Journal of Neuroscience, Vol 13, 45-62, Copyright © 1993 by Society for Neuroscience
Postural force fields of the human arm and their role in generating multijoint movements
R Shadmehr, FA Mussa-Ivaldi and E Bizzi
Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge 02139.
When a perturbation displaces the human hand from equilibrium, arm muscles
respond by producing restoring forces. When a set of displacements are
given at various directions from the same equilibrium position, the
resulting restoring forces form a "postural force field." It is not known
whether these postural forces are related to those generated when a
reaching movement is executed. However, if a movement is a consequence of a
shift of the equilibrium position of the hand toward the target, then, from
the postural force field, predictions can be made regarding the nature of
the elastic forces acting on the hand during the movement. We have taken
the first steps in testing this hypothesis by measuring the postural force
field of a subject's arm over relatively large distances, and comparing
these forces with the static forces generated at the hand while the subject
attempted a reaching movement. Using a robot manipulandum, the hand was
displaced at various directions from an equilibrium position. The measured
restoring forces were fitted to a nonlinear model to define a postural
force field for that equilibrium position. This field was used to predict
elastic forces generated when the subject attempted to move the
manipulandum from a point on the circumference of a circle to a target at
its center--the center corresponded to the equilibrium position at which
the postural field was measured. In some of the movement trials, the
manipulandum was locked during approximately the first 120 msec of the
program for motion and the resulting static "evoked" forces measured. We
found that (1) the evoked forces did not point to the target, but were a
function of the configuration of the arm and rotated with the shoulder
joint, and (2) the magnitude of the evoked forces varied systematically,
even though the movements were of the same magnitude. These patterns were
remarkably similar to those observed in the postural forces. Our results
provide experimental evidence linking maintenance of posture in a
multijoint system to that of generating a movement. The evidence is
consistent with the hypothesis that the CNS programs a reaching movement by
shifting the equilibrium position of the hand toward the target.
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