Limitations to Expert Testimony
Key Points:
- An expert may not testify to matters that fall outside his or her area of expertise.
- Biomechanical engineers are generally qualified to opine about an accident’s forces and the general types of injuries those forces may generate.
- Biomechanical engineers may not offer opinions regarding patient-specific causation without appropriate medical expertise.
Experts play a critical role in litigation. In cases where there is a dispute over whether the forces involved in an accident were sufficient to cause an alleged injury, biomechanical experts are especially valuable. These experts analyze human movement and applied forces to assist in accident reconstruction and injury mechanism analysis.
Under Section 90.702, Fla. Stat. (2025):
[A] witness qualified as an expert by knowledge, skill, experience, training, or education may testify about it in the form of an opinion or otherwise, if:
(1) The testimony is based upon sufficient facts or data;
(2) The testimony is the product of reliable principles and methods; and
(3) The witness has applied the principles and methods reliably to the facts of the case.
The Fifth District Court of Appeal recently addressed the limits of biomechanical expert testimony in Clark v. Hahn, 401 So.3d 550, 551 (Fla. 5th DCA 2024), in which the plaintiff filed a lawsuit against Kermit and Evelyn Hahn for negligence.
The plaintiff alleged she suffered permanent injury as a result of a car accident. During the trial, the plaintiff requested that the court prohibit the defense expert from giving medical causation opinions. The defendants’ counsel assured the court that the biomechanical expert would not offer medical opinions.
However, during the trial, the biomechanical expert was asked whether the forces applied to the plaintiff’s spine could have produced the injury mechanisms at issue. The expert opined that the particular accident would not have generated the mechanisms necessary to cause an intervertebral disc herniation in the cervical spine in a person of the plaintiff’s height and weight, given the specific vehicle involved. The trial court overruled the plaintiff’s objection and permitted the testimony.
The jury ultimately found that the plaintiff did not suffer a permanent injury. The trial court denied the plaintiff’s motion for a mistrial, and the plaintiff appealed. On appeal, the Fifth District Court of Appeal reversed and remanded for a new trial, holding that the expert’s testimony exceeded the scope of his expertise.
The District Court reasoned the expert testified to matters outside of his area of expertise by offering a patient-specific causation opinion. The biomechanical expert was not a medical doctor and, thus, not qualified to render such an opinion. The court explained, “While biomechanical experts may discuss the forces generated by an accident and how a hypothetical person’s body will respond to those forces they are not qualified to render medical opinions regarding the precise cause of a specific injury.”
Biomechanical experts are not generally allowed to render opinions that require medical expertise, such as the permanency or severity of an injury. Such opinions require a medical evaluation of the patient, which is beyond the typical expertise of a biomechanical engineer. This limitation does not necessarily apply if the biomechanical engineer is also a medical doctor. Doctors retained as experts may testify as to specific causation.
The expert’s training, education, and certifications must be analyzed alongside their expected testimony to determine whether their opinions are beyond their expertise. In such cases, a Daubert motion must be timely filed to exclude the opinions that are outside of the expert’s scope. The Daubert standard stems from a United States Supreme Court case that outlines the trial courts’ role as the “gatekeepers” of expert testimony. Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579 (1993). The Daubert standard is codified in § 90.702, Fla. Stat. (2025), and requires the expert’s testimony to be both relevant and reliable. Understanding the limitations of an expert’s testimony plays a vital role in structuring litigation strategy.
Clark reinforces a key evidentiary boundary for expert testimony. Biomechanical engineers may testify regarding the magnitude of accident forces and the general injury mechanisms such forces can produce. However, biomechanical engineers may not cross into medical causation opinions without the appropriate medical qualifications. The case also highlights the importance of timely objecting to improper expert testimony.
Defense Digest, Vol. 31, No. 4, December 2025, is prepared by Marshall Dennehey to provide information on recent legal developments of interest to our readers. This publication is not intended to provide legal advice for a specific situation or to create an attorney-client relationship. ATTORNEY ADVERTISING pursuant to New York RPC 7.1. © 2025 Marshall Dennehey. All Rights Reserved. This article may not be reprinted without the express written permission of our firm. For reprints, contact tamontemuro@mdwcg.com.