In this article, biomechanical engineer, Dr. Valentina Ngai provides an introductory lesson on forensic gait analysis. She discusses how experts use injury data, combined with an understanding of the human gait, to determine what happened in unwitnessed or disputed incidents. Analyses of this type are frequently used to determine the fall mechanism (such as slip or trip), as well as if, how and possibly in what sequence the pathologic loading occurred.


Walking of an average human has been studied by scientists for decades and is termed “Gait Analysis”. A single gait cycle is considered as the motion of one leg, starting at heelstrike (0% of gait cycle), to the next instance of heelstrike for the same leg. “Stance Phase” is the term used to describe the weightbearing portion of gait while “Swing Phase” is the non-weightbearing portion (Figure 1).

Figure 1 - One Gait Cycle
Figure 1 - One Gait Cycle

Human gait is quite complex and involves multiple joints, muscle activation and balance simultaneously working in conjunction to allow movement. Loss of balance occurs when the body’s center of gravity moves outside the established base of support (Figure 2), causing people to fall.

Figure 2: Base of support and loss of balance
Figure 2: Base of support and loss of balance

Falls during gait occur and are defined by the mechanism. The two most common fall mechanisms are trips and slips. A trip is a sudden arrest in the swing leg as the body continues its initial motion trajectory, moving the body’s center of gravity outside the base of support thus causing the person to fall. A slip occurs when there is insufficient friction force between the walking surface and the person’s feet/shoes, causing the foot/feet to slide resulting in a loss of balance and subsequent fall. Trips and slips are distinctly different from one another in cause, loading on the body, path of fall and often injuries.

In incidents where the fall mechanism is unknown, disputed or if there is a question whether the claimed injuries were caused by the specific event, a capable biomechanical engineer is able to examine the diagnosed injuries and event details. By applying a forensic approach utilizing universally accepted fundamental scientific methods, our biomechanical engineers are able to determine the fall mechanism as well as if, how and possibly in what sequence the pathologic loading was provided within the event in question to cause the injuries claimed. Gait analysis is also applied to other forms of human locomotion such as running and stair ascent and descent. The same injury causation and fall mechanism investigations are then applied by our biomechanical engineering experts.

Example Case:

An older adult female fell while walking using a cane in a medical facility and suffered a hip fracture. The incident was unwitnessed and the specific fall mechanism was in question. Did she trip? Did she slip? Our biomechanical engineering expert was engaged and performed a biomechanical investigation to determine how she fell which included analysis of the specific diagnosed hip fracture. Our engineer determined that the fracture mechanism required a combination of axial compression, bending and torsion loading in order to cause the diagnosed fracture. Combined with this information as well as information regarding the incident, the fall mechanism was determined to be neither a trip nor a slip but that the person’s leg had buckled underneath her causing her to fall to the ground onto her buttock/hip area generating the necessary loading to cause her injuries.

An injury causation analysis can be applied to a variety of incidents whether they involve sports, automobile crashes, industrial mishaps, or falls. If your case strategy calls for an understanding of the injury and fall mechanisms, contact Dr. Ngai to discuss your case and how we can help.


The biomechanical experts at Robson Forensic provide scientific and analytical evaluation of injuries to determine causation. Our experts are engaged when the cause or mechanism of an injury is not known, is not understood, or is in dispute.

Injuries tell a story. Could a fall have occurred as testified? Were the forces high enough to have caused the injuries claimed? Was the known mechanism (direction and type of loading) of injury provided in the event? Is the injury consistent with the physical evidence? These are examples of questions our biomechanical experts can help investigate.

For more information visit our Biomechanical Engineering practice page.


Featured Expert

Valentina Ngai, Ph.D., P.Eng.

Biomedical Engineer & Biomechanics Expert

Dr. Ngai is an expert in biomechanical engineering. She provides investigations, reports, and testimony toward the resolution of disputes and litigation involving traumatic injuries including medical implants and devices, motor vehicle crashes, premises liability, in addition to workplace and sports injuries.

Dr. Ngai holds a Ph.D. in Bioengineering from the University of Illinois at Chicago. She has been published in the Journal of Biomechanics, Wear, and the Journal of Engineering in Medicine. She has presented her research at numerous international conferences including the Orthopaedic Research Society, Wear on Materials, World Congress of Biomechanics and was an invited speaker at the 10th International Essen Symposium in Germany. Dr. Ngai is a licensed professional engineer in Ontario, Canada and also holds degrees in Civil and Mechanical Engineering.