Recently, I send this message

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 replies...
Thank you for your response !!!
Mylene Dault

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First subdivide your platform into a discretized square grid. The trick
you want to do here, is to create an pixelized 'image' of your stabilogram.

Represent your COP XY points as a whole series of connected 'dots'.
So, if you have N COP XY points, you will have N-1 line segments. With
simple integer truncation, INT( X_CELLS * ( x- xmin )/ ( xrange ) ),
INT( Y_CELLS * ( y - ymin )/ ( yrange ) ), you can compute what cells
are filled for the line segment end-points. Then, split your segment
in half and test if it's beyond the resolution of your grid. If yes,
do integer truncation of the mid-point and split again. If no, stop
and go to next line segment.

You keep going, line segment by line segment, saving the filled grid
cell numbers. When you are all finished, just add the total
number of grid-cells stored. This will be your area (+/- resolution
of grid cell area size). Once your little program is written, you
might want to try a few different grid sizes and compair results.

You could even devise a simple binary 'image' plot from your data to
double check your results (using something like Matlab).

Ted Morris 612-625-3520
Center For Advanced Manufacturing Design And Control 612-625-9881
Institute of Technology, U of Minnesota FAX: 612-625-8884
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There are mathematical methods to quantify the total sway area
covered by the COP, but you may need a closed geometrical shape from
which to calculate its surface. Therefore many people approximate
this area by calculating either the extrema of the sway (in the x and
y axes) or the standard deviations of the sway in x and y directions.

In your approach is geometrical, you can calculate the surface of any
shape (e.g. rectangle or ellipse) knowing the two major axes (max.
displacement in x and max. displacement in y).

If your approach is statistical, you can assume a normal distribution
in both axes and then calculate the surface of an ellipse, but using
1.96 times the standard deviations of the COP on each axis. This way,
you approximate (under certain circumstances) a surface that covers
about 95^2% of the theoretical COP trajectory.

Ruben Lafuente-Jorge
Institute of Biomechanics of Valencia
P.O. Box 199
46980-Paterna (Spain)
Fax: (96) 1318016
Tel: (96) 1318355

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Il exists plusieurs etudes en france et en belgique sur le patron du
mouvement (sway) avec des standard de normalite et de pathologie.

Dans un prmier temps, la surface (aire) du tracer est calcule, dans un
deuxieme temps, la longeure du tracer est calcule.

Il existe une compagnie (distributer) de ce genre de systeme (platforme
d'equilibre) ici a Montreal.

Norman Murphy, Ph.D.

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You might want to look at amplitude and frequency of sway. You'll probably
need to do this in anteroposterior and mediolateral planes. I'm not sure
of any way to combine these two, unless you want to chart changes in center
of pressure, which you should be able to do with the force plate data.

Paul Fiolkowski, MA, ATC
UF Biomechanics Lab

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