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

************************************************** ***********************

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

tmorris@me.umn.edu 612-625-3520

Center For Advanced Manufacturing Design And Control 612-625-9881

Institute of Technology, U of Minnesota FAX: 612-625-8884

************************************************** ************************

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)

E-Mail: rlafuent@ibv.upv.es

Fax: (96) 1318016

Tel: (96) 1318355

************************************************** ********************

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.

murphyn@ere.umontreal.ca

************************************************** *********************

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

************************************************** *********************

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

************************************************** ***********************

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

tmorris@me.umn.edu 612-625-3520

Center For Advanced Manufacturing Design And Control 612-625-9881

Institute of Technology, U of Minnesota FAX: 612-625-8884

************************************************** ************************

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)

E-Mail: rlafuent@ibv.upv.es

Fax: (96) 1318016

Tel: (96) 1318355

************************************************** ********************

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.

murphyn@ere.umontreal.ca

************************************************** *********************

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

************************************************** *********************