Biomechanical evidence allowed

The Evolution of Expert Testimony in Personal Injury Causation

Personal injury litigation increasingly relies on expert testimony to establish causation between accidents and claimed injuries. One specialized field that has gained significant traction is biomechanical engineering—the study of how mechanical forces affect the human body during accidents. The admissibility of such testimony has evolved considerably, with courts balancing the need for scientific rigor against the practical realities of establishing causation in accident cases.

Under New York law, expert witnesses must satisfy several threshold requirements before they can testify. First, the witness must possess the requisite skill, training, education, knowledge, or experience to render an opinion on the subject matter. Second, the expert’s methodology must be sufficiently reliable to be helpful to the jury. Third, in cases involving novel scientific techniques or principles, the proponent may need to satisfy the Frye standard, demonstrating that the methodology has gained general acceptance in the relevant scientific community.

Biomechanical engineering testimony occupies a unique space in this framework. Unlike traditional medical experts who diagnose and treat injuries, biomechanical engineers analyze the physical forces at play during accidents and opine on whether those forces were sufficient to cause the claimed injuries. This reverse causation analysis—working backward from the accident mechanics to evaluate injury claims—provides defendants with a powerful tool to challenge plaintiffs’ injury allegations.

The growing acceptance of biomechanical evidence reflects courts’ recognition that injury causation involves both medical and mechanical components. While physicians can attest to the nature and extent of injuries, biomechanical experts provide essential context about whether the accident dynamics could plausibly produce those injuries. When properly qualified and applying accepted methodologies, biomechanical engineers can offer opinions that address central causation issues, even when those opinions challenge medical diagnoses provided by treating physicians.

Case Background: Gonzalez v. Palen

In Gonzalez v. Palen, the plaintiff brought a personal injury action arising from a motor vehicle accident, claiming significant injuries caused by the collision. The defendants sought to introduce expert testimony from Dr. Kevin Toosi, a biomechanical engineer, who would opine that the forces generated in the subject accident were insufficient to cause the injuries alleged by the plaintiff.

The trial court conducted a Frye hearing to assess Dr. Toosi’s qualifications and the admissibility of his proposed testimony. At the hearing, evidence established that Dr. Toosi possessed a Ph.D. in biomechanical engineering, held a medical license in Iran, and had accumulated three years of professional experience in accident reconstruction, including specific study of how forces affect vehicle occupants during collisions.

Despite these credentials, the trial court determined that Dr. Toosi was not qualified to render expert opinions on the cause of the plaintiff’s injuries and precluded his testimony. The defendants appealed, arguing that the court applied an overly restrictive standard for expert qualification and that excluding this probative evidence on a central causation issue constituted reversible error. The Appellate Term faced the question of what qualifications suffice for biomechanical engineers to testify about injury causation in motor vehicle accident cases.

The Appellate Term’s Decision

Gonzalez v Palen, 2015 NY Slip Op 51101(U)(App. Term 1st Dept. 2015)

“The trial court erred in determining that defendants’ biomechanical engineer, Dr. Kevin Toosi, was not qualified to render an opinion as an expert as to the cause of plaintiff’s injuries. Even assuming that an evidentiary hearing was warranted to assess Toosi’s professional qualification (see Frye v United States, 293 F 1013 ), the evidence presented at the Frye hearing established that Toosi had the academic and professional qualifications – including a PhD in biomechanical engineering, a license to practice medicine in Iran, and three years’ experience in accident reconstruction, which included the study of the effects of force on occupants inside a vehicle – to render an opinion as an expert on the issue of whether the force generated in the subject motor vehicle accident was sufficient to cause the injuries alleged by plaintiff”

“Inasmuch as Toosi’s testimony was probative of a central issue in the case, the preclusion of this evidence cannot be deemed harmless”

In the realm of biomechanical practice, the Appellate Courts have been expanding the ability of a party to utilize this type of evidence to dispute the central causation issue.

Legal Significance of the Gonzalez Decision

The Gonzalez decision marks an important expansion of admissible expert testimony in personal injury litigation. By reversing the trial court’s exclusion of Dr. Toosi’s testimony, the Appellate Term sent a clear signal that biomechanical engineering qualifications—combining advanced academic training with practical experience in accident reconstruction—suffice to qualify experts on injury causation issues.

The court’s emphasis on Dr. Toosi’s multidisciplinary credentials is particularly noteworthy. His Ph.D. in biomechanical engineering provided the theoretical foundation for understanding force mechanics and human anatomical responses. His medical license, even from a foreign jurisdiction, demonstrated familiarity with human physiology and medical concepts. His three years of accident reconstruction experience, including specific study of forces affecting vehicle occupants, supplied the practical application knowledge necessary to connect theoretical principles to real-world accident scenarios.

The court’s holding that exclusion of this testimony “cannot be deemed harmless” elevates biomechanical evidence from a peripheral or cumulative form of proof to evidence addressing “central issues” in personal injury cases. This characterization protects defendants’ ability to present such evidence and creates stronger grounds for reversal when trial courts improperly exclude qualified biomechanical experts.

The decision also reflects the broader trend Jason Tenenbaum identified: appellate courts increasingly recognize biomechanical evidence as legitimate scientific methodology for challenging injury causation. This evolution parallels developments in other jurisdictions where biomechanical testimony has moved from controversial novelty to accepted practice, provided the expert possesses appropriate qualifications and applies scientifically sound methodologies.

Significantly, the court did not require that biomechanical experts hold medical degrees or licenses to practice medicine in New York. The combination of engineering expertise and accident reconstruction experience, supplemented by Dr. Toosi’s foreign medical credentials, satisfied the qualification threshold. This opens the door for defendants to retain biomechanical engineers with strong technical credentials even if they lack traditional medical backgrounds, provided they can demonstrate relevant experience applying biomechanical principles to accident analysis.

Practical Implications for Personal Injury Practitioners

For defense attorneys in personal injury cases, Gonzalez validates the strategic use of biomechanical experts to challenge causation. When plaintiffs claim injuries from relatively minor impacts or when the claimed injury severity seems disproportionate to accident forces, biomechanical analysis can provide objective, scientifically-grounded counter-evidence. Defense counsel should consider retaining biomechanical experts early in litigation to evaluate whether the accident mechanics support or undermine the plaintiff’s injury claims.

When selecting biomechanical experts, attorneys should prioritize candidates with advanced degrees in biomechanical engineering or related fields, practical experience in accident reconstruction, and familiarity with medical terminology and anatomical principles. While a medical degree or license is not required post-Gonzalez, such credentials strengthen the expert’s profile and may preempt admissibility challenges. Experts should be prepared to explain their methodologies, demonstrate that their approaches have gained acceptance in the biomechanical engineering community, and connect their force analysis to the specific injuries claimed.

Plaintiffs’ attorneys must prepare to counter biomechanical testimony through several strategies. First, challenge the expert’s specific qualifications for the injuries at issue—an expert well-versed in spinal biomechanics may lack comparable expertise regarding shoulder or knee injuries. Second, attack the expert’s assumptions about accident variables like vehicle speeds, impact angles, or restraint system performance. Small errors in these inputs can substantially alter force calculations. Third, retain a competing biomechanical expert or medical expert who can explain why the defendant’s force analysis is incomplete or fails to account for individual vulnerability factors.

Both plaintiffs and defendants should recognize that Gonzalez does not eliminate the need for proper expert disclosure under CPLR 3101(d). Courts can still preclude biomechanical testimony for failure to provide timely and adequate expert disclosure, even when the expert is qualified. The decision addresses qualification and relevance, not procedural compliance with discovery obligations.

Related Articles

• Out of Scope? Establish Proper Foundation for Expert Testimony
• CPLR 3101(d) Preclusion: The Price of Inadequate Expert Disclosure
• Expert Witness Qualification Standards in New York Personal Injury Cases
• Challenging Medical Expert Testimony: Strategies for Cross-Examination
• The Frye Standard and Novel Scientific Evidence in New York

Legal Update (February 2026): Since this 2015 decision, biomechanical expert testimony has become increasingly sophisticated, incorporating computer modeling, crash simulation software, and advanced imaging analysis. Courts continue to apply Gonzalez’s framework for evaluating biomechanical expert qualifications while requiring experts to demonstrate that their specific methodologies and tools have gained acceptance in the relevant scientific community. Recent decisions have addressed the admissibility of finite element analysis, multibody dynamics modeling, and other advanced biomechanical techniques.