Introduction

Gait analysis is known as 'the systematic study of human locomotion.' Forensic gait analysis or forensic gait comparison is defined as 'the assessment and evaluation of the gait patterns and features of the person/suspect and comparing these features with the scene of crime evidence for criminal/personal identification.' In other words, forensic gait analysis may be defined as a contributor to the identification process rather than one of the methods of identification as individualization of the gait of a person has not yet been fully scientifically proved.'[1] The forensic gait analysis's general source or evidence comprises the series of footprints found at the crime scene and the closed-circuit television camera (CCTV) footage. Footprints are one of the pieces of evidence encountered at the crime scene. Footprints can be recovered in the form of bare prints, shoeprints as well as a series of imprints. Footprints can be encountered in several types of cases and crime scenes such as HBT (House Break-in and Theft), robbery, sexual assaults, hit and run, shoplifting, homicides, kidnapping, etc. The science of footprints and gait analysis is a part of an emerging sub-discipline of forensic science known as forensic podiatry. Forensic podiatry is "the application of podiatric knowledge and experience in forensic investigations. It shows the association of an individual with a scene of a crime, or to answer any other legal question concerned with the foot or footwear that requires knowledge of the functioning foot."[2][3] The principles and knowledge of forensic podiatry help the investigating officers and crime scene investigators when they are involved with the foot, footwear, and/or gait-related evidence at the crime scene. This knowledge can further establish the suspect's physical or biological profile for individualization and identification.[2] 

Meanwhile, an investigating officer is always on the look-out for evidence that will link the suspect to the scene of the crime. In recent times, gait analysis has emerged as an important parameter that can help in personal identification during forensic examinations. The advent of CCTV cameras and other surveillance means has generated interest in the practice and research related to forensic gait analysis and its possible use in human individualization. In 1839, in London, gait analysis entered into the court in the case of Thomas Jackson; he was identified by the witness due to his bowed left leg and walking with a limp.[4][5] However, for the very first time, the forensic gait analysis was used as a means of admissible evidence/scientific evidence in the Old Bailey Central Criminal Court, London, UK, in the case of R v. Saunders by the U.K. based forensic podiatrist, Dr. Haydn Kelly., although, the method of forensic gait analysis remains questionable as far as the reliability and accuracy are concerned.[6][1][4][7] The article discusses forensic gait analysis methods, factors affecting the human gait, its forensic applications and its accuracy, reliability, and admissibility in the court of law.

Function

Gait Analysis

Human walking occurs in a specific pattern, including various stages, which is referred to as a 'gait cycle.' It mainly consists of two phases, i.e., the stance phase and the swing phase.[6] The stance Phase constitutes five stages: "initial contact, loading response, mid-stance, terminal stance, pre-swing." While the swing phase constitutes three stages, namely; "initial swing, mid-swing, terminal swing."[8] See Image. Gait Cycle/Walk Cycle.

The dogma of forensic podiatry is to utilize the proper standardized methodology for scientific examination of every evidence. The gait pattern examination includes ACE-VR, which stands for analysis–comparison–evaluation–verification–report.[2] Every step needs to be taken into consideration. After a thorough analysis, the findings are presented in the report format. The forensic gait analysis would depend on the type of gait pattern. The gait pattern may be classified into the following two types:

• Gait analysis on the surface: The analysis of gait pattern on the surface (consisting of a minimum of 3 to 4 consecutive footprints/footwear prints) is performed by considering various parameters including the dimensions, general shape of the prints, patterns in case if ridges are visible, margins of the prints, toe marks, etc. The measurements pertaining to step and stride length are taken into consideration, which may point out the nature of an individual's gait pattern, such as normal gait, running, etc., and associated abnormalities, if any. This analysis may also help approximate stature, sex, age, and the bodyweight of a suspect. A profile of the contributor of the prints may be established. A comparison between the perpetrator of the crime with that of the suspect is made by systematically analyzing and scoring the gait patterns.[9] The information related to the relationship of step length, stride length and footprint length with stature may provide valuable information in the gait comparison process and analysis. Some researchers have found a statistical correlation between footstep length and stride length with the stature of a person taking into consideration various controlled conditions.[2][8][10][11]

• Gait analysis from CCTV footage/video recording: The emergence of CCTV cameras and video recording enabled devices (video cameras, mobile phones, dashboard cameras, surveillance cameras, traffic cameras, etc.) has given a fresh dimension to gait analysis. Currently, video recording and footages from CCTV are used to study the gait of the person.[5] Before the invention and usage of CCTV, the investigating agencies used to rely upon the eye witness who might have 'seen' someone on the spot with a particular 'style of walking.' However, scientific evidence suggests that the discriminative strength of such statements cannot be relied upon as the sole evidence, although forensic gait analysis plays an important supportive role in the absence of other biometric identifiers in the video. The CCTV footage from the scene of occurrence of the offense, surveillance footage, traffic camera footage, dash-cam footage from the dashboard-mounted cameras of the vehicles, mobile phone recording (by the victim, witness or others), etc. should be collected following the established process for collection of digital evidence. Usually, a copy of the recording is secured in a flash drive/hard-disk of suitable capacity. The 'hash value' should be calculated and mentioned in the chain of custody form and the forwarding letter.

Forensic Gait Analysis Approaches

Forensic gait analysis may be human-based/observer-based, as well as computer-based. Both the approaches are very briefly discussed below:

• Human-based: The human-based approach (also known as observer-based approach) falls under three categories: photo-anthropometry, morphometric analysis, and superimposition. Photo-anthropometry takes account of measurements of set landmarks. Morphological measurements are taken from photographs in the morphometric analysis. Then both the measurements are combined and superimposed to establish a match between suspect and standard photographs.[4] In the human-based approach, motion analysis is also performed by running videos in pause and reverse mode. This also helps in identifying the suspect based on observation.[12][13] Nevertheless, this approach is often prone to errors and bias. The inter-observer and intra-observer variabilities are also beyond permissible limits most of the time. Additionally, various measurements are taken either directly on the footprints/gait pattern or with the help of photographs (with a scale of reference). Photogrammetry may also be used. 
• Computer-based human-assisted: The computer-based analysis involves algorithms that can either be model-based or appearance based. The model-based approach work on fixed landmarks for extracting the features of gait. This is done by predefining the landmarks using a human model. Later the appearance-based model function by extracting silhouette sequences of walking individuals. In this, a predefined human model is absent. In both cases, the match score is obtained between questioned and suspected gait.[4][13]

Forensic Gait Analysis Features/Parameters

For any traits or features to be useful for forensic purposes, it has to follow certain principles like universality, permanence, and uniqueness. While some may be grouped under class characteristics and some under individual characteristics. Even if one or two features may not be proof enough for a conclusion, it will result in a more reliable and scientifically sound result when these features are combined. Although it is beyond the purview of this article to detail the parameters/features used and factors affecting the forensic gait analysis, a brief account is given. Some of the features taken into consideration while performing forensic gait analysis (static as well as dynamic) that may contribute to the establishment of identity are as follows: 

• Static measurements: These are the geometrical measurements of the body. For example, the whole leg length, length of knee-foot height, etc.[8]
• Dynamic measurements: These are the measurements related to gait. For example, the distance between both the knees, the length of stride, cadence, etc.[8]

Another classification may include the spatial parameters, the temporal parameters, and the angular displacement parameters.

• Spatial parameter (distance) - These parameters can be measured using the measurement of footprints by drawing a progression line, which is an imaginary line corresponding to the direction of walking. The parameters that may be included are step length, stride length, stride length/leg length ratio, stride or step width, longitudinal dimension, horizontal dimension, dimensions of the foot, foot line, direction line, foot angle, gait line, etc.
• Temporal parameter (time) - In the videos/footage, these parameters are measured using a stopwatch (manually) or with the help of automated/semi-automated software. It includes step time, stride time, stance duration, swing duration, swing/stance ratio, speed, cadence, etc.
• Angular displacement parameter (angle) - Various angles like hip angle, knee angle, are also considered.

Issues of Concern

Factors Affecting Gait Pattern and Analysis

Gait is a biological characteristic of a person, and the gait pattern is simply the manner or style in which a person usually walks. It is highly influenced by many factors (internal as well as external). Walking is often not a conscious behavior and can be used for differentiation if not for identification. Some of the factors that tend to affect the gait and hence the forensic gait analysis are as follows:

• Footwear: It causes gait pattern variability. Some footwears can cause increased hip movement, which results in long stride length, while some may reduce the stride length.
• Fatigue: Fatigue causes depletion of energy, which affects the gait cycle, thus influencing the gait pattern. The speed of walking also decreases when the person is low on energy.
• Use of knee brace: The use of a knee brace also affects the gait of a person.[14]
• Sex: Studies have revealed that the sex of a person can be assessed from the gait pattern, which supports that sex affects the gait pattern.[8]
• Speed: The gait pattern shows changes between the normal walking pattern and the person's running pattern.[8][14][15]
• Disease/disorders/medical conditions: This is one of the common factors affecting a person's gait. A typical and at time-specific gait pattern in the person suffering from such condition is evident, e.g., Parkinsonian/Parkinson's gait in Parkinson's disease (see Video. Parkinson Gait).[9] Choreiform gait presents as the primary feature of Huntington disease (see Video. Choreiform Gait). Other conditions may include chronic rheumatoid disease, Sydenham chorea, involuntary movement, festinant gait, shuffling gait, scissors gait, etc. See Image. Scissors Gait.
• Directed gait: Directional gait is walking under the instruction. For example, in marching, everyone shows a similar gait.
• Age: Age is one of the factors which influence the gait pattern. Walking depends on muscle strength, which gradually changes as the person grown up. The variations in muscle strength affect gait.
• First trimester during pregnancy: It is believed that during pregnancy, the gait pattern shows changes. A study has shown that foot angle exhibits changes during the first trimester of the pregnancy.[16]
• Location: The location where the person is walking also affects the gait because when a person walks on a surface different than usual, he or she will focus on the walking, which makes him conscious and will affect the walking pattern.[15]
• Camera: The type of camera used is also shown to affect gait analysis. The angle and positioning of the camera are also found to be crucial. The quality of recording received for comparison, if not clear, may result in misinterpretation or inconclusive results.
• Lighting conditions: The angle of light, the direction of light, the direction of movement of the person also affects gait pattern analysis.  

Including the factors mentioned above, a few more can affect an individual's walking pattern, such as emotional mind state, music, talking, load carriage, attire, etc.[8][17] The person may try to bring in some gait changes, intentionally to escape being recognized in the footage or to escape suspicion. This artificial 'gait,' i.e., deliberate change in the gait, also needs to be considered by the forensic analyst/examiner.

Clinical Significance

Forensic Significance

According to Birch et al.[6] the forensic gait analysis can be defined as "the identification of a person or persons by their gait or features of their gait, usually from closed-circuit television (CCTV) footage and comparison to footage of a known individual." It means the main source of comparison is the CCTV footage and other videos that can be used in the analysis process. In other words, it is the visual comparison of the videos to identify the individual based on gait patterns. This knowledge has been used in the crime scene investigation, particularly to identify the criminal or perpetrator from the characteristics of the gait pattern. Many forensic cases have been solved, and the criminals have been convicted based on forensic gait analysis throughout the world.[2][18] Presently, Drs. Michael Nirenberg and  Christine Miller in the USA, Dr. Nicholas Harris in Canada, and Professor Ivan Birch, Dr. Jai Saxelby, Dr. Jeremy Walker in the UK regularly practice forensic gait analysis.

Indirect Forensic Significance

The forensic gait analysis may additionally help in the estimation of stature and sex of the person,[2][19] estimation of body weight,[19] estimation of the number of people involved by measuring and evaluating step length/stride length if a series of footprints or gait patterns are available for analysis. The gait analysis may also help comment upon the type of footwear used, the use of any walking aid or support (like a walking stick), underlying disease/disorder/medical condition affecting the gait, etc.

Clinical Significance

Clinical gait analysis helps assess and treat individuals with pathological and/or underlying neurological conditions that may affect their ability to walk. With the help of gait analysis, podiatrists, neurologists, and orthopedic surgeons usually diagnose the issues related to the cause of pain, such as muscles, nerves, and skeletal problems, consequently initiate the treatment for correction of the gait-related abnormalities and develop rehabilitation protocols.[20][21] Gait analysis is being used in several fields, including sports science, rehabilitation and recovery medicine, kinesiology, and orthopedics.

Other Issues

Limitations of forensic gait analysis and criticism about its admissibility in the court of law

In the present era, due to the paradigm shift in forensic science, there is an increased focus on the reliability, accuracy, quality of evidence, and its admissibility in the court of law. Forensic gait analysis has also witnessed criticism regarding its reproducibility, reliability, lack of proper standards, and rules of forensic practice.[7] 

Gait pattern or walking pattern is highly affected by various parameters mentioned above, so it brings the investigating officer into a situation that requires consideration of several factors, which is practically not possible to identify the single suspect. Another limitation of gait pattern analysis is that we do not have databases that can be used for gait comparison. Gait pattern shows high intra-individual variability. It means that a person's gait is highly variable on different occasions and scenarios, which makes it difficult to identify the person.[22] Moreover, the experts are not following any prescribed standard protocol for the analysis of gait; consequently, there are variations in the methodology used for the analysis, which creates confusion amongst the judges and jury to decide the cases based on this parameter of identification.[4] 

Presently, the research is being carried out in the area of automated gait recognition combined with larger & variable datasets, tested variables, and enhanced statistical evaluation of results. Despite these improvements, any form of forensic evidence without fully recommended codes of practice and standardization of procedures and having error rates in reproducibility and repeatability should be treated with caution in forensic scenarios.[7] Forensic gait analysis remained a weak method of identification since the observation of the gait features by the experts also varies in different situations. A study conducted by the forensic gait analysts shows that there is ample difference between the observation of the gait features by the experienced experts as they made precise decisions in 71% of the cases.[23] Studies also indicate a difference in the capability and confidence of the qualified (competent/skilled/experienced/knowledgeable) and inexperienced gait analysis and cautions about the evidence submitted by the non-experienced gait analysts.[1]

Enhancing Healthcare Team Outcomes

Irrespective of the limitations and criticism about individuality and uniqueness and its admissibility in the court of law, forensic gait analysis may be considered supporting/corroborative evidence for identifying the criminals and perpetrators. Many forensic cases have been solved, and the criminals have been convicted based on forensic gait analysis throughout the world.