Should Biomechanical Engineers Be Allowed To Testify In Low-Impact, Soft-Tissue Cases?   10 comments

Should Biomechanical Engineers Be Allowed To Testify In Low-Impact, Soft-Tissue Cases?

By Kevin W. Conner, J.D.



In the interest of full disclosure, I will begin answering the aforementioned question by admitting I hold a strongly biased opinion on this subject (but I am still right). As a personal injury attorney, I have spent too many hours on this earth trying to convince jurors, arbitrators, claims adjusters, and defense counsel that the only logical answer to the question above is a resounding no.I have also spent far too much of my time and my clients’ money reviewing study after study to counteract standard biomechanical engineer opinions regarding g-forces involved in low impact accidents.2

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.  This epiphany was not arrived at as a knee jerk reaction to the persuasiveness of the witness’ testimony, but was arrived at after a thorough understanding of why the testimony always viscerally rubbed me the wrong way.  I came to the conclusion that allowing such testimony is not an effective defense tactic that sheds light on the issues at hand at my client’s expense, but instead, is simply an unfair tactic that distorts the relevant truth and threatens the integrity of our judicial system.  I will follow with the support for my position.

I begin by giving you the 5 step plan of attack the biomechanical engineer will attempt to 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 photograph3, 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”.4 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. The biomechanical engineer expert will be happy to explain. This numerical label will become the cornerstone of the expert witness’ testimony in step #5.

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).5 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 6 inch curb.

Step 3.  The “g- force” comparisons of the forces sustained in our accident compared to non-injuring events are further emphasized by well-rehearsed anecdotes of extremely sophisticated and admittedly impressive high tech tests that involve high speed cameras, test sleds, and human test subjects. The expert will explain that the human test participants were subjected to roughly the same “g-force” as calculated in the accident in the case at bar.  Furthermore, the professor-like, well-spoken expert witness will explain that out of thousands of test subjects he or she has personally tested or read about in other studies, no test subjects were injured.6

Step 4.  The “safety” of being exposed to the “g- forces” involved in the accident in the case at bar will be made even more real with the reminder that it would be “unethical” to utilize test subjects if they would be exposed to forces that could injure them. This point will be highlighted or played up while playing down the fact that the participants of course 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 does involve some risk of injury.

Step 5.  The expert will slow the testimony at this point to make sure the mathematical points are again made clear.7This is the unveiling of the cornerstone of the expert testimony.  The review will essentially say subject X experienced a blank 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 subject X 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 undoubtedly 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 subject X could not have been injured in this accident, he or she will lead the fact-finder to the brink of this unmistakable implicit conclusion.8

Now I will reveal to you why the seemingly logical method of presenting expert testimony in low-impact, soft-tissue cases presented above is inherently unfair and should not be allowed.

The expert witness of course 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 will be erased and the field will expand to include implications pertaining to a diagnosis 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. I say great pains should be taken to insure his or her testimony is narrowly confined. If the testimony is in fact 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-force 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 applicable to the plaintiff.  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?9  Wouldn’t we naturally assume (as we have been led to) the “threshold of injury” must be greater than forces human test subjects are exposed to? 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?

Getting both the calculated g-force involved in the accident in the case at bar admitted into evidence and testimony regarding the g-forces involved in daily activities and/or lab studies admitted is critical for the defense (and thus should be where plaintiffs focus their resistance).10 If allowed, testimony about these forces lays the framework for a comparison that at first glance seems logically dispositive of the question of causation. In fact, without peeling the layers of the testimony and close scrutiny, the carefully scripted flow of the testimonial presentation does not strike anyone as being patently unfair. Therefore, the comparative presentation could logically lead one to conclude that since the g-forces in a given accident were “scientifically”11 established, and since subject X 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?  Is it beyond the realm of possibility that a person could be injured in this fashion?  I would submit to you that the g-force in that scenario is well below the g-forces that the test subjects were exposed to.

Another point to ponder follows. 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.  For example, actress Natasha Richardson died from injuries sustained when she fell from a standing position on a ski slope. How can a force sufficient to break bones or even cause death 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.12 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.13 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 depositions).

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 would simply be wrong.14There 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;15 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);16 position of headrest at time of the accident; type of headrest; and the angle of the impact all affect vertebral displacement.17  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.18This makes sense.  If a linebacker tackles a tailback in a football game, we would typically not expect an injury. If the same linebacker un-expectantly tackles the same unsuspecting tailback with the same force in a grocery store, one would expect an injury to occur more often in that scenario.19 

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 is additionally impermissible given the lack of foundation required before attempting to make the g-force comparisons between the forces experienced in a given accident and the forces experienced by the human test participants. Given the incredibly large number of variables that differ in a real world accident as compared to a laboratory collision, it is simply impossible to make an apples to apples comparison.20 Comparisons between forces in a given accident and forces experienced in daily living should likewise be excluded from evidence on both relevancy and foundational grounds.

This article is of course offered as one plaintiff attorney’s opinion.  I welcome a well thought out response to this position if any defense attorney differs or takes issue with my perspective. Good luck on your motions in limine. Give me a call if you need help.

(1)     All expletives which normally accompany my opinion on this subject have been left out for the benefit of sensitive readers.  Please do not misconstrue my lack of qualifying emphasis as a lack of passion or conviction regarding the issue at hand.

(2)     The biomechanical engineer’s opinion will basically conclude something to the effect that the forces acting upon subject X 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 subject X’s body.

(3)     The viewing of the photograph is in lieu of an actual vehicle inspection where presumably a trained eye could determine if the absorbers inside the bumper were compromised.  Also, crush zones could be viewed upon removal of the bumper allowing for a more accurate calculation.

(4)     The definition of a g-force is a unit of force equal to the force exerted by gravity.

(5)     A good starting point in understanding the physics involved in a low speed automobile accident is an article entitled Low Speed Impacts: Does No Property Damage = No Injuries? Written by attorney Paul E. Godlewski published in the Trial Lawyer Section of the Florida Bar Journal in February, (2000).

(6)     If you read the footnotes of the studies you will find there are some reports of injuries sustained during the studies. Also, please note it warrants finding out how many test subjects refused to continue with the study after the initial sled ride.

(7)     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.

(8)     I have successfully used the following rhetorical question in a motions hearing: “why should the expert be allowed to imply what he cannot say?”

(9)     From my experience, the mechanical engineer will testify there is in fact no minimum “threshold” for injury or in the alternative, he or she will refuse to quantify a threshold.

(10)     Therefore, this is the focus of what should now be a standard motion in limine for all plaintiff attorneys.  I will gladly share my brief in support of a motion to exclude testimony upon request.

(11)     Personally, I do not think viewing a photograph and/or property damage report (as opposed to an actual vehicle inspection looking at internal damage to the vehicle) would pass muster of the “scientific method.” Therefore, my 7th grade science teacher, Mr. Kratzer may not agree the g-force analysis as described above is in fact scientific.

(12)      See Evidence Rule 104.

(13)     Kaneoka K, Ono K, Inami S, & Hayashi K. Motion Analysis of Cervical Vertebrae During Whiplash Loading.  Spine, vol. 24, Number 8 (1999) pp 763-770.

(14)      Croft A & Haneline M. Notable Observations From Three Years of Human-Subject Crash Testing. Dynamic Chiropractic, vol. 20,Issue 17 (2002).

(15)      Kumar S, Ferrari R, & Narayan Y. Looking Away From Whiplash: Effect of Head Rotation in Rear Impacts. Spine, vol. 30, Number 7 (2005) pp 760-768.

(16)     Brault JR, Wheeler JB, Siegmund GP, & Brault EJ. Clinical Response of Human Subjects to Rear-End Automobile Collisions. Arch Phys Med Rehabil vol. 79, January (1998).

(17)      Kaneoka K, Ono K, Inami S, & Hayashi K. Motion Analysis of Cervical Vertebrae During Whiplash Loading.  Spine, vol. 24, Number 8 (1999) pp 763-770.

(18)     See Ryan GA, TaylorGW, Moore VM, Dolonis J. Neck Strain In Car Occupants: Injury Status After Six Months, and Crash Related Factors.  Injury 1994; 25(8); 533-537 which tells us injury is less likely when occupants in rear-impact collisions are braced and unaware patients were 15 times more likely to have long term pain.

(19)     See Freeman MD, Croft AC, Nicodemus CN, Centeno CJ, & Elkins WL. Significant Spinal Injury Resulting From Low-Level Accelerations: A Case Series Of Roller Coaster Injuries. Arch Phys Med Rehabil vol. 86, November (2005), pp 2126-2130.

(20)     Few reports of injury during lab tests is easily explainable when you consider the volunteer human test subjects were healthy individuals, expecting an impact, delivered at a repeatable pre-determined sled angle, with pre-determined body and head positions. (None of these factors occur during a real world crash).

Kevin W. Conner works in the Mount Vernon Office of Tario & Associates, P.S.  Kevin graduated from the University of South Dakota School of Law in 1993.  In addition to working in the private practice of law, Kevin has taught at BrandmanUniversityand in the paralegal program at SkagitValleyCollege.  He may be reached at


Posted September 26, 2011 by tariolaw in Case Studies