Forensic Gait Analysis: Applying Biomechanics in the Legal Realm

In recent years, the field of forensics has captured the imagination of an entire generation through television shows and movies. Watching scientific evidence unfold in the courtroom setting is fascinating and exciting.

Many people are not aware that podiatric medicine has relevance within the world of forensic science, as the profession already has a “foot in the door,” pun intended. One major aspect of forensic podiatry is the ability to perform a forensic gait analysis. Forensic gait analysis (FGA) is the examination, comparison, and evaluation of features of gait to assist in the investigation of crime.¹ Gait by definition is the pattern of movement during locomotion with key elements beings its dynamic and repetitive nature.² Human gait has been a subject of interest for centuries within the legal system, though it may appear to be a more modern concept.

The History of Gait Analysis in the Legal System

The first known application of gait analysis for identification during a legal proceeding occurred in 1839 at the London Central Criminal Court.³ This case was focused on the suspect, Thomas Jackson, being identified by his distinctive “bowed left leg” and “pronounced limp” when walking, which resulted ultimately in his conviction for returning to England after being sentenced to relocation (this was a felony punishable by death).³ In a legal context, during the 19th century, the identification of suspects was primarily made through eyewitness observations as this predated the scientific advancements that would evolve in the next century. In the United States, the first documented use of gait evidence took place during a robbery case in 1908 in Texas.³ The victim testified that though the assailant wore a mask, he could identify him by his voice and manner of walking.³

By 1994, a vital new component was utilized for gait evidence with the introduction of surveillance video footage as noted in a Pennsylvania robbery trial where a suspect was described to have a distinctive “bounce in his step.”³ In contemporary times, the use of closed-circuit television (CCTV) footage to observe gait from a crime scene is commonplace.³ In 2000, the first case employing the comparison of a perpetrator’s gait to that of a suspect’s using video footage from a robbery scene happened in London, with the gait analysis performed by UK podiatrist, Haydn Kelly.³

What You Should Know About the Gait Cycle

The gait cycle is defined as the time between 2 successive occurrences of the repetitive events involved in walking.² In a forensic capacity, this provides an opportunity to identify remarkable characteristics of an individual. Below is an overview of the biomechanical process:

Heel strike—the first contact of the foot with the ground.
Foot flat—first contact where the distal aspect of the foot touches the ground.
Heel rise—when the heel first leaves the ground.
Toe off—point when foot first leaves the ground.

The phases of gait can be broken down into two general components, stance and swing, which alternate for each lower extremity.²

Stance—period of time when lower limb is in contact with the ground (60% of one cycle)
Swing—period of time when the lower limb is not in contact with the ground (40% of one cycle)
Double support—period of time during walking when both feet are in contact with the ground

Parameters of gait fall into two broad categories: spatial (measured in space) and temporal (measured in time).²

Step—the heel strike of one foot to the next heel strike of the other foot.
Stride—the heel strike of one foot to the next heel strike of the same foot.
Base of gait—distance between the rear most point of contact between left and right heels, measured by angles to the line of progression.
Toe out/in angle—angle between the longitudinal axis of the foot and the line of progression.
Speed—distance traveled in a known time period.
Cadence—the number of steps taken in a known time period.

Analyzing Gait: The Sheffield Tool

A proper gait analysis is not only lower extremity-focused but involves a full body systematic approach. Analyzing movement is performed in a “head-to-toe” fashion with the assistance of an assessment tool. The Sheffield Gait Tool is designed to aid in the evaluation of subjective features of gait exhibited by an individual on CCTV footage.⁴ This tool has 15 sections: symmetry, torso frontal, torso transverse, head frontal, head sagittal, shoulders, arm swing, movement at hip, knees at swing, knee at heel strike, knee at heel rise, base of gait, foot at stance, foot at swing, and additional features.⁴ Motion is assessed in the sagittal, frontal, and transverse planes. Scientific evaluation of the Sheffield tool demonstrates a high percentage of repeatability and reproducibility.⁴ It is important to note that these results are of a qualitative nature and not quantitative.  

The questioned footage is analyzed first, as this is the imaging of an individual(s) from the crime scene itself.⁵ The video frames are viewed and copious notes taken while applying the Sheffield tool. Once the questioned footage has been evaluated, then the reference footage is reviewed by the same methodology. The reference footage is that of a person of interest in the case (suspect).⁵ 

Performing a Gait Comparison Report

The comparison process is based on notes and lists of features from both the questioned patients and reference footage. The questioned footage is always viewed first to diminish the chance for any cognitive bias by the forensic gait analyst.⁶ The Analysis, Comparison, Evaluation, and Verification (ACE-V) methodology is utilized for conducting comparison studies.⁶ When evaluating features, compatibility, incompatible, and differing characteristics between the two sets of footage are examined.⁶ Forensic gait analysis does not provide an absolute identification of a suspect but rather a level of certainty that the same individual was in both the questioned and referenced footage.⁶ The scale of support delineates from very strong evidence to support, strong evidence, moderate evidence, and limited evidence.⁶

The initial report will have to be verified by another forensic gait analyst to ensure that a suitable process has been followed, and the conclusions reached are within probability.⁶

From the Clinic to the Courtroom

Knowledge of the gait cycle and videography alone are not enough to be successful in the forensics arena. The politics of the courtroom are tricky to navigate and very different from the dynamics of everyday clinical practice. Being an expert witness requires great understanding of the legal system as well as being a content authority. There is governance regarding the role of expert witnesses, particularly Article VII of the US Federal Rules of Evidence.⁷ Rule 702 defines an expert witness as qualified by knowledge, skill, experience, training, or education.⁷ The trial judge has the ultimate say over the admission of expert evidence. Expert witnesses must be ethical, impartial, and appreciate the responsibility that comes with their testimony. It is highly recommended that anyone considering becoming an expert witness should attend preparatory sessions that are available and not solely rely on the attorneys who provide a brief information session before entering the courtroom.⁸

Conferences such as the International Association for Identification (IAI) offer workshops in expert witness testimony. The IAI is a multidisciplinary expansive association offering opportunities for collaborative learning in every aspect of forensics. Mastering the witness stand is an acquired skill that takes time and experience.

What You Should Know About the American Society of Forensic Podiatry

The American Society of Forensic Podiatry (ASFP) was created to advance the cause of forensic podiatry and maintain the highest standards of practice via research, discussion, education, publications, while working with other organized agencies.⁹ The objective of this organization is to promote the use of podiatry in forensics cases, utilizing the analysis and evaluation of evidence related to the human foot, while maintaining the highest standards of practice. The organization promotes these objectives through continuing education for its members by way of educational seminars, study, newsletters, discussion, publications, and liaison with other established disciplines.⁹

In September 2003, the American Society of Forensic Podiatry was formally created. In July 2007, with the help of ASFP members, a forensic podiatry sub-committee was established within the structure of the International Association for Identification (IAI).⁹

The field of forensic podiatry deals with pedal evidence. Pedal evidence can comprise several different forms relative to the static or dynamic foot as well as the footwear.⁹ This can include bloody footprints, footprints in a substrate such as dirt or dust, or the foot impression on the sock liner of the shoe.⁹ Forensic podiatry also includes the analysis of gait from crime scenes as well, using such evidence as X-rays and medical records, to assist with the identification of an unknown individual.⁹

Currently, ASFP members have been active expert witnesses in several cases, even working with the Innocence Project helping to exonerate individuals using forensic gait analysis. The ASFP offers education in several different concentrations including forensic anthropology and paleopathology. There are scholarship opportunities offered for podiatric students and residents to learn more about this unique field. This small society has dedicated members who are willing to mentor those with genuine interest in forensic practice. Forensic podiatry is an interesting subspeciality at the intersection of biomechanics and forensic science.

Dr. Miller is a certified wound specialist by the American Board of Wound Management and a Fellow of the American College of Clinical Wound Specialists. She currently serves as the Co-Director of the Limb Salvage Program at the University of Florida, College of Medicine-Jacksonville.

References
1.    Birch I, Nirenberg M, Vernon W, Birch M. Chapter 1: Introduction. In Forensic Gait Analysis: Principles and Practice. CRC Press, 2020, pp. 1–18.
2.    Birch I, Vernon W, Walker J, Young M. Terminology and forensic gait analysis. Science Justice. 2015; 55(4):279–284. https://doi.org/10.1016/j.scijus.2015.03.002
3.    Birch I, Nirenberg M, Vernon W, Birch M. Chapter 2: the history of the use of gait analysis as evidence. In Forensic Gait Analysis: Principles and Practice. CRC Press, 2020, pp. 19–38.
4.    Birch I, Birch M, Lall J. The accuracy and validity of the Sheffield features of Gait Tool. Science Justice. 2020; 61(1):72–78. https://doi.org/10.1016/j.scijus.2020.08.001
5.    Birch I, Nirenberg M, Vernon W, Birch M. Chapter 7: Analyzing the questioned and reference footage. In Forensic Gait Analysis: Principles and Practice. CRC Press, 2020, pp. 107–116.
6.    Birch I, Nirenberg M, Vernon W, Birch M. Chapter 8: Comparison of gait and evaluation. In Forensic Gait Analysis: Principles and Practice. CRC Press, 2020, pp. 117–128.
7.    Birch I, Nirenberg M, Vernon W, Birch M. Chapter 4: The legal context of forensic gait analysis. In Forensic Gait Analysis: Principles and Practice. CRC Press, 2020, pp. 59–69.
8.    Birch I, Nirenberg M, Vernon W, Birch M. Chapter 10: Presenting gait evidence in court. In Forensic Gait Analysis: Principles and Practice. CRC Press, 2020, pp. 147–158.
9.    About Us: The ASFP. The American Society of Forensic Podiatry. (n.d.). Retrieved March 8, 2023, from https://www.theasfp.org/about