H. Hatze And A.baca
05-26-1994, 09:55 PM
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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