When a collision leaves relatively little visible vehicle damage, the defense may try to turn the case into a numbers exercise: the impact was “minor,” the Delta-V was low, the computer model shows limited occupant movement, and therefore the plaintiff could not have sustained the claimed injury.
That argument can sound scientific. But a biomechanical opinion is not self-proving. It depends on a chain of assumptions, measurements, models, and scientific literature. If one of those links is unreliable—or if the expert takes population-based research and applies it to a particular plaintiff without adequately accounting for that plaintiff’s circumstances—the opinion may not support the conclusion the defense wants the jury to reach.
For plaintiff’s counsel, the objective is not to argue that biomechanics has no legitimate role. It is to determine exactly what the expert can reliably say, identify the assumptions underlying the opinion, and expose the analytical gap between a collision in the real world and the conclusion that this particular plaintiff could not have been injured.
What a Biomechanical Expert Actually Does
Biomechanics sits at the intersection of engineering and biology. In collision litigation, a biomechanical expert may evaluate the forces generated by an impact, the movement of an occupant, and the mechanical effects those forces can have on the human body.
A commonly described forensic approach has three stages:
1. Accident reconstruction. The expert determines the severity and direction of the collision and estimates the forces generated during the event.
2. Occupant-motion analysis. The expert uses those inputs, sometimes through computer simulation, to estimate how the occupant moved and what loads were applied to a body region.
3. Comparison with injury criteria. The estimated loads are compared with research-based injury thresholds, risk curves, or other published biomechanical criteria.
That framework is important because it gives counsel three separate places to examine the defense opinion. The question is not simply whether the expert’s final conclusion sounds reasonable. The question is whether each step supports the next one.
The First Legal Question: What Is the Expert Actually Qualified to Say?
One of the most important issues is the distinction between general causation and specific causation.
General causation asks whether a particular level or type of force can cause a particular type of injury in a population. Specific causation asks whether the forces generated in the particular collision caused the particular injury suffered by the particular plaintiff.
Courts have not adopted a uniform rule governing how far a biomechanical engineer may go on specific causation.
In Smelser v. Norfolk Southern Railway, 105 F.3d 299, 305 (6th Cir. 1997), the Sixth Circuit held that a biomechanical engineer was not qualified to testify about the precise cause of an individual’s injury where the opinion required consideration of individual tolerance levels and pre-existing medical conditions. The court limited the testimony to the forces generated in the collision and general discussion of the types of injuries those forces could produce.
Other courts have taken a different approach. In Eskin v. Carden, 842 A.2d 1222, 1230 (Del. 2004), the Delaware Supreme Court focused on whether the expert’s methodology reliably connected the general response of the human body to the forces of the accident with the individual plaintiff.
That disagreement makes the expert’s precise assignment critical.
Before the deposition, identify every opinion in the report and ask what qualification is being relied upon for each one.
Is the expert reconstructing the crash? Estimating forces? Describing occupant movement? Explaining general injury mechanisms? Or actually diagnosing an injury and deciding that the collision did or did not cause it?
Those are different opinions. A professional engineer may have substantial expertise in reconstruction and mechanics without having medical training that permits the witness to diagnose pathology, interpret a patient’s complete medical history, or determine the medical cause of a particular condition.
Attack the Reconstruction Before Attacking the Biomechanics
The biomechanical analysis is only as reliable as the collision inputs supplied to it.
That makes the accident-reconstruction file one of the most important discovery targets. Do not settle for the biomechanical expert’s summary of the reconstruction. Obtain the underlying photographs, measurements, repair records, vehicle specifications, event-data information, calculations, software inputs, assumptions, and reconstruction report.
Ask:
• What vehicles were inspected personally?
• Were the vehicles available for inspection?
• Did the expert personally download any event-data or onboard information?
• What actual vehicle weights were used?
• What measurements were taken rather than estimated?
• What repair information was available?
• What assumptions were made about crush, stiffness, speed, direction of force, or vehicle configuration?
• Were alternative reconstruction methods considered?
• What happens to the biomechanical opinion if one of those assumptions changes?
The point is not to force a disagreement. It is to determine whether the expert’s force estimate is grounded in measured facts or depends heavily on assumptions. When the reconstruction changes, the biomechanical analysis built upon it may change as well.
Delta-V Is Not a Magic Injury Number
Delta-V is often central to the defense presentation, but counsel should resist turning the case into a debate over one number.
Delta-V is a measure of change in velocity. It can be an important data point in reconstructing a collision, but it does not, by itself, determine whether a particular person suffered a particular injury.
The forces experienced by an occupant depend on more than Delta-V. Direction, duration, location, acceleration characteristics, vehicle structure, occupant position, restraint use, and individual characteristics can matter. A collision can also involve a complex crash pulse rather than a single instantaneous force.
The 2018 analysis by Rami Hashish, Omid Komari, and Manon Limousis-Gayda is particularly useful on this point. Their review of biomechanical literature concerning low-speed rear-end collisions identified limitations in reducing injury potential to a simple comparison between experienced force and an injury-tolerance metric.
The authors emphasized factors including loading rate, muscle contraction, pre-existing conditions, age, weight, sex, and the magnitude, duration, location, and direction of applied forces.
Accordingly, the better deposition question is not simply: “What was the Delta-V?”
It is: “What assumptions allow you to move from that Delta-V to a conclusion about this plaintiff’s injury?”
The Computer Model Is an Estimate—Not a Videotape of What Happened
The second stage of the analysis may involve computer simulation of occupant kinematics.
Computer modeling can be useful, but a simulation is still a model. It depends upon inputs and assumptions about the collision, occupant characteristics, seating position, restraint use, and other variables.
The published literature discussed by Loren Peck identifies validation error rates for certain computer simulations and emphasizes the importance of fit: whether the simulation actually represents the facts of the case.
A single computer run does not establish that the simulated movement is exactly what occurred.
That creates a practical line of questioning:
• What program was used?
• What version?
• What inputs were entered?
• Where did each input come from?
• Which inputs were measured and which were assumed?
• Was the plaintiff’s actual height and weight used?
• Was the plaintiff’s actual seating position used?
• Was restraint use modeled accurately?
• Were alternative simulations run?
• If multiple reasonable inputs were possible, did the expert test them?
• What published validation study supports the program for this particular application?
• What is the error rate or uncertainty associated with the analysis?
Make the expert show the jury the bridge between the model and the plaintiff. Generally speaking, Federal Rule of Evidence 702 and Daubert do not permit an expert to bridge an analytical gap merely by asserting that the conclusion follows from the data.
The Injury-Threshold Problem
The third stage—comparing estimated forces to published injury thresholds—may present the most important scientific issue in a low-speed collision case.
Biomechanical injury thresholds are generally developed from experimental research. Because researchers cannot ethically subject humans to potentially injurious testing, the literature may rely on cadavers, limited human-volunteer testing, animals, or other surrogates.
The problem is not that such research has no value. The problem is whether the research supports the particular inference the expert wants to make in the courtroom.
External Validity
Tolerance limits derived from cadavers or healthy younger subjects may not translate cleanly to people with different ages, body sizes, BMIs, or pre-existing conditions.
Loading Rate and Viscoelasticity
Intervertebral discs are viscoelastic. Their response depends in part on how quickly a load is applied.
The Hashish, Komari, and Limousis-Gayda paper noted that real-world rear-impact crash pulses and laboratory loading conditions may differ substantially, which can affect the applicability of published tolerance values.
Surrogate Testing
Some injury criteria were developed using animals or other human surrogates.
Those surrogates cannot necessarily reproduce all aspects of human response to a real collision, including active muscle contraction and other occupant responses.
Those limitations do not establish that every biomechanical injury criterion is invalid. They establish something more useful for litigation: the expert must explain why the particular research applies to the particular collision and the particular plaintiff.
That is where the fit analysis becomes critical.
Use the Expert’s Foundation Against the Opinion
The most effective biomechanical depositions often focus on foundation rather than confrontation.
The expert may have inspected only photographs rather than the vehicle, may not have downloaded vehicle data, may not have actual vehicle weights, may not have reviewed the plaintiff’s complete medical records, may lack peer-reviewed crash data supporting a particular model, or may not have accounted for individualized medical conditions or seating position.
A strong deposition outline should therefore proceed from the witness’s own report:
• Qualifications and scope of opinions.
• Every document and data source reviewed.
• Every document or data source requested but not received.
• Vehicle inspection and physical measurements.
• Reconstruction methodology and assumptions.
• Computer program and validation.
• Occupant characteristics and seating position.
• Injury criteria and underlying studies.
• Plaintiff-specific medical information.
• Opinions the expert is unwilling or unable to give.
The goal is to leave the witness with a narrower, more defensible opinion than the one presented in the report.
Research the Expert Before the Deposition
Do not begin your expert deposition with the expert’s report. Begin with the expert.
Review prior deposition testimony, prior reports, published articles, presentations, fee schedules, testimony lists, and opinions in oth`er cases.
Look for positions the expert has taken about injury thresholds, collision mechanisms, pre-existing conditions, and the limits of biomechanical analysis.
Prior testimony can be especially valuable when the expert has adopted different thresholds for different collision configurations or different anatomical regions. It can also reveal occasions when the expert relied upon research that the expert now criticizes.
Compensation and litigation history can be relevant to bias, but they should be handled carefully. The objective is not to imply that a paid expert is automatically unreliable. Experts are paid for their work.
The useful question is whether the witness’s litigation history, client mix, compensation, or repeated opinions bears on credibility or consistency.
The same principle applies to the retaining firm. The more you know about the witness before the deposition, the less likely you are to discover important information for the first time at trial.
The Pre-Existing-Condition Argument: Do Not Concede Too Much
A plaintiff with a pre-existing condition may face a familiar defense argument: the collision did not cause an injury; the plaintiff simply has degenerative “wear and tear.”
That argument creates an important distinction between causation and aggravation.
The question may not be whether the plaintiff had a perfectly healthy spine before the collision.
The question may be whether the collision caused a new injury, aggravated a pre-existing condition, accelerated symptoms, or resulted in treatment that otherwise would not have occurred.
The Power Hour materials specifically emphasize obtaining an admission concerning aggravation where a pre-existing condition exists and cite Texas Pattern Jury Charge 8.8 for the distinction.
For deposition purposes, ask the defense expert to identify exactly what the expert is saying:
• Is there a pre-existing condition?
• What evidence establishes that it was symptomatic before the collision?
• What evidence establishes that the plaintiff would have required the same treatment absent the collision?
• What part of the opinion is biomechanical?
• What part requires a medical diagnosis?
• Can the expert exclude aggravation?
Those questions can move the case away from the false choice of “perfectly healthy before the crash” versus “completely unrelated degenerative disease.”
A Better Way to Frame the Case for the Jury
The most effective trial presentation is usually not an argument that “biomechanics is junk science.” That overstates the point and gives the defense an easy response.
A better argument is that biomechanics has a legitimate role, but the defense expert has gone beyond what the science and the expert’s qualifications can reliably establish.
The jury can understand the distinction:
The reconstruction estimates what happened to the vehicles.
The biomechanical analysis estimates what may have happened to the occupant.
The medical evidence addresses what injury the plaintiff actually sustained and the medical cause of that injury.
If the first step depends on disputed assumptions, the second step depends on a computer model that does not fully reproduce the plaintiff, and the third step depends on population-based thresholds developed under different conditions, the jury should be cautious before treating the final conclusion as a scientific certainty.
That is the analytical gap counsel should expose.
The Takeaway for Practitioners
A minor-impact defense built on biomechanical testimony is only as strong as the reconstruction, the simulation, and the injury-threshold literature underneath it—and each of those layers has been shown, in both the legal and biomechanical literature, to have real limits. Attorneys who press on those limits, rather than arguing generally that the expert is biased, tend to get further with juries and judges alike. For attorneys facing this issue, the experienced personal injury attorneys at Ammons Law Firm are a resource worth consulting.
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