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I'm presently working on postural sway with head injured subjects.
I'm having some troubles to find a way to quantify the surface of the sway.
The results shown a multi-form pattern. Subjects have been tested on a AMTI
force plate-form and the acquisition is done by the Peak system.

Here are some responses.

Thank you for the numerous responses !!!

Mylene Dault

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I have the following references for you

Dual-task Assessment of Reorganization..
ACH Geurts et al. Arch Phys Med Rehabil 72, 1991,p 1059.

Attention Demands in Balance...
ACH Geurts et al. J of Motor Behavior 26(2), 1994, p 162.

Identification of Static and Dynamic..
ACH Geurts et al. Arch Phys Med Rehabil 77, 1996, p639.

In this artikles they have done postural and balance studies.
May be it will be a help for you.

"Rico de Visser"

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Currently a graduate student I advise is working on postural sway using an
AMTI force plate. To quantify the amount of sway he sums the amount of
displacement of the ground reaction forces in the X and Y directions
(lateral and fore/aft, respectively, according to AMTI's definition) during
a five sec. trial with sampling at 100 Hz. For example if the there are five
consecutive samples of 0.5, 0.10, 0.5, -0.5, -0.10 cm, for the X values he
adds 0.5+0.5+0.10+0.5=2.5 cm (differences between the values). This is also
calculated for the Y direction. The velocity can be determined by dividing
by the amount of time to get cm/sec.

The calculations for the difference between the 500 numbers can easily and
quickly be derived using a spreadsheet like Microsoft Excel. If the X data
is in column A (cells A1:A500) of the worksheet, enter the formula in cell
B2: =ABS(A2-A1) and fill the formula down to B500. In cell B501, calculate
the sum by entering the formula: =SUM(B2:B500) or by clicking on the sum S
button on the toolbar.

This is like an integration of a full-retified EMG signal. A
root-mean-square (RMS) calculation would also work, I believe. If your Peak
system is recording the force plate data, it should be able to export the
files to a spreadsheet. We use an Ariel system which exports data into the
Excel spreadsheet.

I have not verified this method with other people studying postural sway and
would be interested in knowing if this is a valid method. I would also like
to see other responses you get. If you don't post replies, please forward
them to me.

Thank you.


Bruce Etnyre, Ph.D., P.T. _/_/_/_/ _/_/_/_/ _/_/_/ _/_/_/_/ _/ _/
Human Performance and _/ _/ _/ _/ _/ _/ _/ _/
Health Sciences Dept. _/ _/ _/ _/ _/ _/ _/
Rice University _/_/_/_/_/ _/ _/ _/_/_/ _/ _/
6100 Main MS 545 _/ _/ _/ _/ _/ _/ _/
Houston, Texas 77005 _/ _/ _/ _/ _/ _/ _/
USA _/ _/ _/ _/ _/ _/ _/ _/ _/ _/ _/_/_/_/ _/_/_/ _/_/_/_/ _/_/ _/
FAX: (713)285-5329
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We resolved the problem as follow supposing you have about 1000 points
passed by sway. All points have a pair of coordinates (x , y) :

1. taking the average of x and y gives X0 and Y0
2. calculate the distance D and the angle A of each point vs X0 and Y0
3. divide the area in 24 sectors with X0 and Y0 as center
4. sort the data so that you find which point is in wich sector
5. sort the data in each sector to the biggest D
6. calculate the surface of each triangle obtained and make the sum.

The more sectors gives more chance to be accurate but also increase
the chance of finding a area without points...

I found the algorithm in :
Arch. Phys. Med. Rehabil vol 75 September 1994

================================================== ========
ing Pierre Van Cleven
Dept of Kinesiology Institute of Physical Education
University of Gent
Watersportlaan 2 B-9000 Gent (Belgium)
tel (32) fax (32)
================================================== ======

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>I'm having some troubles to find a way to quantify the surface of the sway.
>The results shown a multi-form pattern.

You might try looking at the data in terms of fractional brownian motion,
as in these papers:

Riley, M. A., Mitra, S., Stoffregen, T. A., & Turvey, M. T. (1997).
Influences of body lean and vision on unperturbed postural sway. Motor
Control, 1, 229-246.

Collins, J.J. & De Luca, C.J. (1995). The effect of visual input on
open-loop and closed-loop postural control mechanisms. Experimental Brain
Research, 103, 151-163.

About a dozen additional ways to characterize postural sway are presented in

Riccio, G. E., & Stoffregen, T. A. (1991). An ecological theory of motion
sickness and postural instability. Ecological Psychology, 3, 195-240.

Good luck.

Tom Stoffregen

Thomas A. Stoffregen, Ph.D.
Department of Psychology, P.O. Box 210376
University of Cincinnati
Cincinnati, OH 45221-0376

(513) 556-5569
Personal homepage:
Postural stability laboratory:

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We are just developing a program for testing postural sway
in knee patients. So far, we have the subjects stand on the
force plate (Kistler) in a single leg stance for 15 seconds
with eyes open and eyes closed. We record the vertical ground
reaction force Fz and the location of the force vector on the
platform in both directions (ax and ay). We sample 600
measurements, i.e. we have a frequency of 40 Hz. We calculate the
standard deviation of ax and ay, the range between min and max
values in both directions and the total distance that the force
vector "traveled" during the whole period. Dividing distance by
time gives you something like the sway velocity.

We have yet to find out which parameter(s) is (are) most reliable
and can help to distinguish between normal and pathologic
conditions. I hope this helps a little.



__________________/\ ________/\ _____________________

\/ \/

Dr. Dieter Rosenbaum

Abt. fuer Orthopaedische Physiologie (Kinesiology Lab)

Klinik und Poliklinik fuer Allgemeine Orthopaedie

Westfaelische Wilhelms-Universitaet Muenster

Albert-Schweitzer-Str. 33

D-48129 Muenster, Germany

Phone: (...49) 251 - 834 7981 or 7995 (fax 7989)


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Thank you to Stephen J. Kinzey, Ph.D. for his response.

Stephen J. Kinzey, Ph.D.
Assistant Professor / Director of Biomechanics Laboratory
The University of Mississippi
Department of ESLM
University, MS 38677
office: (601) 232 - 5540
fax: (601) 232 - 5525