Gait analysis

Gait analysis is the study of animal locomotion, including locomotion of humans.

The study encompasses quantification, i.e., introduction and analysis of measurable parameters of gaits, as well as interpretation, i.e., drawing various conclusions about the animal (health, age, size, weight, speed, etc.) from its gait.

History
With the development of photography, it became possible to capture image sequences which reveal details of human and animal locomotion that are not noticeable by watching the movement with the naked eye. Eadweard Muybridge and Étienne-Jules Marey were pioneers of this in the early 1900s. It was photography which first revealed the detailed sequence of the horse "gallop" gait, which is usually mis-represented in paintings made prior to this discovery, for example.

Although much early research was done using film cameras, the widespread application of gait analysis to humans with pathological conditions such as cerebral palsy, Parkinson's disease, and neuromuscular disorders, began in the 1970s with the availability of video camerasystems which could produce detailed studies of individual patients within realistic cost and time constraints. The development of treatment regimes, often involving orthopaedic surgery, based on gait analysis results, advanced significantly in the 1980s. Many leading orthopaedic hospitals worldwide now have gait labs which are routinely used in large numbers of cases, both to design treatment plans, and for follow-up monitoring.

The forefathers of this research are Murali Kadaba, HK Ramakrishnan, and Mary Wootten. Their main papers, dealing with Euler Angles, led to the development of a marker system. This marker system, referred to as the Helen Hayes Marker System is the predecessor of modern marker systems, such as the ones used in movies.

Equipment and techniques
Gait analysis commonly involves the measurement of the movement of the body in space (kinematics) and the forces involved in producing these movements (kinetics).

Kinematics can be recorded using a variety of systems and methodologies:

1) Photography is the most basic method for the recording to movement and strobe lighting at known frequency has been used in the past to aid in the analysis of gait on single photographic images.

2) Video recordings using footage from single or multiple cameras can be used to measure joint angles and velocities. This method has been aided by the development of analysis software that greatly simplifies the analysis process and allows for analysis in 3 dimensions rather than 2 dimensions only.

3) Passive marker systems, using reflective markers (typically reflective balls), allow for very accurate measurement of movement using multiple cameras (typically up to 8 cameras simultaneously). The cameras send out infra red light signals and detect the reflection from the markers placed on the body. Based on the angle and time delay between the original and reflected signal triangulation of the marker in space is possible. These are typically used for motion capture in movies.

4) Active marker systems are similar to the passive marker system but use "active" markers. These markers are triggered by the incoming infra red signal and respond by sending out a corresponding signal of their own. This signal is then used to triangulate the location of the marker. The advantage of this system over the passive one is that individual markers work at predefined frequencies and therefore, have their own "identity". This means that no post-processing of marker locations is required, however the systems tend to be less forgiving for out-of-view markers than the passive systems.

Therefore, a typical modern gait lab has several cameras (video or infra-red) placed around a walkway or treadmill, which are linked to a computer. The patient has markers applied to anatomical landmark points, which are mostly palpable bony landmarks such as the iliac spines of the pelvis, the malleoli of the ankle, and the condyles of the knee. The patient walks down the walkway or on the treadmill and the computer calculates the trajectory of each marker in three dimensions. A model is applied to compute the underlying motion of the bones. This gives a full breakdown of the motion at each joint.

In addition, to calculate movement kinetics, most labs have floor load transducers, also known as force-plates, which measure the ground reaction force, including both magnitude and direction. Adding this to the known dynamics of each body segment, enables the solution of equations based on Newton's laws of motion and enables the computer to calculate the forces exerted by each muscle group, and the net moment about each joint at every stage of the gait cycle. The computational method for this is known as inverse dynamics.

This use of kinetics however does not result in information for individual muscles but muslce groups, such as the extensor or flexors of the limb. To detect the activity and contribution of indivudual muscles to movement, it is necessary to investigate the electrical activity of muscles. Some labs also use surface electrodes attached to the surface of the skin to detect the activity of for example the muscle of the leg. In this way it is possible to investigate the activation times of muscles and to some degree the magnitude of their activation and thereby assess their contribution to gait. Deviations from normal kinematic, kinetic or EMG patterns are used to diagnose specific conditions and predict the outcome of treatment.

Medical diagnostics
Pathological gait may reflect compensations for underlying pathologies, or be responsible for causation of symptoms in itself. The study of gait allows these diagnoses to be made, as well as permitting future developments in rehabilitation engineering. Aside from clinical applications, gait analysis is widely used in professional sports training to optimise and improve athletic performance.

Biometric identification and forensics
Gait analysis techniques allow for the assessment of gait disorders and the effects of corrective Orthopedic surgery. Options for treatment of cerebral palsy include the paralysis of spastic muscles using Botox® or the lengthening, re-attachment or detachment of particular tendons. Corrections of distorted bony anatomy are also undertaken.

It is heavily used in the assessment of sports and investigations into the movement of a large variety of other animals.

Minor variations in gait style can be used as a biometric identifier to identify individual people. The parameters are grouped to spatial-temporal (step length, step width, walking speed, cycle time) and kinematic (joint rotation of the hip, knee and ankle, mean joint angles of the hip/knee/ankle, and thigh/trunk/foot angles) classes. There is a high correlation between step length and height of a person. 

Gait analysis was proposed as authentication for portable electronic devices. 

For slip and fall investigations, the incident walking surface slip resistance can be measured. The surface can be tested to identify if it is above or below accepted levels or slip thresholds. 

the English XL slip meter, also known as a VIT (Variable Incidence Tribometer) is a leading edge portable "slip tester", which is designed to test the coefficient of friction or "slip index" on various walking surfaces, level or incline (even steps), under dry and wet (or otherwise contaminated) conditions by mimicking certain pedestrian biomechanical parameters. The objective measurements that can be analyzed and compared with "normal" walking forces and industry standards regarding flooring slip resistance. 

Popular media
G.K. Chesterton premised one of his Father Brown mysteries, "The Queer Feet", on gait recognition.

Dr. Nick Stergiou - Gait Analsyis, Biomechanics lab. University of Nebraska at Omaha.