Research‎ > ‎Papers‎ > ‎

Bizzi - 1991 - Computations underlying the execution of movement: A biological perspective


Bizzi E, Mussa-Ivaldi FA, Giszter S. Computations underlying the execution of movement: a biological perspective. Science. 1991 Jul 19;253(5017):287-91. PUBMED

10 Word Summary

Endpoint force-fields give evidence for the equilibrium point hypothesis.


To execute voluntary movements, the central nervous system must transform the neural representation of the direction, amplitude, and velocity of the limb, represented by the activity of cortical and subcortical neurons, into signals that activate the muscles that move the limb. This task is equivalent to solving an "ill-posed" computational problem because the number of degrees of freedom of the musculoskeletal apparatus is much larger than that specified in the plan of action. Some of the mechanisms and circuitry underlying the transformation of motor plans into motor commands are described. A central feature of this transformation is a coarse map of limb postures in the premotor areas of the spinal cord. Vectorial combination of motor outputs among different areas of the spinal map may produce a large repertoire of motor behaviors.


  • Frogs have brain-stem severed from spinal cord, spinal cord is stimulated and the forces at the ankle were measured.  This procedure was repeated for many different configurations of the frog leg in order to construct an effective force-field of the "resting" leg.
    • Can the force field be described by geometry?  Constant stiffness would change endpoint stiffness for different configurations.
  • CFF = convergent force field
  • Four areas of the spinal cord elicit unique force fields that form rostrocaudally oriented stripes.
    • Stimulating two regions shows a LINEAR combination of the two fields.