This is the second in a series of articles that will address cross-examining defense biomechanical experts and the field of biomechanics generally. Last month, I left off by indicating that this series would provide the tools to competently cross-examine the defense medical expert in the field of biomechanics.

Like most areas of cross-examination, it is important to identify and understand the literature upon which the expert is relying in forming his or her opinions. There are certain texts that seem to be universally relied upon by experts in the field. A classic examination of techniques used to measure and analyze all body movements was written by David A. Winter, called Biomechanics and Motor Control of Human Movement, published in 1990. Another well-established text in the field was co-authored in 1988 by James G. Hay, and J. Gavin Reid, called Anatomy, Mechanics and Human Motion.

Still another well-established text was authored in 2001 by Margareta Nordin and Victor Frankel. This text, which I prefer, is titled Basic Biomechanics of the Musculoskeletal System. The authors promote the book as providing "a working knowledge of biomechanical principles for use in the evaluation and treatment of musculoskeletal dysfunction." I agree with this assertion.

The book encompasses a variety of disciplines, including orthopedic surgery, physical therapy, occupational therapy, hand surgery, physical medicine and rehabilitation, sports medicine, biomechanical engineering and anesthesiology. Each of these disciplines is particularly relevant to a personal injury attorney. The book also includes separate chapters that provide a detailed analysis of the biomechanics of bone, articular cartilage, nerves and nerve roots, knee, hip and shoulder.

Aside from these university-level publications, useful reference sources can be found through the U.S. Department of Transportation and through ASTM, which publishes a Standard Practice for Safe Walking Surfaces.

While it is certainly necessary to read these books and reference sources to get a full understanding of the issues involved, I would like to highlight certain terms and facts that are universally important in conducting a biomechanical analysis. It should be understood that many of these facts also overlap into the field of accident reconstruction as well, but this article is not focusing on that perspective.

Invariably, in a case where the defendant is using a biomechanical expert in any variety of slip-and-fall case, the term center of mass or center of gravity will arise. Mass is defined as the quantity of matter composing a body. In every object, there is a unique point called center of mass (or, interchangeably, the center of gravity), around which the object’s mass is equally distributed in all directions. In other words, mass is balanced at the center of mass in all directions.

Finding the center of mass of the plaintiff is an important task in biomechanics because, in many cases, the center of mass of the plaintiff is the point that effectively represents the actions of the whole body. For purposes of cross-examination, it is important to understand that the center of mass is slightly different for all individuals; however, most biomechanical experts proceed under the working assumption that the center of mass is the point slight below the belly button. For women, the center of mass is slightly lower on the body than men. For men, the center of mass is generally at about the 56 percent point of height and for women it is about the 54 point of height.

Another important fact when cross-examining a biomechanical expert in any variety of slip-and-fall case is typical walking patterns, otherwise known as gait pattern. The typical human will walk anywhere from 2.5 miles per hour to 3.5 mph. Obviously, if the person was rushing, it could be even faster or, if carrying something heavy, even slower. Many biomechanical experts will manipulate speed to fit their theory of the events.

The plaintiff’s attorney needs to be aware of the manipulation of the variables and be prepared to act. For instance, 1 mph essentially amounts to 1.467 feet traveled per second. If the plaintiff describes an event happening over the course of 10 to 20 seconds, the speed applied by the biomechanical expert can change the outcome by as much as 15 to 30 feet. In a slip-and-fall case, friction of the walking surface is an important variable that is many times not available to the expert and he or she inserts a friction that is favorable to his or her theory.

Still another important factor to consider is gait patterns. According to the U.S. Department of Transportation, the minimum ground clearance of the toe when the foot is swung forward has been observed to average 0.6 inches and range between 0.375 inches and 1.5 inches. This is important when discussing tripping hazards.

An important mathematical formula to know is how fast an object falls. Typically, an object, including humans, will fall from its center of mass. The rate at which the object falls from its center of mass is 32 feet per second squared. In other words, as one might think intuitively, the objects picks up velocity as it falls. To apply this equation, absent air friction, a human will fall 32 feet in one second and pick up speed from there.

In a slip-and-fall case, there are competing mathematical formulas as the center of mass is falling down because the body still has the forward momentum of the gait speed. All of this is designed to show that the manipulation of the variables is dangerous and needs to be carefully explored by the plaintiff’s attorney on cross-examination. Always remember that your client was there and testifying about how the accident happened in reality, not in a theoretical biomechanical universe. Never lose sight of that fact.

In this article, much of the particular focus was on slip-and-fall cases. In the next article, I will focus on biomechanical analysis in motor vehicle cases. •

Brandon Swartz is a founding partner of Swartz Culleton. His practice focuses on all types of personal injury cases, including wrongful death, medical malpractice, civil rights violations, motor vehicle, premises liability, products liability, trucking accidents and workers’ compensation.