A Practical Review of the Biomechanical Parameters Commonly Used in the Assessment of Human Gait

The analysis of human gait is a potential diagnostic instrument for the early and timely identification of pathologies and disorders. It can also supply valuable data for the development of biomedical devices such as prostheses, ortho-ses, and rehabilitation systems. Although various research papers in the literature have used human gait analyses, few studies have focused on the biomechanical parameters used. This paper presents an extensive review and analysis of the main biomechanical parameters commonly used in the human gait study. The aim is to provide a practical guide to support and understand of the choices and selection of the most appropriate biomechanical parameters for gait analysis. A comprehensive search in scientific databases was conducted to identify, review and analyze the academic work related to human gait analysis. From this search, the main biomechanical parameters used in healthy and pathological gait studies were identified and analyzed. The results have revealed that the spatiotemporal and angular gait parameters are the most used in the assessment of healthy and pathological human gait. advances. keywords terms related to human gait (gait and walking), type of gait (normal, healthy, pathological, hemiplegic, parkinsonian, paraparetic, dancer, apraxic, ataxic, vestibular, equinus, steppe,


INTRODUCTION
Human gait is a complex process that involves many systems such as bones, joints, muscles, peripheral nerves, spinal cord, and the brain. When one, or some, of these systems fail or present some limitations, the gait process is altered and becomes a pathological gait [1] [2] [3] [4] . Therefore, the analysis of the human gait represents an important instrument for the early and timely identification of pathologies, and a component of follow-up rehabilitation programs [5] [6] [7] [8] [9] . In addition, gait analysis is also important to the design of equipment, devices, rehabilitation systems, prostheses, orthoses, and humanoid robots [10] [11] [12] [13] [14] .
Moreover, it can be used in sports science to improve the techniques and performance of athletes. However, such a diagnostic tool requires the use of biomechanical parameters to characterize and evaluate the human gait performance.
Several research works have undertaken the assessment of human gait as a prevention, monitoring, and diagnostic tool. For example, it can be possible to avoid foot ulcers in diabetic patients by increasing foot motion during the intermedia stance phase of the gait cycle [15] . Patients with knee osteoarthritis (KOA) often adopt a type of antalgic gait as their disease progresses [16] . These patients generally try to protect the damaged knee by biomechanical adaptations that may affect the movement of the joints of the lower extremities and the lower back [17] [18] . Gait and balance abnormalities have been described even at early stages in the diagnosis of Alzheimer's ailment [19] . Similarly, a fall during gait can be the result of specific characteristics of gait disturbances related to neurological disorders [20] . Additionally, it has been observed that robotic gait rehabilitation can improve the biomechanical parameters in chronic hemiplegic patients [21] .
Numerous biomechanical parameters have been used in the analysis of the human gait. It was reported that gait patterns and parameters values often vary with the gait velocity [22] , a parameter used to identify groups of elderly patients who are at increased risk of death and serious morbidity after cardiac surgery [23] .
The spatiotemporal parameters have been related to the gait mechanics of the KOA progression [24] [25] [26] .
Cadence, stride length, and gait velocity are the main gait parameters that provide a general idea of how well patients can walk [27] . In addition, the joint angles of the knee flexion and leg shortening effect parameters are very relevant when analyzing the kinematics of the knee and designing knee prostheses [28] .
Over time, the interest in analyzing the performance and characteristics of human gait through biomechanical parameters has increased because human gait alterations can be associated with pathologies known to cause bad body postures and muscle imbalance [19] [29] [30] [31] [32] . However, in the clinical area the gait analysis still relies on the knowledge and experience of the specialist, which represents a certain degree of subjectivity. In addition, few studies have correlated the gait parameters and metrics with clinical problems such as musculoskeletal, neurological, and circulatory problems [33] . Furthermore, there is not a comprehensive baseline study to ease the understanding and selection of biomechanical parameters for normal and pathological gait analyses. Therefore, the objective of this research work is to identify and analyze the main biomechanical parameters commonly used in the assessment of human gait. In the first search, 380 publications were selected.

Literature search
Next, the title, abstract, and conclusions of each paper were reviewed and the following selection criteria applied: 1) the study should be based on a quantitative analysis of pathological or healthy gait, 2) the study should report the analysis of pathological gaits caused by neurological, musculoskeletal and circulatory problems, 3) the study should use kinematic and kinetic parameters, and other particular parameters according to the case study or pathology, 4) the study should indicate application areas, and 5) priority was given to papers that included the use of three-dimensional digital measurements in gait analysis. The first three criteria were mandatory and the last two were desired.
Two independent researchers (authors) conducted searches by applying the criteria and resolving discrepancies. A total of 144 papers were finally selected.

Demographic and anthropometric data
The demographic and anthropometric information of the patient is essential when conducting a gait analysis. Table 1 summarizes the demographic and anthropometric parameters reported in the literature.
The demographic data comprises general information about the group of people such as age, gender, place of residence, as well as social or economic characteristics such as occupation, marital status, etc. The most common demographic data used for gait analysis are age and gender. Table 1 also describes the anthropometric data used in gait analyses. Since computer vision systems, tracking systems, or force plates are commonly used in gait studies, it is important in the collection of anthropometric data to have a robust benchmark that can be used to calibrate the vision system, define the dimensions of the walkway, locate the force plates, etc. [34] [35] [36] . A tendency to decrease the amount of anthropometric information required in gait analyses was also observed [37] [38] [39] .

Gait analysis applications
In general, six main application areas were identified and the percentage of each calculated,

Data Description
Anthropometric Ankle width The anthropometric length between the lateral and medial malleolus of each leg.

ASIS width
The horizontal distance between the anteroposterior iliac spines.

Body mass index
A key index for relating weight to height.

Bodyweight
The force that induces the gravity on the human body.

Foot length
The anthropometric length between the heel and the big toe.

Foot width
The anthropometric length between the first and fifth metatarsal heads.

Knee width
The anthropometric length between the lateral and medial epicondyles of the femur.

Lower leg length
The anthropometric length between the lateral femoral condyle and the lateral malleolus.

Pelvic height
The anthropometric length between the superior iliac crest and the ischium when seated on a stiff chair.

Pelvic width
The anthropometric length between the right and left superior iliac crests.

Standing height
The height of a person from foot to head.

Thigh-length
The anthropometric length between the greater trochanter and the lateral femoral condyle.

Age
The time that a person has lived (in years).

Gait parameters
In the literature, human gait biomechanical parameters have been classified in several ways. However, all the classifications converge on the use of spatial, temporal, angular, and strength biomechanical parameters. Table 3

Clinical gait parameters
The disability due to gait problems represents approximately 55% of the total disabilities worldwide [50] . These gait disorders may be a consequence of various pathologies, which are known to disturb human motor functions. The most common semiology of the pathological processes that affect human gait is pain, limitation of movement, muscular weakness, and deficit of neurological control [29] [51] . Consequently, the study of the characteristics of human gait by means of biomechanical parameters represents a potential tool for the diagnosis, treatment and monitoring of some pathologies. In the literature, many gait pathologies have been analyzed and classified according to the root cause of the functional impairment [27] [29] [33] [51] . Based on a preliminary study [52] , three large groups of pathologic causes are proposed: neurological, musculoskeletal, and circulatory. the work presented by Muro-de-la-Herran [33] . On the other hand, Table 5 presents the biomechanical parameters and the demographic and anthropometric data used in pathological gait analyses.

X
Step length SL X X X X Step width SW X X X

Ranking of parameters
In order to rank the biomechanical parameters used in the analysis of pathological and healthy gaits, two criteria were defined: 1) the relevance of the parameter in the research studies, and 2) the frequency, which was calculated using the following expression: where f is the frequency percentage, n is the number of times the parameter was used in the selected papers.
A normalization process was then considered, given a value of 1 to the most used parameter. Regarding the f = 100n/144 (1) relevance criterion, a value of 1 was given to those parameters with the largest reliability, the most common in different gait analysis (set theory), and the most cited. A relevance value of 0 was assigned to the rest of the parameters. Table 6  tively. Notice that although these parameters are not very popular parameters, they may be as important as the spatiotemporal parameters for some particular pathologies. The results also revealed that the temporal parameters are the most popular with 37% of use, followed by the spatial parameters with 24%, the angular parameters with 18%, the force parameters with 8%, and others with 13%.
According to Whittle MW [27] , the cadence, gait velocity, and stride length parameters provide a general idea of how well a patient walks. However, the results of this research work suggest that the stance time is also important. For instance, in the study of musculoskeletal-related gait pathologies [57] [60] [140] [142] , the stance time is an indicator of the improvement or deterioration of the pathology. On the other hand, although the stride length of both limbs may be the same, the step length of each limb may be different, as reported by Kirtley C [148] ; for this reason, many studies have considered it relevant. The step width is another relevant parameter, [67] [68] [143] [148] , which tends to increase with a balance disorder.

General discussion
The proposed analysis and classification of the gait parameters represent a general and practical guide to select the parameters for healthy or pathological gait studies. In general, the spatiotemporal and angular parameters are the most used because they allow a detailed and objective study of the human gait.
However, other parameters are specific and important for particular pathologies, such as the cartilage thickness [149] , which is used in the study of antalgic gait due to knee or hip osteoarthritis. In this work, only the most general pathologies and parameters reported in the literature have been considered.
It was also observed that most of the pathological gait studies (90%) reported in the literature were conducted on a walkway instead of an electric treadmill. This is because in the analysis of pathological gaits, the gait velocity is a parameter that depends on the pathology, and the evolution and progress of the patient. In contrast, in the studies related to healthy gait, the gait velocity is usually an independent variable that is defined and varied using an electric treadmill.
The stance time parameter is indicative of the improvement, or severity of pain, in the antalgic gait caused by knee osteoarthritis [131] . For example, an increment of the stance time is an indication that the pain experienced by the patient is less and therefore the support of the loading has improved. The use of the step height parameter has not been reported in the literature; however, it can be used in the study of musculoskeletal pathological gaits since it is closely linked to the angular parameter of the knee articulation. The step height parameter may vary due to musculoskeletal pathologies or due to the use of certain footwear, such as the use of high heels, as reported by Arellano [28] .

AUTHOR CONTRIBUTIONS
The

CONFLICTS OF INTEREST
The authors do not report any conflict of interest.