3 Direct Measurement Techniques

direct Measurement
Techniques
Dr. Amal HM. Ibrahim
Professor of Physical Therapy
aebrahim123@hotmail.com

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
 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
F= ma


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-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 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.


 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.


 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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