View Full Version : Re: Inverse Dynamics

Anatol Feldman
01-11-2001, 05:11 AM
Here are my responses to the comments (quated here) by Pat Riley, point
by point.

1. Where are the equilibrium states in walking and running?

Response. A single step is a transition from one postural (equilibrium)
state to another. One can also say that a step results from changes in
specific parameters that transform the equilibrium configuration of the
body in such a way that eventually the body establishes approximately
the same (initial) posture but in another part of external space. All
forces (torques) required for such a transition emerge in response to
the shifts in the equilibrium body configuration and are not programmed
by the nervous system. The faster the shifts, the faster the step. If
you repeat the control shifts, you get walking. By speeding the shifts,
you get running. For more details, please consult section Response in
our article (Feldman & Levin 1995).

2 &3. I think control theory has long recognized that the dynamics of
the system are part of the control loop.

Comment. Generally, you are right but we discuss not a general but a
specific theory called the inverse dynamics. My point was that this
theory is inconsistent with the intrinsic dynamics of the neuromuscular
system and, therefore, it is an example of a theory which failed to make
“the dynamics as a part of the control loop”.

3, 4 Stonehenge, ancient diagrams of circles and epicycles, and
Keppler's equations attest to the fact you can describe undisturbed
motion accurately without Newtonian mechanics. If everything worked as
well fortunately, we live in a world where we want to do things that
have never been done before; and, unfortunately, we live in a world
where many people cannot move according to the heavenly plan. For both
problems, you need f=ma physics. I doubt that the Druids or Ptolemy or
Keppler or the lambda hypothesis could have put people on the moon and
got them back. I hope we do not update the education of young scientist
and engineers to the point that we totally loose the ability to solve
real problems.

Comments. I did not understand the point of this philosophy. The famous
names you listed were the precursors of Newton’s and modern science.
In particular, Newton first tested his laws by looking whether or not
they were consistent with Keppler’s laws. So to be fare, we should blame
not only Newton but also Kepler for what “we could put people on the
moon and get them back”. Concerning the lambda hypothesis, do not you
want to use this theory to simulate locomotion? Maybe, this is a way to
advance our knowledge on how the brain controls it? This might be not
only theoretically significant but might give you the practical ability
you want - “to solve real problems”, such as the understanding of basic
movement pathologies (I heard the opinion that this is more essential
than putting people on the moon). Somebody can use inverse dynamics to
create walking robots. Fine. In fact, we have already thechnical
realizations- airplans- immitating locomotion of birds. Beyond
airodynamics, the creation of airplains did not advance our
understanding of how the brain of birds controls flight. This is an
essential lesson showing that technical immitations of biological
phenomena are devices that are helpless without the brain of the

All best
Dr. Anatol Feldman
Neurological Science Research Center
Department of Physiology
University of Montreal and
Rehabilitation Institute of Montreal
6300 Darlington, Montreal, Quebec, Canada H3S 2J4
Tel (514) 340 2078 ext. 2192
Fax (514) 340 2154
Web Site: http://www.crosswinds.net/~afeldman/

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