The engineer hired to say you cannot be hurt
In almost every contested soft-tissue auto case, the defense reaches for the same witness: a biomechanical engineer who will testify that the forces in the collision were too low to have caused the plaintiff's injuries. The opinion usually arrives dressed in physics, complete with delta-V calculations, occupant-kinematics diagrams, and citations to low-speed crash studies. It sounds authoritative. Handled correctly, it is also one of the most vulnerable opinions in the defense arsenal.
The vulnerability is a matter of scope. A qualified biomechanical engineer can competently describe the general forces a body experiences in a rear-end impact. What that engineer usually cannot do, and frequently is not licensed to do, is render a medical opinion that this plaintiff's cervical spine was not injured. The line between describing forces and diagnosing an individual is where most of these opinions should be trimmed or excluded.
Where the opinion overreaches
Start with qualifications. Many defense biomechanics hold engineering degrees, not medical ones. Under the rules governing expert testimony, an expert may opine only within the bounds of their expertise. An engineer who slides from generalized force description into a causation opinion about a specific human injury has stepped outside that boundary, and a growing number of courts have said so, limiting biomechanical witnesses to biomechanics and leaving medical causation to physicians.
Then attack the fit. The low-speed studies these experts rely on were often conducted on small numbers of healthy, young, braced volunteers in staged impacts. Your plaintiff may be older, may have had a degenerative predisposition, and did not know the impact was coming. An opinion built on volunteer data that does not match the plaintiff's age, health, position, or awareness fails the fit requirement no matter how sound the underlying study was for its own purpose.
Building the challenge
The deposition is where you set up exclusion. Pin the engineer down on a short list of questions:
- Are you a physician? Are you offering an opinion that the plaintiff was not injured, or only about the forces involved?
- Did you examine the plaintiff? Review the imaging? Speak to any treating physician?
- What are the demographics of the subjects in the studies you rely on, and how do they compare to this plaintiff?
- Have you accounted for the plaintiff's specific position, awareness, and pre-existing condition?
The answers usually establish that the witness never examined the plaintiff, relied on inapposite study populations, and is offering a medical conclusion without medical training. That record supports a motion to exclude or, at minimum, to limit the testimony to general biomechanics and bar the causation leap. Even a partial win reshapes the trial, because a jury hears a very different message from an engineer describing general forces than from one flatly declaring the plaintiff was not hurt.
The photographs problem
Bound up with the biomechanical defense is the minor-damage photograph. Defendants like to show the jury a bumper with a scuff and imply that a small dent means a small injury. The counter is both legal and evidentiary. Property damage is not a proxy for bodily injury, and modern bumper systems are engineered to absorb low-speed energy without visible deformation, which means the energy went somewhere other than the sheet metal. If the defense biomechanic relies on the photographs to bootstrap a causation opinion, that reliance is another exclusion argument, because it substitutes an untested visual inference for actual analysis.
The affirmative case still runs through the treating physicians. A biomechanical rebuttal expert can be useful, but the center of gravity should be the treaters who examined the plaintiff, documented the mechanism, and can explain why a low-speed rear impact produces exactly the injury pattern they saw. Our auto coverage has walked through the low-impact defense before, and the through-line is consistent: keep the medical questions with the physicians and confine the engineer to engineering.
Practical posture
Do not treat the biomechanical report as something to fear. Treat it as an opinion that has usually reached past its own competence. Depose to the scope limits, move to exclude the causation leap, and where the court lets the engineer testify, cross-examine on the mismatch between the study subjects and your client. The sourcing discipline that governs medical experts, which our medical-malpractice coverage examines in depth, applies with equal force here: an expert is only as good as the fit between the data relied on and the person in the courtroom. For how appellate courts have policed the biomechanics-versus-medicine line, see our case-law coverage.