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About postural sway

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  • About postural sway

    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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