Dear List readers,

Following Stevan Harnad's recent posting, I think that it might be useful
to re-post one abstract onto the list, considering the claim on `computa-
tional complexity' of the inverse dynamics problem.

I hope that some of the readers will retrieve the full manuscript(s) and
propose some comments to Dr Harnad!

Sincerely -- Herman J. Woltring, Eindhoven/NL


E. Bizzi, N. Hogan*, F.A. Mussa-Ivaldi and S. Giszter
Department of Brain and Cognitive Sciences and
*Department of Mechanical Engineering
Massachusetts Institute of Technology
Cambridge, MA 02139

ABSTRACT: The hypothesis that the central nervous system (CNS)
generates movement as a shift of the limb's equilibrium posture has
been corroborated experimentally in single- and multi-joint motions.
Posture may be controlled through the choice of muscle length tension
curves that set agonist-antagonist torque-angle curves determining an
equilibrium position for the limb and the stiffness about the joints.
Arm trajectories seem to be generated through a control signal defining
a series of equilibrium postures.

The equilibrium-point hypothesis drastically simplifies the requisite
computations for multijoint movements and mechanical interactions with
complex dynamic objects in the environment. Because the neuromuscular
system is springlike, the instantaneous difference between the arm's
actual position and the equilibrium position specified by the neural
activity can generate the requisite torques, avoiding the complex
"inverse dynamic" problem of computing the torques at the joints.

The hypothesis provides a simple unified description of posture and
movement as well as performance on contact control tasks, in which the
limb must exert force stably and do work on objects in the environment.
The latter is a surprisingly difficult problem, as robotic experience
has shown.

The prior evidence for the hypothesis came mainly from psychophysical
and behavioral experiments. Our recent work has shown that
microstimulation of the spinal cord's premotoneuronal network produces
leg movements to various positions in the frog's motor space. The
hypothesis can now be investigated in the neurophysiological machinery
of the spinal cord.

KEYWORDS: spinal cord, force field, equilibrium point,
microstimulation, multi-joint coordination, contact tasks, robotics,
inverse dynamics, motor control.