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Re: Inverse Dynamics

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  • Re: Inverse Dynamics

    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:

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