Dr. Chris Kirtley
01-07-2002, 03:29 AM
Dear all,

This BIONET project and discussion promises to be of great interest to all of us. Thanks to Prof.
Hatze for getting us started.

I would firstly like to try to establish some perspective on this first debate, since it seems to me
that we might be in danger of discrediting a great deal of useful previous work otherwise. It seems
to me that there are basically three types of researcher in biomechanics:

#1. The clinical gait analyzer, who is concerned with relatively slow motion with almost negligible
accelerations, and who has access to the ground reaction forces via force platforms.

#2. The sports biomechanist, who looks at much higher velocity activities with correspondingly
greater second derivatives, and who, for practical reasons, often does not have the luxury of force
platform recordings.

#3. The advanced researcher, who is concerned with highly accurate and valid measurement.

I would contend that many of the problems that Prof. Hatze outlines (invalid rigid body modelling
assumptions, motion artefacts, problems computing second derivatives, inaccuracies in body segment
parameters) apply only to cases #2 and 3. The clinician need not worry unduly, because most of these
problems only become significant when large accelerations are present in the motion, which
fortunately is not the case for pathological gait (though I would welcome a debate on this point).

Having said that, there is one important consideration that does indeed apply to the clinical realm:
that of the estimation of joint (especially the hip) centers of rotation. To my mind this is the
single greatest source of error in clinical inverse dynamics, and I hope this topic will be
addressed. I note that Prof. Hatze mentions that joints generally have six degrees of freedom, and
would hope that this will generate discussion.

Please note that I am not saying that these issues are not relevant, but only that there needs to be
some perspective in the debate. Otherwise I can well imagine that some people (especially those
without a deep understanding of the mathematics involved) may come away totally disillusioned with
inverse dyanmics, which I think would be wrong and unfortunate.

Dr. Chris Kirtley MD PhD
Associate Professor
HomeCare Technologies for the 21st Century (Whitaker Foundation)
NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
Dept. of Biomedical Engineering, Pangborn 105B
Catholic University of America
620 Michigan Ave NE
Washington, DC 20064
Tel. 202-319-5440, fax 202-319-4287
Email: kirtley@cua.edu

Clinical Gait Analysis: http://guardian.curtin.edu.au/cga
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