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1.1 Gait Analysis Basics


Gait analysis is a sophisticated laboratory technique by which modern electronics are used to incorporate information from a number of inputs to illustrate and analyze the dynamics of gait. It describes for the clinician (physician, surgeon, therapist) in quantitative and dynamic terms the movement of the body and its limbs and the changing relationships of one extremity to other extremities during motion (e.g.: walking, leg raising). It helps the clinician judge what are the forces resulting in a dysfunction in movement of a limb and what is the result of compensating for the dysfunction; to put it another way, what is cause and what is effect.

Gait analysis has become particularly useful to the surgical team when decisions need to be made about the applicability of a surgical procedure for correction of a faulty gait. It is also valuable after surgery to learn whether the dysfunction has been corrected and how motion of the treated limb is now effecting the dynamics of walking.

Although the technology of gait analysis continues to be studied, it has moved from an "experimental" methodology to the realm of a tool in clinical practice. We can anticipate future improvements in the technology of gait analysis and in the interpretation of its results; however, gait analysis has reached the status of a clinical laboratory test used by clinicians when specific information is necessary as a guide to therapy.

Gait analysis relies upon the use of sophisticated equipment by a team of experts from several disciplines. Because of this, gait analysis is a relatively expensive laboratory examination. However, when used judiciously it has become a useful instrument in appraisal of gait dysfunction and in the evaluation of the results of clinical therapy

Gait analysis is the quantitative measurement of locomotion. It provides objective analysis of pathological locomotion and understanding of the mechanisms which lead to optimum walking ability.

Using advanced three-dimensional gait analysis systems it is possible to:


Measure accurately deviations from normal in the gait pattern. 


Determine the cause of gait abnormalities. 


Provide information to assist clinical decision making in the areas of surgery, therapy and orthotics. 


Evaluate the outcome of intervention to maximize the result.

Why Use Gait Analysis?

Gait is complex and difficult to assess visually. Gait analysis provides a repeatable and reliable method of measuring walking. Surgical, orthopedic and therapeutic recommendations are commonly based on clinical examination alone. Since visually similar gait patterns can be generated from a variety of underlying pathologies, the outcome of intervention can be disappointing and subject to error. A more scientific evaluation can be provided by the use of clinical gait analysis. Interpretation of kinetic and kinematic data using advanced computer technology can, for example, distinguish between primary gait abnormalities and compensatory mechanisms. This allows a more certain outcome from selective surgical intervention, more precise prescription of orthopedic provision and therapeutic recommendations, leading to improved walking ability.

The Foundation of APAS/Gait - Equipment and Methodology

Human gait analysis with motion capture video systems and interactive 3d modeling systems help doctors analyze, diagnose, and document aberrant gaits with tools that compare the gaits of patients before and after treatment to historical case studies of normal and abnormal gaits.

The primary mission of the gait laboratory is to provide objective quantitative information on normal and pathological function which can be ultimately applied to the improved evaluation and treatment of musculoskeletal disease and injury.

Gait analysis combines expertise from orthopaedics, physical therapy, and biomechanics to objectively evaluate the walking patterns of children with gait disturbances. These walking problems may be caused by cerebral palsy, amputation, myelodysplasia, clubfoot or other neuromuscular problems, or traumatic injury.

A gait evaluation consists of a number of tests performed during a single outpatient visit. A typical analysis will consist of a clinical evaluation, visual documentation of your child's current gait status, and measurement of gait performance using highly advanced technology. Each of these tests can help to identify the possible causes of gait-related problems. This data is reviewed by an orthopaedic physician to help determine treatment options for your child, and later to help evaluate your child's response to the treatment.

Every Gait System includes 3 primary components

    1. Kinematics: Analysis of movement of the body and its segments, without calculation of forces

    2. Kinetics: Analysis of Forces and Moments acting on the body segments.

    3. Neuromuscular Activation measured by Electromyography.


screen00050.jpg (60741 bytes)

The Kinematics process involve capturing of video or markers location in order to determine the coordinate of calculated joint center. These coordinates allow the calculation of body’s joint positions relative to Global and Local Coordinate system. Further derivatives allow calculation of velocities and accelerations in 3D space.

Kinematics Basics

Kinetics Basics

Basic Technology Required for Gait Analysis

Available equipment:


These equipment consist of potentiometer placed between two bars. One bar is strapped to the proximal segment and the second to the distal segment. The potentiometer gives a varying voltage output correlate with the angle between the two bars. Any combination of 3 potentiometers can give three angles of rotation. This method of analysis require long preparation time, repeatability problem in placement, cross talk and reliability problems.

Imaging Systems: 


In the old time, only 15 years ago, film was the most versatile means of recording Human Movement. However, today, almost exclusively, video is used to capture Human movement. For 2D analysis, only one camera is required. For 3D, the requirement is at least 2 cameras. However, any number of cameras can be used.


The most frequently employed approach to automated motion analysis is the use of video technology. Typically these systems automatically track reflective markers covered with a highly reflective material. Each camera has a source of light, normally infrared LED's, surrounding the lens. The markers reflect the light back to the cameras. The video signal is either fed into a computer via a "frame grabber" or is analyzed by a hardware video processor. The computer or the video processors examine the video data looking for objects above a threshold of light. In these systems the reflective markers are the brightest objects and the thresholding can be set to automatically discriminate the markers. The cluster of 'pixels' seen above the threshold are then examined to form markers and the centroids of the markers are automatically computed in the two dimensional image plane of the cameras. The difficulty with these system is that any marker can be visible at any time therefore the objects have to be sorted. This is normally done in the 3D reconstruction of the object space. In this process the 3D coordinates of markers seen in at least 2 camera are generated. In human motion such markers generally form 3D trajectories that are easily distinguished and link together. The process of forming 3D trajectories after data collection, in post processing, in known as tracking. Modern algorithms for tracking have drastically reduced the data turn around time.

The use of markers to track body kinematics is not without problems: Identification of standard bony landmarks can be unreliable. The soft tissue overlying the bony landmarks can move, giving rise to noisy data. The marker itself can wobble due to its own inertia Markers can even come adrift completely. Children are often irritated by the markers and pull them off.

marker sets Discussion by the Experts

Accuracy requirements

Electromagnetic systems

The commonly heard names in this area are "flock of birds" and Polhemus. Electromagnetic devices have emerged from the growth of virtual reality in the computer industry. These system detect the motion of sensors placed on each segment in an electromagnetic field. They provide real time six degree of freedom data. The limitations appear to be the cabling necessary to connect the sensors, sampling frequency, the number of sensors that can be followed at one time and the cost. The use of this equipment is growing in areas of human motion analysis, other than gait.

The underlying fundamentals that are central to successful determination of kinematics in gait analysis

  1. Marker sets must have defined relationship to the underlying anatomy.

  2. Three dimensional techniques must be employed.

  3. Quality of the results of gait analysis depends upon the care with which the marker set is applied and necessary measurement are made.

  4. Joint centers and segmental coordinate systems must be accurately determined

Sources of Error in Gait Analysis:

Primary sources of error:


Limitations of measuring equipment


Resolution (number of pixels broken into)


Field size (change for differing needs)




Recognizing markers size (merging or obstructed markers)


Centroid calculations


Resolution and accuracy (bottom line 1-3mm, 2-4 mm more typical)


Skin motion (skin marker movement 2-4mm)


Distance between markers


Anthropometric measurements


Joint center and axes of rotation calculations


Repeatability of identifying axes


Movement of axis


Errors in locating can greatly affect kinetics


Sample hip center determination (not the best for inferior/superior and a/p determination)

APAS/Gait was designed to maximize analysis efficiency and minimize any errors. APAS/Gait integrates video with markers sets. It allows the investigator to select any of the markers sets available. 

In addition, where the average cost of any gait system is around $200,000, APAS/Gait is priced only $5000. You can download the system for free and try it for yourself. Try it, use it, and then let us know. We are sure you will love it.

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