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Control of Equilibrium Position and Stiffness Through Postural Modules.

R. Shadmehr1

  • 1Department of Brain and Cognitive Sciences, E25-534, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

Journal of Motor Behavior
|September 1, 1993
PubMed
Summary

Muscles act as spring-like torque generators, and controlling postural modules allows for precise joint stiffness and equilibrium. This system explains limitations in voluntary control of human arm force fields.

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Area of Science:

  • Biomechanics
  • Motor Control
  • Robotics

Background:

  • Muscles function as torque generators, with their energy integral representing potential energy.
  • The musculoskeletal system's energy landscape, defined by muscle and limb potential energy, dictates equilibrium positions.
  • Postural control involves generating desired torque fields through muscle activation.

Purpose of the Study:

  • To explore how postural modules, defined as muscle synergies, can control torque functions and limb equilibrium.
  • To investigate the control of stiffness and equilibrium positions in single-joint and multi-joint systems.
  • To explain the limitations in voluntary control of human arm force fields using a simulation-based approach.

Main Methods:

  • Defining muscles as spring-like torque generators and formulating an energy function for the musculoskeletal system.

Related Experiment Videos

  • Introducing postural modules as muscle synergies controlling torque functions and stiffness.
  • Simulating a multi-joint system to analyze mechanical constraints and voluntary force field control.

    • Main Results:

    • Two postural modules can independently control stiffness and equilibrium position in a single-joint system.
    • Mechanical constraints on restoring force fields in multi-joint limbs were derived.
    • Simulations showed that the organization of postural modules as local stiffness controllers explains the inability to voluntarily control force field shape and orientation.

    Conclusions:

    • Postural modules act as local stiffness controllers, explaining limitations in voluntary force field manipulation.
    • The coactivation of postural modules provides a coarse encoding of workspace.
    • This framework suggests postural modules serve as an intermediate control system in the motor control hierarchy.