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Re: BSP parameters and human gait

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  • Re: BSP parameters and human gait

    In a recent posting, H. Hatze and A.Baca suggested a "discussion forum on
    the accuracy required in determining human body segment parameters"
    (BSP's). I would like to address this issue in the context of using the
    Inverse Dynamic Approach (IDA) to calculate joint moments. (In this
    approach, kinematic, Ground Reaction Forces (GRF's) and BSP data are all
    utilized.) The question that has interested me (and others) for some time
    is, "How important are uncertainties in each type of data set--e.g., does
    an error of 1% in GRF data degrade the accuracy of hip moments as much as
    say a 10% error in BSP data?

    If one uses an approach described by Cappozzo, Leo and Pedotti (Equation
    17 in their article in J. Biomechanics, v.8, 307-320, 1975), one can assess
    the contribution of different errors on the overall uncertainty of a
    calculated value for a joint moment. I have used the sagittal plane data
    described in Winter's 1979 book on gait and, as a starting point, assumed
    the following uncertainties;
    1. BSP and acceleration data: 10% of nominal values
    2. GRF data: 1% of nominal value (based on forceplate specifications)
    3. Position of resultant GRF vector: 6.3 mm (based on Bobbert and
    Schamhardt's article in J. Biomechanics, v23, 7, 705-710, 1990)
    4. Coordinates of joints: 10mm

    The results of this exercise showed that uncertainties in BSP and
    acceleration data had little effect on the overall uncertainty of the hip
    moment during the stance phase of gait, accounting for less than 2% of the
    total uncertainty. Errors in Center of Pressure (CoP) location, and joint
    axes had much greater ramifications, accounting for over 64% of the
    uncertainty. Percentage-wise, the contributions of BSP and
    accelaration errors became greater during swing (accounting for up to 50%
    of the average uncertainty), but this must be seen in light of the fact
    that moments during swing are extremely small. (The only people who
    placed great importance on the moments during the swing phase were Braune
    and Fischer who at the turn of the century were testing the theory of the
    Weber brothers that the leg acted as a pendulum!)

    The above statements are directly in line with the findings of Capozzo, Leo
    and Pedotti who stated "Errors in determining the position of the
    structure, with respect to the vector of ground reactions, are major
    determinants of the errors on the muscular moments".

    In summary, one could make a 50% error in one's estimates of BSP
    parameters and hardly effect the accuracy of calculated values for joint
    moments. So, in the context of human gait, it makes little sense in
    striving to get perfect BSP data. It makes more sense to obtain reliable,
    accurate data for joint centers of rotation and GRF data.

    Brian L. Davis, Ph.D.
    Dept. Biomedical Engineering (Wb3)
    Cleveland Clinic Foundation
    9500 Euclid Avenue
    Cleveland, Ohio 44195, U.S.A


    Ph: (216) 444-1055 (Work)
    Fax216) 444-9198 (Work)