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Summary of Replies: Kistler Bioware CoM Calculation

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  • Summary of Replies: Kistler Bioware CoM Calculation

    Biomch-L Readers

    I have included the responses to my Kistler Bioware CoM calculation
    question below. Thanks to all those who replied. From all of the
    feedback I am expecting to be able to further understand and resolve my
    original issue.

    In appreciation,


    Dr Peter Clothier
    Lecturer - Sport & Exercise Science
    Building 24 Level 4 Room 7

    School of Biomedical & Health Sciences
    University of Western Sydney
    Campbelltown Campus
    Locked Bag 1797
    Penrith South DC NSW 1797

    Ph. +61 02 46203743
    Fax. +61 02 46203792
    Hi Peter,

    I played around with this a while ago and put the spreadsheet here: (third row from the bottom of the

    Integration is always a fiddle because you don't know the integration
    coefficient - in this case the initial vertical velocity. Luckily, the
    vertical direction is unique, in that you know that it must average zero
    (otherwise the runner would start flying or sink into the track), so the
    trick is:

    1. Run the integration with initial velocity set to zero
    2. Calculate the mean vertical velocity, Vm
    3. Re-run the integration with initial velocity set to -Vm

    Note that the subject's body mass is also quite critical, so needs to be
    measured accurately (or fiddled!). Also, you can't know the absolute
    height of the CoM so you have to estimate it (the NASA rule of thumb is
    54.3% of height from the ground).

    Have fun!

    Dr. Chris Kirtley MB ChB, PhD
    608 Dockside
    44 Ferry St.
    Kangaroo Point
    Queensland 4169
    Australia (GMT+10)
    Tel. 61+7-3891 6644 x 1608
    China mobile: 13418732724
    Fax 3891 6900
    You would need to ensure you have included the initial conditions
    (velocity and position) before you perform the double integration
    otherwise your calculations will be inaccurate. These initial conditions
    have to be determined at the point of impact on the platform. I'm not
    sure whether Bioware v4 allows you to include these values as I don't
    use this.
    Dr Drew Harrison
    Senior Lecturer and
    Head of Dept of Physical Education and Sports Science University of
    Tel: +353 61 202809
    Dear Peter,
    One step that you have not described is that of subtracting the
    bodyweight of the subject from the vertical force component before
    integrating it. This may have been done, but not written in the previous
    Vertical force - subtract bodyweight to get acceleration of COM -
    integrate to get velocity of COM - integrate to get displacement of COM.
    Hi Peter,

    I have been working on measuring COM displacement in my research. I am
    not familiar with the Bioware software, but I am familiar with kinematic
    and kinetic approaches for measuring COM displacement. Double
    integration of the force plate signal will result in vertical COM
    displacement, but only if you take into consideration the mass of the
    subject and the integration constants that emerge when you perform the
    integration. The following article gives a good overview of different
    approaches for measuring COM displacement.

    Gard SA, Miff SC, and Kuo AD. Comparison of kinematic and kinetic
    methods for computing the vertical motion of the body center of mass
    during walking. Human Movement Science; 2004; 22(6): 597-610.

    Good luck!

    Ben Roewer
    MSME/MBA student
    University of Delaware
    That's true, double integration of vertical force should be related to
    the vertical displacement. The problem are the numerical errors that
    grow like time^2. However, since running is a cyclical motion then
    perhaps a Fourier method would work? FFT the force data, replace the
    frequency coefficient a(f) with -a(f)/(2 \pi f)^2 [corresponds to double
    integration; f = frequency] and then take the inverse FFT.
    Regards Frank Borg
    Hi Peter,

    I have never used the kistler software to perform the double
    integration. So, no idea what's going on with the software. However, one
    thing to keep in mind is that the constants required to obtain exact
    solutions from the integration steps, being the initial velocity and
    position respectively, are critical to obtain realistic CM traces.
    Attached is a paper that could point you in the right direction, if this
    indeed is part of the problem.

    I don't know if it is possible for you to plot the raw data and look at
    the pattern first.
    The double integration is very sensitive to noise in the signal,
    therefore the signal is usually conditioned/filtered first. The nature
    of the forceplate instrument is that it typically have some inherent
    signal noise, in our case, it's about 0.25%. So, that might have to be
    resolved before you can compute for accurate CoM. I hope this helps.