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By Gary J. Willnauer and Julie A. Jorgenson

Biomechanical engineers are being used with more and more frequency in litigation and an excellent biomechanical expert can certainly create an advantage for a party. Biomechanical engineers are most frequently used in a civil context in low speed car accident cases, but they can also be of great use in other types of cases such as: slip/trip and fall cases, unrestrained driver cases, cases involving a crush injury, and product liability cases. However, biomechanical engineers are frequently the subject of federal court Daubert Motions, and foundational objections in both federal and state courts. Sometimes these challenges are based on the facts and research the expert is using, but sometimes the expert’s testimony is put at risk based on the attorney’s misunderstanding of the scope of the expert’s expertise and elements of analysis that must be included in the disclosed opinion.

Scope of Biomechanical Testimony

Biomechanical engineers are trained to analyze and describe the mechanical modalities of the crash, the fall, the movement, or the compression that cause a mechanical failure of the human body. Therefore, in terms of causation a biomechanical engineer can be used in a complementary way with a physician, with the biomechanical expert testifying as to mechanical causation and the doctor testifying as to medical causation. The biomechanical expert then serves as a bridge between the pure mechanics of the injury and the ultimate diagnosis. For instance in a products case where suffocation is alleged, a physician may opine as to the cause of death, but a biomechanical engineer could offer expert testimony regarding the forces on a person’s chest that are necessary to restrict breathing to the point of suffocation, and whether such forces were present in the case at issue.

Problems begin to occur when the biomechanical expert is asked to go one step further and actually opine as to the cause of death or the medical cause of the injury or when there is insufficient factual detail given as a basis for the opinion. Problems also arise when there is a significant gap between the studies used and the conclusions reached.

A review of some of the standards for admission of expert testimony can help assure that your biomechanical expert is using an adequate factual basis for his or her testimony and to assure that the analysis and methodology used meets standards for admission.

Standards for Admission – Federal

As stated by one federal district court with regard to its role with regard to experts: “the district court is the gatekeeper. It is an important role: [E]xpert witnesses have the potential to be both powerful and quite misleading[;] the court must ensure that any and all scientific testimony . . . is not only relevant, but reliable. In carrying out this role, I need not determine that the proffered expert testimony is irrefutable or certainly correct—[a]s with all other admissible evidence, expert testimony is subject to testing by ‘vigorous cross-examination, presentation of contrary evidence, and careful instruction on the burden of proof.'” Tyree v. Boston Sci. Corp., 54 F. Supp. 3d 501, 516 (S.D. W. Va. 2014) (internal quotations and citations are omitted).

Under Federal Rule of Evidence 702, a witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:
(a) the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
(b) the testimony is based on sufficient facts or data;
(c) the testimony is the product of reliable principles and methods; and
(d) the expert has reliably applied the principles and methods to the facts of the case.

When the admissibility of expert testimony is challenged, the Court must them consider whether: (1) The testimony is based upon sufficient facts or data; (2) The testimony is the product of reliable principles and methods; (3) The witness has applied those principles and methods reliably to the facts of the case. It is important to remember that when your expert is challenged, it is the proponent of the expert testimony that has the burden to prove its admissibility by a preponderance of the evidence. Lauzone v. Senco Products Inc. 270 F.3d 681, 686 (8th Cir., 2001) citing Daubert v. Merrell Dow, 509 U.S. 592 (1983).

Under Rule 702 and Daubert the trial judge acts as a gate keeper with regard to the admission of expert testimony. The trial judge evaluates the testimony and only allows such testimony if it is both relevant and reliable. In the case of Kudabeck v. Kroger Co., 338 F.3d 856, 860 (8th Cir. 2003) the 8th Circuit discussed the trial court’s gatekeeper function under Daubert and under Rule 702. It stated:

First, the trial court must make a ‘preliminary assessment of whether the reasoning or methodology underlying the testimony is scientifically valid and of whether that reasoning or methodology properly can be applied to the facts in issue.’ Daubert, 509 U.S. at 592-93, 113 S.Ct. 2786, 125 L.Ed.2d 469. The Court cautioned that the trial court must focus ‘on [the] principles and methodology, not on the conclusions that they generate.’ Id. at 595, 509 U.S. 579, 113 S.Ct. 2786, 125 L.Ed.2d 469. Second, the court must ensure that the proposed expert testimony is relevant and will serve to aid the trier of fact. Id. at 592, 509 U.S. 579, 113 S.Ct. 2786, 125 L.Ed.2d 469. Expert testimony assists the trier of fact when it provides information beyond the common knowledge of the trier of fact. Id. at 591, 509 U.S. 579, 113 S.Ct. 2786, 125 L.Ed.2d 469. The Court, in Kumho Tire Co. v. Carmichael, 526 U.S. 137, 119 S.Ct. 1167, 143 L.Ed.2d 238 (1999), clarified that the district court’s gatekeeper function applies to all expert testimony, not just testimony based in science. Id. at 147, 526 U.S. 137, 119 S.Ct. 1167, 143 L.Ed.2d 238.”

The factual basis of an expert’s opinion can also form the basis for exclusion. While the factual basis of an expert’s opinion generally goes to the credibility of the testimony and is addressed on cross examination, the gatekeeping inquiry must analyze whether the opinion is truly “tied to the facts” of a particular case. Kumho Tire Co., Ltd. V. Carmichael, 119 S.Ct. 1167, 1175 (1999). “If the expert’s opinion is so fundamentally unsupported that it can offer no assistance to the jury, such testimony must be excluded.” Hartley v. Dillards Inc., 310 F.3d, 1054, 1061 (8th Cir., 2002)(citations and quotations admitted).

Standards for Admission – Missouri

In Missouri admissibility of expert opinions is governed by state statute. Section 490.065 of the Missouri Revised Statutes governs the admission and exclusion of expert testimony, and has four requirements: 1) the expert witness must be qualified; 2) the testimony will assist the trier of fact; 3) the expert’s testimony is based on facts or data of a type reasonably relied on by other experts in the field; and 4) the facts or data used by the expert are otherwise reasonably reliable. See, State Board of Registration for the Healing Arts v. McDonagh, 123 S.W.3d 146, 152-153, (Mo banc 2003) While this certainly allows a less rigorous inquiry than federal standards, common challenges still exist.

Exclusion Based on Inadequate Factual Basis

Both the federal and state standards allow for the exclusion of expert opinions if the opinion does not have a reliable or adequate factual basis. A challenge to an opinion based on an unreliable or inadequate factual basis is essentially a foundation challenge and the case law supporting such a challenge can lie in both state and federal law.

A case that clearly demonstrates the potential foundational factual pitfalls for many matters in a biomechanical opinion is the Illinois case of Martin v. Sally, 341 Ill. App. 3d 308, 313-314 (Ill. App. Ct. 2d Dist. 2003). The case involved low speed rear end collision in which the plaintiff Martin alleged the accident resulted in her need for back surgery. It is a case where the court found that the expert had not considered all of the relevant facts. It is a cautionary case, as the factual basis for the biomechanical expert’s opinion in the Martin case looks very similar to the basis found in many biomechanical reports.

The accident at issue occurred when Martin was stopped at a red light. It had been raining and the pavement was wet. Sally testified that he saw the red light and Martin’s van in front of him. He applied his brakes, but he skidded on the wet pavement and his pickup truck struck the rear of Martin’s van. Sally estimated that his truck was traveling between five and seven miles per hour when it “squarely” impacted the rear end of Martin’s van. Martin eventually underwent a lumbar discectomy at the L4-L5 level.

There were competing physician opinions as to whether the surgery was causally related to the accident. In addition to physician testimony, Defendant Sally offered the expert testimony of Mark Strauss an “accident reconstructionist and biomechanicist.” The Illinois Appellate Court found that Strauss had training and professional experience in “the study of how humans may or may not be injured in different types of vehicular accidents.”

Strauss testified that the factual basis for his report included his review of the police report, deposition transcripts, and photographs taken of both vehicles after the accident. “He also researched measurements of an exemplar Dodge Caravan such as the one operated by plaintiff, and he testified about studies of whether the human body could be injured in low-speed, rear-end impacts.” Id. at 313.

Strauss opined that based on the material he had reviewed, the defendant’s estimation that his vehicle was traveling between five and seven miles per hour when it struck plaintiff’s vehicle was supported. “He stated there was nothing in the photographs that would indicate that the contact between the two vehicles was anything other than a low-speed impact and that the “delta V” of plaintiff’s vehicle (the maximum speed that plaintiff’s vehicle achieved due to the impact) was approximately three to five miles per hour.” Id.

Strauss then utilized biomechanical studies and tests to determine what types of injuries would result from this type of low-speed impact. According to Strauss the studies and tests utilized in his opinion “were all subjected to a peer review process, used methodology or techniques firmly established in the field of biomechanical engineering, and were scientifically performed. He also testified that the tests were performed not only by independent researchers, but also by university labs and by the government to establish the safety of vehicles.” The tests utilized in his opinion “showed that in rear-end impacts involving speeds of 10 miles per hour from one vehicle into the back of another, the human volunteers showed either no symptoms or only muscle tension or slight damage to the tissues in the neck and the shoulder, and sometimes the volunteers experienced headaches.” Id. at 314. “Strauss noted that in none of the human volunteers was there any injury or damage sustained to the lower back, even in volunteers who had abnormalities of disc degeneration in their lumbar spines according to pretesting MRIs.” Id. Based on his review of the materials and scientific research, Strauss opined that the impact between the two vehicles in the accident at issue was “not sufficient to aggravate any condition in plaintiff’s back that existed prior to the accident, including her protruding disc at the L4-L5 level.” Id.

The court found that “Strauss was improperly allowed to testify because he rendered an opinion as to individuals in general, which had no relevance to plaintiff. That other individuals might not suffer injuries in low-impact vehicular crashes has no bearing on whether this particular plaintiff might have suffered injury in this particular crash.” Id. at 315. It stated that the proponent of an opinion must show that there is adequate foundation establishing that the information upon which the expert bases the opinion is reliable.

The court pointed to the specific factual bases that were lacking in Strauss’ opinion: “There is no indication in the record that, in rendering his opinion, Strauss considered plaintiff’s weight, height, and age, whether plaintiff wore a seat belt, how she was seated, or how extensive her preexisting injuries were at the time of the crash. In short, the focus of his opinion was based on generalities that were not tied to plaintiff specifically.” Id. at 315-316.

The Martin case demonstrates how easy it is to lose focus on the individual plaintiff in a biomechanical analysis. While it would be highly unusual to see a biomechanical report lack specifications and other information about the mechanics of the vehicles or the product in question, reports often lack specific information about the person involved in the accident. At least with some courts, the expert merely saying that medical records were reviewed may not be sufficient to tie the facts to the specific case.

In Missouri courts, the factual underpinnings of an expert opinion can also be the basis for exclusion. In the case, Anglim v. Mo. P. R. Co. 832 S.W.2nd 298 (Mo. 1992) the Missouri Supreme Court upheld in a FELA action, the trial court’s withdrawal instruction relating to exclusion of the testimony of an expert in ergonomics which incorporated a biomechanical model, who concluded that the plaintiff’s description of his slip and fall when he slipped and fell on grease, oil, and water that had accumulated on the steps of a locomotive, would not have occurred as described by the plaintiff. The court found that the expert’s testimony was based upon assumptions of facts not in evidence and that his tests did not duplicate the conditions existing at the time of the claimed event, particularly the “nature and extent of the oil and grease and the type and condition of the soles of plaintiff’s boots…”. The court found that experimental evidence is admissible, but only if the experiment is made under substantially similar conditions to those at the time of the accident, although the conditions need not be identical. Id. at 308. Given that there was “no such substantial similarity” shown between the facts pertaining to the plaintiff’s claimed slip and fall and the computer model/test on which the expert based his opinions, the Court supported the trial court’s withdraw instruction.

However, not all factual weaknesses form the basis for exclusion. Although not a bio-mechanical case, the case of Spalding v. Stewart Title Guar. Co., 463 S.W.3d 770 (Mo. 2015) demonstrates how the Missouri Supreme Court may handle potential ruling regarding the admissibility testimony. The Court stated that “[a]s a general rule, ‘[a]ny weakness in the factual underpinnings of the expert’s opinion . . . goes to the weight that testimony should be given and not its admissibility.’” Id. citing Elliott v. State, 215 S.W.3d 88, 95 (Mo. banc 2007). Dealing with an appraiser expert, Stewart Title argued that the factual basis of the Plaintiff’s appraiser was speculative and unreliable. Stewart had several objections to the testimony including that “it was based on assumptions contrary to the facts in evidence, including ‘the existence of a governmental permit to create a lake development, title insurance coverage and ownership of all required property, and the availability of financing.’” The Court rejected Stewart’s argument and found:

All of these assumptions may have been weaknesses in the factual underpinnings of Reardon’s opinion, but such weaknesses go to the weight of his testimony and not to its admissibility. Stewart Title had the opportunity to cross-examine Reardon extensively about his assumptions and did so, but the record establishes that Reardon’s assumptions were not so speculative, unreliable, and unsupported by the evidence as to render his opinion inadmissible. Id.

Even if a lack of specificity does not lead to exclusion, it opens up a gapping hole for cross examination. Allowing opposing counsel to attack the opinion because your expert did not address or consider the fact that the injured party was 82 years old and weighed 105 pounds, or was 6’5” and weighed 400 pounds, opens the door to a loss of credibility. For most jurors (and many attorneys) the equations and calculations that are used by biomechanical experts are not easy to understand and it is important for the expert to establish personal credibility with the fact finder and maintain it throughout their testimony. This type of cross examination can lead to a juror deciding that the entire opinion is not credible, including estimates of speed which are likely completely unrelated to considerations such as height and weight of the occupant.

Gaps in Analysis

Another basis for exclusion of a biomechanical opinion is a fundamental gap in analysis. In response to the Supreme Court decision in Daubert the advisory committee added notes to the 2000 amendments to Rule 702. In their notes the advisory committee discussed certain factors relevant in determining whether expert testimony is sufficiently reliable to be considered by the trier of fact. One of those factors is “Whether the expert has unjustifiably extrapolated from an accepted premise to an unfounded conclusion.” The committee referenced General Elec. Co. v. Joiner, 522 U.S. 136, 146 (1997), for the principle that in some cases a trial court “may conclude that there is simply too great an analytical gap between the data and the opinion proffered”.

The analytical gap challenge is seen most often in the contexts of toxic tort opinions and opinions on specific causation. An exclusion based on a gap in analysis is somewhat rare as most courts find that gaps or other flaws go to the weight and credibility to be accorded the expert opinions and not to the admissibility of those opinions.

One way an analytical gap could occur in a biomechanical opinion is the use of subjective evidence only, such as the statement of a plaintiff, as the factual basis for the opinion. It is frequently the case in low speed accidents that no measurements are taken at the scene of what was initially thought to be a minor accident with no injuries. With an expert deadline looming, a report might be produced based on an interview with only the plaintiff or defendant, or based on statements taken. Depositions are later taken of all parties and known witnesses, and photos are produced, but there is no analysis of the additional information or supplementary opinion offered. The expert can also fail to take the step of testing the interviews and statements against other available physical evidence and information. It can then be argued that a critical error or gap in analysis was created by using the subjective information only which then propagated itself throughout all calculations and conclusions. The use of subjective information only when making a scientific conclusion can form the basis of the exclusion. In a case where an expert failed to follow up initial subjective information with objective scientific measures the court excluded the opinion and stated: the gaps in expert’s analysis were “illustrative examples of the perils inherent in applying subjective experience instead of a proper scientific approach.” Brown v. Burlington Northern Santa Fe Ry., 765 F.3d 765, 773 (7th Cir. Ill. 2014)

The gap in analysis can also occur based on the type of studies used or the lack of studies on the subject. This type of attack on an expert opinion was the basis for the findings in the General Elec. Co. v. Joiner case cited above. In that case Joiner linked his development of cancer to his exposure to PCBs and their derivatives. Joiner’s experts testified that PCBs alone can promote cancer and that furans and dioxins can also promote cancer. Joiner’s experts “opined that since Joiner had been exposed to PCBs, furans, and dioxins, such exposure was likely responsible for Joiner’s cancer.”

In their argument to the Supreme Court the Defendants argued that “Joiner’s experts regarding causation were nothing more than speculation.” They further argued that the opinions were “’not supported by epidemiological studies . . . [and were] based exclusively on isolated studies of laboratory animals.’”

Joiner argued that according to Daubert’s language, when the district court analyzes expert opinions for admissibility the “focus, of course, must be solely on principles and methodology, not on the conclusions that they generate.” 509 U.S. at 595. Joiner argued that because the district court excluded evidence based on the conclusion that the experts drew from the studies, it was properly reversed by the Court of Appeals.

However the Supreme Court in Joiner found that “conclusions and methodology are not entirely distinct from one another. Trained experts commonly extrapolate from existing data. But nothing in either Daubert or the Federal Rules of Evidence requires a district court to admit opinion evidence which is connected to existing data only by the ipse dixit of the expert. A court may conclude that there is simply too great an analytical gap between the data and the opinion proffered.” 522 U.S. at 146.

Certain types of biomechanical studies open themselves up to an analytical gap attack similar to the attack in Joiner. For instance, the use solely of volunteer studies that were made up of only healthy, young volunteers (which is often the case) could be attacked in an aggravation case with an elderly occupant. A study that analyzed only the subjective complaints of the volunteers could also be vulnerable to an attack that the studies lack and objective scientific approach. It is important to assure that there is not too great an analytical gap between the data used and opinion proffered. It is therefore important to assure that there are multiple types of studies and statistics cited and that the studies used relate to the actual injured party in a meaningful way. As stated in the Ingram v. Solkatronic Chem., Inc., 2005 U.S. Dist. LEXIS 38304, 31-32 (N.D. Okla. Dec. 28, 2005) case a court “may not, consistent with the mandate of Daubert and its gatekeeping function, permit this litigation to become a forum in which an expert may offer what is, presently, mere speculation on the possibilities of …injury.”

Exclusion Based On Flawed Methodology

Flawed methodology is a less common challenge to a biomechanical opinion. Generally, the same equations, and the same values and constants in the equations are used. Flawed methodology challenges generally occur when an expert proposes a new type of analysis or develops a new method or analysis for a specific case. Conceivably a biomechanical engineer could be attacked on the basis that their methodology has been rejected or supplanted by newer techniques or because the analysis deviated from the methods employed by the authors of the various studies cited in the opinion. However, the factual basis and gap in analysis are more likely methods of attack.


While biomechanical engineers are being used with more frequency in civil actions, an attorney using the biomechanical expert must ensure, whether handling case in federal or state courts, that the expert’s opinion is based on the facts involving the case and the circumstances involving the particular party in the accident. The opinions must not only ensure “substantial similarity” with the facts of the case, but must narrow the gap in analyzing all of the information upon which the calculations and conclusions are based. Finally, the attorney must ensure that the biomechanical expert is using a recognizable methodology in developing the expert’s opinions and conclusions in order to survive motion to strike the expert from the opposing party.