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    Dear BIOMCH-L community,

    The last thing we want on this network is a certain type of
    low-level discussion including unfounded accusations and
    untrue statements that harm the spirit of BIOMCH-L and
    could possibly be repeated, if we were to respond to the
    unqualified attacks recently advanced by Zvi Ladin in his
    May 23-Biomch-L-posting, which posting has apparently been
    placed on the order of OsteoKinetics Corporation. This is
    decidedly below our standards and below the standard of the
    BIOMCH-L community as well. Rather, we would like to initiate
    a scientific discussion forum on the accuracy actually re-
    quired in determining the values of human body segment

    Practically all anthropomorphic models assume that human body
    segments are rigid, have clearly defined intersegmental
    boundaries, and possess an invariant endo-structure. All of
    these assumptions are convenient for modeling purposes but
    are basically incorrect: First, body segments change their
    exo- and endomorphologies under the influence of external
    (inertial) and internal (muscular and passive viscoelastic)
    forces substantially, sometimes even dramatically (such as the
    thorax during extensive breathing, and the "wobbling" of soft
    tissue structures during running). Secondly, segment boundaries
    are by no means fixed but vary as a segment moves through its
    range of motion. In this process, mass particles are transferred
    from one segment to the adjacent one and vice versa. Other
    contributors to changing segment boundaries are non-stationary
    joint axes of rotation. Thirdly, the constantly changing volume
    of body fluids (mainly blood) within a segment also
    substantially alters the segment's mass distribution.

    In a 1980-publication, one of us (H. Hatze, Journal of Bio-
    mechanics 13, pp. 833-843) described all of these phenomena
    and estimated their influence on the accuracy of the computed
    parameter values to be no more than 6%. This estimate may,
    however, be completely wrong. To our knowledge, there exists no
    reliable study at present which would allow us to give a more
    accurate estimate. However, we may not be aware of pertinent
    studies that appeared recently, such as the apparently important
    MS thesis of Ori Sarfati on which, according to Zvi Ladin, the
    recent publication by Ori Sarfati and Zvi Ladin of the video-
    based system for the inertial parameter determination (J. of
    Biomechanics 26(8), 1011-1016, 1993) is based. (Strangely
    enough, however, Sarfati and Ladin did not find it worth
    quoting Sarfati's MS thesis in the reference list of their own
    publication, although this MS thesis is allegedly fundamental
    to this publication. Under these circumstances, it is
    obviously difficult for others to discover such hidden

    Considering the above arguments, one wonders to what extent
    precision in determining segmental parameter values ( masses,
    principal moments of inertia, mass centroid coordinates, etc.)
    is really of relevance. At present, this question is difficult
    to answer. It is certainly not of much use to the biomechanics
    researcher or to the clinician when error bounds of 2% are
    quoted for parameter determinations of artificial uniform test
    bodies, as is the case with a recently announced system. The
    user is not interested in errors that pertain to unrealistic
    uniform test bodies. He wants to know what accuracy he can
    expect in determining inertial parameter values of real (and
    much more complex) body segments, the densities and shapes of
    which are by no means uniform but vary across the cross-section
    of the segments as well as along their longitudinal axes,
    facts, which all have to be taken into account in an appropriate
    anthropomorphic model for inertial parameter determination.

    Reasonable estimates of error bounds relating to realistic
    techniques for the determination of inertial segment parameter
    values exist. The question remains what relevance the
    stipulation of such error bounds might have in view of the
    fuzziness of intersegmental boundaries and the inaccuracies
    introduced by the apparent non-rigidity of real body segments.
    This is the topic on which we would like to initiate a
    discussion on this forum.


    Herbert Hatze and Arnold Baca