View Full Version : Re: Summary of marker sets and gait models

07-05-2007, 02:40 AM

I want to add my two cents and some venting to the maker set discussion.
Rather than give my opinion on which marker set is best, I want to
highlight a criticism that is common to most, if not all, of them.

We all know about the problems of skin movement and accuracy of landmark
palpation. To repeat those problems would be redundant. However, many
seem to accept these problems as the "price of doing business." Recent
advances in technology are making resigned acceptance of these issues an
unnecessary roadblock to scientific advancement.

A more vexing problem with marker set systems is the unquestioned
association of an anatomical landmark with important functional
qualities (joint centers, muscle insertion points, etc). Many of the
analytical systems we use calculate joint centers based upon marker
location. Let's assume for a second that the problems of marker location
and skin movement have all been solved -- most of the software systems
we use incorporate these assumptions anyway. What is the basis for the
joint center calculation? Rarely do our software manuals explain this
calculation. Even worse, they assume that this calculation will apply to
all persons at all times. For example, there is a common assumption that
the femoral epicondyles will indicate, or can be used to calculate, the
axis of knee flexion/extension for all people at all times and in
exactly the same way. A statement like that is contrary to the
principals of biological science. The growth formation of the femoral
epicondyles are not related to the axis of the knee and therefore do not
have to have a consistent relationship to that axis. Consistent
correlation of palpation landmarks to joint centers might apply to
robots, but it is not true for humans.

Ok, sure, but what else are you supposed to do? And yes, there is a
little straw in the man I've just created. I don't have a good answers.
I am, nonetheless, rankled by the blind acceptance of the Helen Hayes
marker system, or any other system, that incorporates these types of
non-biological assumptions. Data collection technologies have advanced
to the point where we can start investigating the implementation of
subject specific maker placement in a way that will be more biologically
sound without losing the mechanical qualities that we have come to
expect. Researchers should be encouraged to investigate systems that
adopt this type of perspective, rather trying to improve the mechanical
qualities associated with the biologically inappropriate uniform marker
set approach.

Thomas M. Greiner, Ph.D.
Assistant Professor of Anatomy
Dept. of Health Professions
University of Wisconsin - La Crosse