted" as reliable in the relevant scientific community. 293 F. 1013 (D. C. Cir. 1923). This remained the standard of review for seventy years. As long as an expert was qualified within the field in which he was to testify, the expert's testimony was generally admitted.

In 1975, Federal Rule of Evidence 702 was adopted. It provides that "if scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training or education may testify thereto in the form of an opinion or otherwise." In Daubert v. Merrill Dow Pharmaceuticals, 509 U.S. 579 (1993), the Supreme Court revisited the Frye standard in light of Rule 702.

The Court recognized the liberalizing intent of Rule 702 and explicitly cited the Rule's "general approach of relaxing the traditional barriers to 'opinion' testimony." 509 U.S. at 588. The Supreme Court ruled that Rule 702 superceded the Frye "general acceptance" standard. The Supreme Court pronounced that the new standard required the trial judge to ensure that "any and all scientific testimony or evidence admitted is not only relevant but reliable." Id. at 589.

In order to accomplish this, the trial judge would function the "gatekeeper": [Applying Rule 702] involves a preliminary assessment of whether the reasoning or methodology underlying the testimony is scientifically valid and of whether that reasoning or methodology properly can be applied to the facts at issue. Id. at 592-93.

This gatekeeper obligation is your tool for attacking the admissibility of the defense biomechanical expert. 

In Daubert, the trial court had to determine whether an expert in epidemiology could base his opinion testimony on a novel theory in his discipline concerning whether the drug Bendectin could cause birth defects. It was in this narrow context of forensic epidemiology that the Supreme Court defined the "four factors" trial judges may apply in assessing an expert's methodology:

   1. Has the theory been tested, or can be it tested?
   2. Has the theory been subjected to peer review and publication?
   3. What is the technique's known or potential error rate?
   4. Is the theory "generally accepted" [i.e. similar to Frye]?

Id. at 593-95. The Court emphasized that these factors are to be applied flexibly, depending upon the nature of the principle proposed.

What applied narrowly in Daubert to forensic epidemiology was substantially expanded in Kumho Tire v. Carmichael, 119 S. Ct. 1167 (1999), in which the Supreme Court ruled that the trial judge's "gatekeeper" duty applies to all experts in all cases, not just purely scientific experts. Kumho Tire involved an expert in mechanical engineering whose opinion regarding a blown tire was based on experience and observation rather than pure science.

The Supreme Court's ruling in Kumho Tire was that the trial judge's gatekeeper obligations to apply the Daubert factors under Rule 702 applies to all experts, not just epidemiologists. Even though the Court found the expert qualified to offer testimony in this field, it ruled that his methodology was unreliable and his testimony therefore excluded. Id. at 1176
B. Flaws in Defense Biomechanical Engineering Testimony - Attacking the Foundation

Defense biomechanical testimony is virtually always stated as an absolute - NO injury was possible in this accident.

This absolute position is a critical weakness in this kind of expert testimony. If the expert backs down from this extreme position one iota, you can slip your plaintiff through the hole. If he doesn't back down, the defense has to demonstrate a basis for the opinion that satisfies Daubert.

To reach the position that NO injury to ANY person was possible in a particular accident, most defense biomechanical experts follow a similar logic path: there is a threshold of force below which no human being will sustain injury. This accident delivered a force upon the claimant that was less than the threshold of force necessary to cause injury. Therefore, this accident did not cause injury to the claimant.
In support of this conclusion defense biomechanical experts cite from a narrow list of studies that conclude that "low speed" accidents do not cause injury to human beings.

It is natural to be intimidated by such an impressive-looking list of citations, but don't be. Those studies are not what they seem. 

Any lawyer facing defense biomechanical expert testimony should read the article authored by Michael Freeman entitled "A Review and Methodologic Critique of the Literature Refuting Whiplash Syndrome." Published in SPINE (Vol. 24, No.1 pp 86-98) in 1999, this peer-reviewed article addresses the methodological flaws in the studies relied upon by defense biomechanical experts. Dr. Freeman provides invaluable ammunition for your Daubert attack on the defense biomechanical expert who cites studies in support of the "low speed impact = no injury" argument. His critique of the defense studies includes:

  • Inadequate sample size in the study.
  • Inappropriate study design.
  • Selection bias.
  • Inappropriate use of technology.
  • Conclusions unsupported by the study results.
  • Misquoted and/or biased literature cited in the study.
  • Unsubstantiated and/or unreferenced claims.
  • Study sample not representative of real-life claimants.
  • Crash conditions not representative of real-life conditions and/or overly generalized.

C. Misleading illustrations

Plaintiff lawyers facing defense biomechanical testimony should use Dr. Freeman's article to evaluate the strength of the underlying citations. Defense biomechanical experts rely on reports produced by others for the proposition that certain thresholds of force are required before any human being could suffer injury.

As discussed above, the reports on which biomechanical experts rely are subject to attack on Daubert grounds.

Even assuming that the reports underlying biomechanical testimony were valid, the expert still must apply the generalized statements in the studies upon which he relies to the specific facts of the individual case. To state that the forces inflicted on the claimant were insufficient to cause injury, it is necessary to calculate the amount of this force. This is a ripe area for attack.

When a vehicle collides with an object, there is a change in the vehicle's velocity.

Delta V is engineering term for the change in velocity of the vehicle. Delta V is used to calculate the amount of "g force" created by a collision. When two moving objects come into contact with one another, calculating g-forces requires factoring many variables. Yet defense biomechanical engineers always manage to come up with precise g-force figure, and that figure is invariably below a particular level of force they claim is necessary to cause injury in human beings.

By looking at photographs and forming opinions of force, the expert is performing "crush damage analysis." The principle of crush damage analysis is "conservation of energy".

This principle of physics states that all energy in a collision must be conserved (i.e., it has to go somewhere). The expert concludes that because the energy in the accident didn't cause visible property damage, the energy level was insufficient to cause injury.

This is a flawed conclusion. 

Energy in an accident may be transferred in a number of ways other than damage to property. Sometimes the vehicles will move after the impact. The vehicles themselves absorb some of the energy. Energy is lost in heat and friction. And, a factor usually ignored by defense biomechanicists, some of the energy is transferred to the occupants.

As a plaintiff lawyer, don't worry - you do not need to be able to determine precisely how the energy was transferred. The point is that the defense biomechanical expert cannot do so.

In reaching conclusions about force imposed upon occupants of a target vehicle, the defense biomechanicist relies on flawed assumptions. For example, he assumes that you can calculate g forces imposed on a car based on reviewing photographs of it. He also assumes that g forces imposed on a car will impose the exact same force on the occupants of that car, and that the same force will be imposed on each portion of the occupant's anatomy. These assumptions are not supportable.

Unlike static lab studies, real world accidents involve infinite variables. Unless the expert knows the precise accident conditions, conclusions drawn are not reliable. Merely looking at property damage photographs simply doesn't give an expert enough information to produce the opinions offered. The expert's deposition should be used to explore the information the defense expert relied upon in reaching conclusions about the force imposed upon the claimant in the accident.

The less specific the expert can be in filling in the blanks, the more unaddressed variables remain.
Some areas to explore with the expert include:

  1. What was the angle and direction of collision? Too often it is assumed that a rear end collision occurred from straight behind or at exactly ninety degrees from the plaintiff's vehicle. Yet sometimes the defendant's vehicle swerves or turns before impact, imparting a twisting force to the plaintiff's vehicle. Similarly, if the target vehicle's wheels were turned at the time of impact, that vehicle will likely move in the direction of the tires, not the direction of the defendant's vehicle. A rear-end collision that is off-center or at less than 90 degrees may cause twisting of the seat back, decreasing the seat's ability to cradle the occupant and possibly causing the head to miss the head restraint entirely. An off-angle impact also increases the likelihood that the plaintiff's head may have struck something on the car's interior, such as a door frame or side window.
  2. What was the plaintiff's body position at the moment of impact? It is common for people riding in cars to be sitting in a less than optimum position. Twisting to face a fellow passenger, leaning on one side or another, reaching for something inside the car, and even sleeping are common. Studies suggest that the position of the plaintiff's head just before impact can be critical to the post-accident outcome. The neck's range of motion is greatest when the head is in the neutral position. When the head is moved in various planes of motion, not surprisingly, the neck's range of motion is decreased. When the head is extended, rotated, or both, the strains on the supporting ligaments is greater and the capacity for injury is greater.
  3. What effect, if any, did the expert ascribe to the plaintiff's gender? There are enough studies indicating that women are at greater risk of injury from rear end accidents than are men that this proposition should be considered beyond challenge. If your client is a woman, look at the subjects who took part in the studies cited by the defense expert. Were they healthy middle-aged males?
  4. What does the expert know about the plaintiff's body type? There are studies suggesting that people with slight muscular development are at greater risk of injury from a particular accident than those with more developed musculature. Also, neck flexibility and strength decrease with age. Thus, an older woman of slight build may fare worse in an accident than a relatively young man of average build.. Again, compare your client to the subjects used in the studies upon which the defense expert relies. 
  5. What was the position of the seat back and head rest at the time of the accident? Related to the question of the plaintiff's body position is the question of the position of the seat and head restraint. The biomechanical engineer's report will often refer to the vehicle as being equipped with a head-restraint, implying that this device would prevent injury. This argument is flawed for several reasons. First, the injury may occur before the head ever strikes the restraint. In fact, "whiplash" in the low-speed impact case may be a misnomer. The medical evidence suggests that it is not an excessive flexion/extension movement but rather the initial movement of a portion of the cervical spine relative to the rest of the spine that causes the injury. The fact that the head does not move to the point of excessive flexion or extension, due to head restraint or otherwise, may be irrelevant in analyzing the neck injury in the rear end case. Second, it is rare that the biomechanical expert will know the precise position of the headrest and the seat back relative to the plaintiff's anatomy. Without this information, it is impossible to know the extent of movement of the plaintiff's body before striking the head restraint and/or the seat back, if in fact the plaintiff's body struck the head restraint at all. Finally, as we know, the existence of head restraints does not guarantee safety. A check on the website of the Insurance Institute for Public Safety will tell you that relatively few of the passenger cars on our country's roads today have head restraints rated as "good" or even "acceptable." This same website confirms that even minute differences in adjustment of the head restraint and seat back can make a significant difference in the amount of force imposed upon the occupant in a rear-end collision. Quite an admission, given the source of the information!
  6. Was the plaintiff ready for the impact? Just as a denser, more powerful body type may be less susceptible to injury in an accident, so can a body tensed in anticipation of impact protect better against injury.
  7. What were the road conditions? This is important for several reasons. First, plaintiff's vehicle's tires will adhere better to a dry road, causing less forward movement of the target vehicle on impact. There is less adhesion on an icy or slippery road, so the target vehicle can accelerate further and faster with the impact. Second, a collision on an icy, slippery road can cause unpredictable movement of the target vehicle, with exaggerated rotational force combining with the above-mentioned increase in acceleration upon impact. Other than generalizations such as "wet", "dry" or "snow-covered", the information typically available to the biomechanist is inadequate to allow any precise calculation of either g force or direction.
  8. What is the construction of the seat back? Often overlooked in the biomechanical analysis is a discussion of the elastic energy available from the seat back itself. Since the rear end collision projects the upper body forward, it is the seat back that creates the initial movement. The seat back can create a "trampoline" effect that accentuates the acceleration. One study suggests that this effect can double the g force imposed upon the cervical spine. Because there are no known studies of the construction of seat back construction in even a small sampling of the vehicles available for sale in this country, it is virtually impossible for a biomechanical expert to have this information.
  9. Did the expert consider the ramping effect of the rear end collision? It is typically assumed that the occupant's body moves linearly backwards during a rear end impact. This is not so. Depending upon a number of the factors discussed above, such as seat back angle, occupant position, seat back construction, seat surface friction (leather is slipperier than velour), as well as vertical motion of the target vehicle during impact and seat belt use, the occupant's body may move upward as well as backward during the impact. This ramping effect can increase the initial travel of the torso and head backwards into the seat back and head restraint, which increases the likelihood of cervical injury.
  10. What about seat belts? Seat belts have proven to be a mixed blessing. Part of the problem is that, like head restraints, seat belts are useless or possibly even harmful if not positioned properly. Particularly at risk are shorter people who wear the shoulder portion higher toward the neck.
  11. What are the relative sizes of the vehicles? Since mass and velocity are multiplied to calculate momentum, the relative sizes of the vehicles is important. Basic math tells you that a 5000 lb vehicle delivers twice the force of a 2500 lb vehicle at an equal speed. 
  12. What were the individual's physiological limits and pre-existing medical conditions? Biomechanical engineers typically have little or no knowledge of medical issues generally and the claimant's medical condition specifically. Yet the specifics of the individual claimant can make all the difference.
  13. How did the vehicles move after the impact? Superficially, the notion that a bumper with no visible damage proves that the impact was "minor" makes sense. In fact, all it really means is that the bumper did its job. Automotive bumpers are designed to protect the vehicle itself, not the cargo or occupants. Bumpers work by absorbing the energy of the impact into a spring or shock absorber. Basic physics tells us that after the initial action of absorbing the energy, there will be a reaction as the spring or shock absorber releases its load. This is seen in the form of a "bounce." Sometimes this bounce is seen as the target vehicle is propelled forward. Other times the vehicle is seen to "shake" without actually moving forward, particularly if the brakes are applied. Considerable energy is conserved and dissipated within the car itself - including its cargo and occupants - even as the bumper performs as designed in protecting the vehicle itself from expensive property damage.
  14. How did the expert calculate delta V? Accepted accident reconstruction principles hold that the forces in a collision between vehicles are imparted over a period of time rather than instantaneously. If you were to look at a graph with time on one axis and force on the other, you would seem some variant of a bell curve. At some point in the collision, delta V is at its highest, meaning the change in velocity is greatest. This is the point at which the accident imposes the greatest g-force upon the vehicle and its occupants. Frequently, however, defense biomechanical experts take the average of delta V rather than the peak delta V as appropriate. Frequently overlooked, this use of the average rather than the peak can understate the amount of force by as much as 100%. Some points to keep in mind when considering this list: First, it is not exhaustive. Reading on the subject will give you more ideas and areas of inquiry. Second, and more importantly, it is not necessary that you know the answers to any, let alone all, of these questions. Remember, it is the defense's burden to show a sufficient foundation for the expert testimony it will offer. Your job is to illuminate as many variables to the equation as possible, and demonstrate that the proffered expert does not know the answer to these questions. Remember, few if any defense biomechanical experts actually perform any of their own tests, instead relying on tests by others. Can they really tell so much just from looking at photographs?

III. What is a "biomechanical expert"?

Finally, what is a "biomechanical expert" anyway?

Is it an engineer with a doctorate in biomechanics? Or is it a retired police officer who may have accident reconstruction experience and who attended a couple seminars on rear-end collisions sponsored by insurance companies?

Find out.

You may like what you find.

Your discovery should include disclosure of the defense biomechanical expert's medical experience and training. Typically it will be minimal. This will go a long way in your effort to exclude his opinion testimony. At a minimum, even if his education and/or experience permits him to talk in general terms about engineering principles, you should be able to exclude testimony correlating forces to specific propensities for injuries. This is medical testimony which the biomechanical expert is not qualified to give. It is also useful for cross-examination in front of a jury. The only basis such an expert has to comment on propensity for injury is reference to the flawed studies discussed above. 
Don't confuse accident reconstruction and biomechanical testimony. While there may be some overlap between the specialists, an accident reconstructionist is not a biomechanical expert. The accident reconstructionist can tell you how the vehicles moved before, after and sometimes during the accident sequence, but the accident reconstructionist is not qualified to describe the forces actually imparted to the occupants. Nor is he qualified to discuss the medical and physiological ramifications of such force on a particular individual in such an accident.

Many defense biomechanical "expert" mills achieve economies of scale by incorporating boiler-plate analysis of so-called "low speed impact" cases into every one of their reports. Showing through deposition testimony at a minimum and preferably through examples that these are "one-size-fits-all" reports can seriously impair an expert's credibility, both with a judge on the Daubert motion in limine and with a jury. This can be particularly devastating if the reports and studies cited within the boilerplate portion of the report fall outside the area of expertise of the so-called expert biomechanicist.
IV. Conclusion

These are proven ways to either exclude or impeach the testimony of a defendant's biomechanical expert.

The key to getting such evidence excluded or impeaching it through cross-examination is comprehensive knowledge of how the expert reached his opinion and through thorough examination of the basis for the opinion in discovery.


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