Measurement of Gait Kinematics

By : Khan Muhammad Saad

Gait Analysis by ObservationThe simplest form of Gait Analysis is done by

unaided Human eye . It is the most complicated and versatile form of analysis available. Many Clinicians use this observation type.

This method has four serious limitations Record is not Permanent Eye cannot observe high speed eventsIts only possible to observe movements, and

not forcesIts greatly dependent on the skills of

individual observer.

General Gait ParametersSpeedCadence ( Steps/ min)Stride Length

KinematicsKinematics describe the gait in

terms of

◦Angles◦Positions( displacements)◦Velocities & Accelerations

Normal Gait Kinematics

Techniques for MeasurementElectrogoniometersOptical (Visual ) SystemsElectromagnetic SystemsUltrasonic (zebris) SystemsInertial ( Using MEMs)

ElectrogoniometersElectrogoniometer is a device for

making continuous measurement of the angle of a joint. The basic electrogoniometer consists of a potentiometer, mounted on two brackets ,which are strapped to the body segments . Its mostly used for knee joint.

The output is plotted as chart of joint angle with respect to time or the cyclogram

One arm of goniometer is attached to one limb segment, the other to adjacent limb segment and the axis of goniometer is aligned to joint axis.

A constant voltage E is applied across the outside terminals and the wiper arm picks of the fraction of that voltage. The fraction of voltage depends on joint angle.

The voltage proportion of joint angle requires a potentiometer .

Limitations of ElectrogoniometersThe fundamental limitation lies in need for

accurate alignment of potentiometer spindle with the joint axis of rotation

The electrogoniometer is fixed by cuffs around the soft tissues, not to the bones, so the output doesn’t exactly relate to the bone movement of the joint

It is difficult to identify the joint axis ,e.g. in the case of hip joint

The joint axis may not be fixed, e.g. in the case of knee.

Electrogoniometer can only record relative motion between adjacent body segments.

Flexible GoniometerBiometrics Ltd made an

improved version known as flexible goniometer in which two small blocks are connected to a strain gage metal strip. The end blocks are simply attached to the each side of joint and there is no need to align them.

Ultrasonic System ( zebris)Zebris Ultrasonic system is a

motion analysis system, which utilizes the delay in sound transmission through air to triangulate the position of ultrasound emitting markers.

Electromagnetic SystemsIf magnetic coils are attached to

a subject’s limbs, kinematics measurement can be done using the orientation and variation of magnetic field.

Optical ( Imaging ) SystemsOptical systems can measure Kinematics

in 2-D or 3-D . Simplest measurements require single

camera. 3-D measurements require two or more cameras

A single camera can measure joint angles in sagittal plan. The camera is positioned at right angle to the planes of motion and as far away as possible. Some commercial systems are available to measure joint angles from television pictures.

Cinematography Many sizes of cameras are available,

from 8mm to 70mm. 16 mm cameras are mostly used. Most high speed cameras are 16mm

Cine films are used to measure outdoor activity usually as they have the ability to film the subject in the field.

If a calibration has been used, it is also being filmed. The images are digitized usually by an operator using conversion techniques.

Television/ Video RecordingTelevision cameras has fixed frame

rate and its usually low for quantitative analysis. The major advantage of Television camera is immediate replay and it conveniently digitized.

Video recording can be used in same way as cinematography , as it is cost effective, speedy and convenient. But its accuracy and resolution is not good compared to cine films.

Infra Red CamerasInfra-red camera does not use

visible light. It only picks up the reflected infrared light, which is reflected from the markers. The light is pulsed at 120Hz , and the marker images are frozen in time.

But these are mostly used for 3-D analysis.

Most commercial 2-D systems can measure the positions, but its difficult for them to calculate acceleration and velocity, due to differentiation required.

Limitations of 2-D systemsParallax Error :It occurs when the subject moves

away from the optical axis of the camera. It should be minimized by aligning the optical axis of the camera to the central part of motion and using a zooming lens.

Parallax error

Perspective Error:It is apparent change in length of

an object when it moves out of the calibrated plane.

In 2-D system, it is supposed that patient walks in sagittal plane , while some disorders make it difficult for the patient to walk in sagittal plane.