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  • about potural sway

    This is the message I sent:

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

    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)
    ================================================== ======

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

    >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:

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


    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)


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

    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