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Burlington Attorney Kevin Conner published in October WSAJ Trial News


Why Biomechanical Experts Should Not Be Allowed To Testify In Low-Impact, Soft-Tissue Cases

by Kevin W. Conner

In the interest of brevity, my simple answer to the question above is “because it is not fair.” The support for my conclusion follows.  The sole purpose of this article is to break down the biomechanical engineer’s expected testimony to help shed light on the exact part of the testimony that should be the focus of an objection.  I begin by giving you the five step plan of attack the biomechanical engineer will probably employ during his or her expert testimony.  I will then point out why allowing this expert testimony is a travesty of justice that must be prevented.

                Step 1.  Based upon viewing a photograph, a property damage estimate, or both, a calculation will be made by the biomechanical engineer of the force involved in the collision in the case at bar that will be expressed in terms of a “g-force.” Since few of us outside the scientific community can comprehend what a g-force is, whatever number that is used in relation to a given situation (expressed with a g-force) is essentially an arbitrary label.  With no common point of reference in appreciating what a proffered g-force really means, the jurors are totally dependent upon the biomechanical engineer for the explanation of the relevance of his or her g-force analysis. By design, this numerical label will become the cornerstone of the expert witness’ testimony in step five.
                Step 2.  The g-force calculation will be analogized to events in everyday life we all experience (with no differentiation between vertical and horizontal vectors, a crucial distinction).  For instance, a 3.1 g-force determination in a given accident may be said to be roughly equivalent in g-force to sitting down on a low couch, or a 5.0 g-force determination may be said to be roughly equivalent in g-force as briskly walking and stepping off of a six-inch curb.
                Step 3.  The g-force comparisons of the forces sustained in the accident in the case at bar compared to non-injuring events are further emphasized by well-rehearsed anecdotes involving extremely sophisticated and admittedly impressive high-tech tests that use high-speed cameras, test sleds, and human test subjects. The expert will explain that in thousands of test cases, the human test participants were subjected to roughly the same g-force as calculated in the accident in the case at bar.  The expert witness will explain that out of the thousands of test subjects he or she has personally tested or read about in other studies, no test subjects were injured.
                Step 4.  The lack of potential danger from being exposed to the g- forces involved in the accident in the case at bar will be emphasized by the expert witness by reminding the jury that it would be “unethical” to expose test subjects to forces that could injure them. This point will be highlighted or played up while simultaneously playing down the fact that the participants were required to sign a waiver or release of liability, which by definition and by the terms set forth therein, recognizes that participation in the study involves some risk of injury.
                Step 5.  As the expert witness’ testimony builds towards a dramatic crescendo, he or she will slow the testimony at this point to make sure the “mathematical points” resonate with their jury.  The cornerstone of the expert testimony is unveiled. The review will essentially say subject Plaintiff experienced a g-force (probably in the range of a 1.9 to 5.0 g-force) which is lower or equal to forces we all experience in daily life. Likewise, it will be pointed out Plaintiff experienced a g-force equal to or less than the forces the test subjects were exposed to in the dramatically referenced, major university endorsed, incredibly sophisticated, “ethical” studies which the expert witness focused on earlier in his or her testimony.  Although the expert biomechanical engineer (who will most likely not also be an M.D.) stops clear of offering a medical opinion that Plaintiff could not have been injured in this accident, he or she will lead the fact-finder to the brink of this unmistakable, implicit conclusion.  THIS IS UNFAIR. Why should the expert be allowed to imply what he or she could not say?               

The expert witness is only allowed to testify regarding matters within his or her field of expertise.  If allowed, with the artfully deceptive misdirection of a street magician, the lines of the expert’s field are erased and the field will expand to diagnose regarding the presence or absence of physical injuries (as opposed to a mathematical calculation).  The biomechanical engineer is theoretically limited to only conducting a biomechanical analysis.  Great pains should be taken to ensure his or her testimony is narrowly confined. If the testimony is limited to a mathematical calculation, little or no impact is made by the testimony.  The danger lies in allowing the expert to explain the complex concept of g-forces acting upon the plaintiff by making anecdotal analogies to other forces experienced by other people in other situations (i.e. other people in lab studies, or general information about g-forces other people experience in daily activities).

The logical fallacy at play (which is being intentionally promoted) during the biomechanical engineer’s expert testimony is the seemingly common-sensical assumption that there is a minimum g-force threshold of injury.  Said another way, after hearing the testimony of the expert witness, wouldn’t the average person believe there is a minimum “g-force” that must be present before an injury could possibly occur?  Wouldn’t we naturally assume (as we have been led) the “threshold of injury” must be greater than forces human test subjects are exposed to in “ethical lab studies?” In other words, if we are told in thousands of studies, no human participants were injured (which is not true, read the footnotes of the lab reports) while being subjected to a 4.0 g-force, and the plaintiff was only subjected to a 2.9 g-force then……(do I really need to spell out the implied answer?) Think about it, if bolstering this implication is not the ONLY reason for the expert’s testimony, then why mention the studies or the g-forces involved in daily activities at all? The comparative presentation logically leads (and in fact is designed to lead) one to conclude that since the g-forces in a given accident were “scientifically” established, and since Plaintiff was not exposed to a g-force greater than the minimum injury threshold impliedly set by the daily activities and/or studies, it stands to reason no bodily injury could have occurred as a result of the accident.

Let’s examine why the fact-finder should not drink from this well to which they have been so carefully led. Start the analysis by asking yourself hypothetically, what g-force would be involved if a person reached for an item on the ground and “threw out” their back or sleeping wrong on a large pillow?  I submit to you that the g-forces in those scenarios are well below the g-forces that the test subjects were exposed to.

Another point to ponder: If asked, the expert biomechanical engineer will testify that forces upon the human body experienced from falling from a standing position are generally in the range of 4 to 5 g-forces, (i.e. within the g-forces experienced in daily life). No one would testify that it would be impossible for a person to fall from a standing position and break their hip or wrist or cause other injuries. After all, this is in fact a common occurrence. How can a force sufficient to break bones not be sufficient to cause soft tissue injury? Clearly, I have just demonstrated there is in fact no empirically expressable g-force threshold of injury.

Washington Courts have the discretion (and the obligation) to exclude evidence lacking a necessary preliminary fact or connection to the case at bar. (ER 104).  The best support that the g-force comparisons (i.e. the forces sustained in the crash in the accident at bar compared to g-forces experienced in daily life and/or g-forces experienced by human test subjects during studies) lack foundation will come directly from the biomechanical engineer.  If asked if there is a direct correlation between g-forces and injuries, you will be informed that evaluating injury criterion is complicated because injuries are not based on simple one number g-force factors, but the best correlative indicator of injury is shear displacement between vertebrae.  Let me say that again, in spite of the care taken to lead you down the g-force to g-force comparison, the real predictor of injury has everything to do with vertebral movement and little or nothing to do with a simple g-force calculation. This irrefutable point will be endorsed by the expert witness.  Just ask (I suggest during a deposition).

So the last grasp in support of including or allowing biomechanical engineer testimony in low-impact, soft-tissue cases would be to take the position that there must be a direct correlation between a g-force involved in an accident and the amount of movement between vertebrae in a given individual. In other words, the argument simply goes the greater the force of a collision, the greater the movement of the bones in one’s spine. Logical, right? This position is simply wrong.  There are simply too many factors that affect vertebral displacement. Body position at time of impact such as sitting straight or leaning forward; head position at time of impact such as looking forward as opposed to having one’s head turned and fully torqued;seatbelt position; type of seatbelt; height and weight of individuals; pre-accident health of accident victims (specifically muscle tone); length of the neck; ratio of mass of head to body (specifically the neck);position of headrest at time of the accident; type of headrest; and the angle of the impact all affect vertebral displacement.  However, the largest factor affecting vertebral displacement is whether the accident victim was aware of the impending impact which would allow a person to brace themselves and reduce the body movement reducing the vertebral shear (Citations omitted). This makes sense.  If a linebacker tackles a tailback in a football game, we don’t typically expect an injury. If the same linebacker un-expectantly tackles the same tailback with the same force in a grocery store, one would expect an injury to occur more often in that scenario.

In conclusion, there is no relevancy to testimony regarding a calculation of g-force in an accident because there is no scientifically demonstrable correlation between a specific g-force and injury.  The anecdotes regarding test subjects and/or examples of forces experienced in daily activities is additionally impermissible given the lack of foundation required before attempting to make the g-force comparisons between the respective groups.  Given the incredibly large number of variables that differ in a real world accident as compared to a laboratory collision or experienced in daily living, it is simply impossible to make an apples to apples comparison.

I had a career altering epiphany on this subject the moment I realized I should not be trying to counteract testimony of biomechanical engineer’s mathematical opinions (which I liken to challenging Aquaman to an underwater fight), but instead I should be fighting to prevent biomechanical engineers from testifying in the first place.  On behalf of all WSAJ members’ clients, I extend an invitation to have you join me in this noble cause.  I will be glad to send any WSAJ member a stock brief I drafted on this issue.

  1. The biomechanical engineer’s opinion will basically conclude something to the effect that the forces acting upon Plaintiff in this accident were low, relative to forces experienced in daily living and thus the accident is not a likely source of significant forces acting on Plaintiff’s body.

  2. Note, the mathematical calculation may or may not be accurate for reasons that are beyond the scope of this article.  Regardless if the math is correct, the unfair application of the mathematical finding is the objectionable issue at hand.

Kevin W. Conner, WSAJ EAGLE member, is an associate attorney at Graham Lundberg Peschel working in the firm’s Burlington, Washington office.  He is a Million Dollar Advocates Forum member.  He graduated from the University of South Dakota School of Law in 1993.  He may be reached at Mr. Conner has served his community as a school board member and has worked as a Tribal Court Judge as well as a College Instructor at Skagit Valley Community College and Chapman University.

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