3 Direct Measurement Techniques

direct Measurement Techniques

Dr. Amal HM. Ibrahim

Professor of Physical Therapy

@hotmail.com123aebrahim

Goniometers

10/28/2011 aebrahim123@hotmail.com

Goniometers A goniometer is a special name given

to the electrical potentiometer that can be attached to measure a joint angle.

One arm of the goniometer is attached to one limb segment and the other to the adjacent limb segment.

The axis of goniometer is aligned to the joint axis

Goniometers A constant voltage is applied across

the outside terminals, and the wiper arm moves to pick off a fraction of the total voltage.

The fraction of the voltage depends on the joint angle, θ.

Note that the voltage proportional to θ requires a potentiometer whose resistance varies linearly with θ.

Goniometers Potentiometers: A potentiometric

element is attached to a joint's rotation point. The potentiometer's electrical resistance can be used to determine the angle between the joints.

These types of electrogoniometers are somewhat bulky and restrict patient movement

Advantages of Electrogoniometer

A goniometer is inexpensive.

Output signal is available immediately for recording or conversion into a computer.

Disadvantages of Electrogoniometer

1. Relative angular data are given, not absolute angles, thus severely limiting diagnostic value.

2. It may require an excessive length of time to fit and align.

3. If a large number are fitted, movement can be encumbered by the straps and cables.

4. More complex goniometers are required for joints which do not move as hinge joint.

Agreement between an electrogoniometer and motion analysis system measuring angular velocity of the knee during

walking after stroke V.M. Pomeroy a,∗, E. Evansa, J.D. Richards- Physiotherapy 92 (2006) 159–165

Objectives: There is a need to measure quality of movement to assess effectiveness of physical therapy interventions after stroke. Knee angular velocity may be a sensitive measure of change in performance of movement during functional activity, but not all clinical centers have access to a movement analysis laboratory.

The aim of this study was to investigate the concurrent validity of using an electrogoniometer and

a laboratory-based movement analysis system (Vicon Motion Systems Ltd, Oxford, UK) to measure knee angular velocity.

Design: Prospective agreement study.

Setting : Movement analysis laboratory.

Participants: Fifteen adult volunteers (mean age 55.6 years) at least 6 months after stroke and able to walk at least 4.5m indoors.

Interventions: Kinematic data were collected simultaneously from the electrogoniometer and TheVicon system whist subjects walked forwards in the laboratory.

Main outcome measures:

Electrogoniometer and Vicon data were filtered.

Angular velocity was calculated for both sets of data, and the values for peak flexion and peak extension velocity for each stride were extracted for each subject trial.

Results : The intraclass correlation coefficient (ICC) and lower 95% confidence interval was 0.90 (0.87) for peak flexion angular velocity and 0.92 (0.8), and 0. 9 for peak extension angular velocity. The limits of agreement were −50.64 to 80.28 ◦/second for peak flexion angular velocity and −30.59 to 86.27 ◦/ second for peak extension angular velocity.

Conclusions: Despite high ICC values, the limits of agreement were wide. These data indicate that the Vicon system and an electrogoniometer may not be used interchangeably to measure knee angular velocity in stroke subjects.

Movement Analysis using VICON computer generated image

Motion Analysis Lab An adult male types a paragraph as motion data is

collected to evaluate motor performance during typing.

accelerometers

accelerometers An accelerometer is a device that measures

acceleration.

Most accelerometers are nothing more than force transducers designed to measure the reaction forces associated with a given acceleration.

If the acceleration of the limb segment is a and the mass inside the accelerometer is m, then the force exerted by the mass is

accelerometers The force is measured by force transducer, usually a

strain gauge or piezoresistive type.

The mass is accelerated against a force transducer which produces a signal voltage, V, which is proportional to the force, and since m is known and constant, then V is also proportional to the acceleration

accelerometers The acceleration can be toward or away from the face

of the transducer; this is indicated by a reversal in sign of the signal.

In the most movement there is no guarantee that the acceleration vector will act at right angles to the force of the force transducer.

The accelerometer measures the aⁿ component.

Nothing is known about a t or a unless a triaxial accelerometer is used.

accelerometers Advantages

1- output signal is available immediately for recording or conversion to a computer.

Disadvantages

1-Acceleration signal is relative to its position on the limb segment.

2- Cost can be excessive if a large number are used.

3- If a large number are used they can encumber movement.

4- Many types are quit sensitive to shock and are easily broken.

Imaging Measurement Techniques

Imaging measurement techniques

There are many types of imaging systems that could be used:

1- Movie camera.

2- Television.

3- Multiple exposure.

4- Optoelectric

Film Acquisition 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.

Video Camera System 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 camera systems. A video camera is a camera used for electronic motion picture acquisition, initially developed by the television industry but now common in other applications as well.

Hi Cam Intensified Camera A gated intensified high-speed camera that

records images up to 100.000 fps, at high time resolution. Also offers single photon detection. Fast moving objects are recorded sharply by shuttering the image

Hi Speed Camera Ultra-compact high-speed camera with 1,3

megapixels at 1.000 fps.

Basic Lens Optics A simple converging lens is one which creates an

inverted image in focus. The typical focal length is 25 mm

Basic simple lens type There are TWO basic simple lens

types:

CONVEX or POSITIVE lenses will CONVERGE or FOCUS light and can form an IMAGE

CONCAVE or NEGATIVE lenses will DIVERGE (spread out) light rays

Camera lens This complex lens has 6 simple lens

elements

A professional TV zoom lens used to broadcast sports could have 40 elements.

f-stop sitting and field of focus The amount of light entering the lens is controlled by

lens opening, which is measured by its f-stop setting (f means fraction of lens aperture opening).

f-stop (or f-number) is a measurement of the size of the aperture (opening) of the camera iris. If the aperture is wider, more light enters the lens and the picture is brighter.

The larger the opening, the lower the f-stop setting.

f-stop sitting and field of focus F-stops can be a little confusing at first as the numbers

do not behave as you would expect. Higher f-stop numbers indicate a smaller aperture diameter - in other words, as the iris opening decreases in size the f-stop number increases.

Example:

a lens may have the following settings: 22,16,11,8,act, f/22 is 1/22 of the lens diameter and f/11 is 1/11 of the

lens diameter.

Thus f/11 lets in four times the light that f/22 does.

f-stop sitting and field of focus F-stop settings are normally written with a forward

slash like so: f/16. Common f-stops are:

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22.

Note that a small f-stop (large aperture) results in a small depth of field and a larger f-stop (smaller aperture) gives a large depth of field.

f-stop sitting and field of focus

The amount of light entering the lens is controlled by the lens opening which is measured by its f-stop sitting (f means fraction of lens aperture opening).

The larger the opening the lower the f-stop sitting.

f-stop sitting and field of focus

to keep the lighting requirements to a minimum it is obvious that the lens should be opened as wide as possible to a low f setting.

Cinematography Many different sizes of

movie cameras are available.

8mm 16mm, 35mm, and 70mm cameras.

16 mm cameras are reasonable compromise and most high speed camera.

Cinematography There are several types of 16mm either spring driven or

motor driven (by batteries or AC source).

The type of the film required depends on the lighting available.

The ASA rating is a measure of the “speed” of the film; the higher the rating, the less light required to get the same exposure.

Cinematography Higher ASA ratings are available and

are good for a qualitative assessment of movement, especially fast moving sport.

The higher ASA films introduces inaccuracies in quantitative analyses.

Cinematography The shutter speed of the camera

Shutter speed is the amount of time the shutter remains open to allow light to reach a digital camera sensor. Shutter speed is measured in seconds, or fractions of seconds.

The higher the frame rate, the less time is available to expose the film.

Most high speed cameras have rotating shutters that open once per revolution for a period of time to expose a new frame of unexposed film.

Cinematography Example:

at 60 frames/sec using a 3 factor shutter the exposure time is 1/180 sec.

the amount of light entering will be the same as a normal (still) camera set to a speed of 1/180 sec.

Television The television has a fixed frame rate .

The name given to each television image is a field ranged 60 field/sec (USA) and 50 field/sec (Europe).

Television has a high enough field rate for most movements, but too low for a quantitative analysis of rapid athletic events.

The advantage of television is the capability for instant reply, which serves both as a quality control check and as an initial qualitative assessment.

Multiple Exposure It is economical and simplest

imaging system.

It uses a still camera in a darkened room with its shutter open for the entire time of the event.

a bright flashing light (strobe light) illuminates the subject for few milliseconds at a regular interval (say 20 times/sec).

Multiple Exposure Reflective strips

or markers are placed on the subject, and once every flash a new exposure is made on the film.

Multiple Exposure Reflective strips or markers are placed on the subject,

and once every flash a new exposure is made on the film.

Over a period of a few seconds the film is re-exposed many times, but each time with the body in a new position.

Multiple Exposure

advantages

This technique is not expensive.

Give rapid assessment if a Polaroid type camera is used.

Multiple Exposure

Disadvantages

There is overlapping of the images on top of each other, so the assessment are limited to crude measures as ROM.

Due to the need of darken room and limited field of view of a fixed camera, the technique have restricted applications.

The flashing strop light can be very distracting to the subject.

Optoelectric Technique

Two types of system have evolved:

1- Selspot technique which requires the subject to wear special light on each desired anatomical landmark. The lights are flashed sequentially and the location (x,y) of the light flash is picked up on a special camera.

Optoelectric Technique

The location of the image of the light flash gives two signals, one indicating the x coordinate of the image , the other indicating the y coordinate.

As each light flashes in sequence a series of x and y coordinate signals are fed to the tape recorder or high speed computer.

Motion analysis lab Experimental set-up during motion capture of typing

styles

Cameras in motion analysis lab

Clinical movement analysis lab

Reflected marks

Movement Analysis Movement analysis has now become

an important clinical tool to diagnose movement disorders. Specialist physicians are able to diagnose problems by inspecting the pattern of ground reaction forces, the nature of anatomical joint angles, joint torques and power. However, an estimation of muscle forces during a gait cycle will give precise information for the treatment of gait pathologies, particularly, in surgical cases.

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3 Direct Measurement Techniques

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