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Testing Validity Responses

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  • Testing Validity Responses

    Thank you for all of the responses that I received on my posting “Testing
    Validity”. Following is the original query and the responses:

    This query is about test validation.

    We have developed a sitting reach test with individuals with spinal cord
    injury to measure trunk control and have had high test/re-test reliability
    with the 3D kinematics. Now we want to validate this test. We began this
    quest by sitting the patients directly on a force plate to measure the COP
    excursion in the direction of the reaching. The subject is positioned in an
    unsupported sitting position with feet on the floor. A problem has now
    arisen. If the patient leans far enough, the COP reaches the edge of the
    force plate, thus truncating the excursion.

    Are there any suggestions for remedying this problem or alternative for
    validating this test?

    Stephen Lindley MS, PTA


    Have you tried sitting the subject on a Swiss Ball, BOSU, or Dynadisk? I've
    been thinking about doing something like this for years for entirely
    different reasons, but this might be applicable in your scenario as I
    understand it. I'm admittedly not clear on how the force plate will "read"
    movement occuring on such a device, and specifically wonder if one will lose
    measurement sensitivity. Nonetheless, at first glance, the Dynadisk might
    give some valuable measures on center of pressure while simultaneously
    limiting the excursion. Please summarize and post the responses to your
    query: I'll be among those very interested in learning about the suggestions
    from around the world.

    Don Don Hoover
    Add a force plate under their feet & use the COP of their entire contact
    area rather than just under their rear. It would move through 3D space, but
    could never pass beyond the edge of a force plate.

    Brian Schulz, Ph.D.
    Program Specialist
    VISN 8 Patient Safety Research Center (118M)
    James A. Haley Veterans' Hospital
    11605 North Nebraska Ave.
    Tampa, FL 33612-5738
    Phone: (813) 558-3944
    Fax: (813) 558-3990
    Perhaps a metal plate, bigger in dimension than the surface of the force
    plate, could be clamped or bolted to the top of the force plate. The
    plate would need to be re-calibrated to get a new coordinate system
    center after the attachment. Then the change in center of pressure
    could occur over the dimensions of the metal plate. I am not sure if
    this would work or not.

    Ray Burdett Burdett, Ray
    If you measure the COP with the force plate, the COP can't really go beyond
    edge. Unless you have tensile forces acting on the plate.

    Is the chair placed on the force plate? If so, what you may be seeing is
    COP reaching the outline of a rectangle formed by the legs of the chair.
    would cause the signal to look truncated.

    van den Bogert, Ton

    what about building a construction (like a seat) and mount that on the force
    plate, so that the patient has his COP near to the rear edge of the plate
    while sitting in an upright position? The feet of the patient could rest on
    the platform. The construction must not touch the ground outside the

    Best regards

    Lars Dr. Lars Janshen
    Dean et al. (Sitting balance I: trunk-arm coordination and the
    contribution of the lower limbs during self-paced reaching in sitting.
    (1999) Gait and Posture, 10(2): 135 - 146) used two force plates, one
    under the feet and one under the buttocks, to capture CoP displacement
    during seated reach. This allowed them to look at the transfer of loading
    from the seat to the feet, and could give you characterisation of CoP
    outside the buttock-only base of support.

    Hope this helps,
    Jon Doan, M.Sc., P.Eng.
    Balance Research Laboratory
    University of Lethbridge
    Please call me at the numbers listed below, I have a couple of suggestions
    that may help.
    Gary Blanchard
    Product Manager
    Tel: 617-926-6700, mobile: 617-216-5397
    Fax: 617-926-5045

    Once again, thank you all for your responses
    Stephen D Lindley
    Senior Research Associate
    The Miami Project to Cure Paralysis
    University of Miami – Medical Campus