Experimental Brain Research

, Volume 88, Issue 2, pp 433–442 | Cite as

Time-varying stiffness of human elbow joint during cyclic voluntary movement

  • D. J. Bennett
  • J. M. Hollerbach
  • Y. Xu
  • I. W. Hunter
Regular Papers


The objective of this study was to determine the extent to which subjects modulate their elbow joint mechanical properties during ongoing arm movement. Small pseudo-random force disturbances were applied to the wrist with an airjet actuator while subjects executed large (1 rad) elbow joint movements. Using a lumped parameter model of the muscle, tendom and proprioceptive feedback dynamics, a time-varying system identification technique was developed to analyze the phasic changes in the elbow joint's mechanical response. The mechanical properties were found to be time-varying, and well approximated by a quasi-linear second-order model. The stiffness of the arm was found to drop during movement. The arm was always underdamped, with the damping ratio changing during movement. Inertia estimates were constant and consistent with previous measurements. Overall, the moving arm was found to be very compliant, with a peak stiffness value less than the lowest value measured during posture, and a natural frequency of less than 3 Hz. Changing the speed of movement, or the load from gravity, changed the stiffness measured, but not in strict proportion to the change in net muscle torque.

Key words

Movement Neuromuscular control Time-varying Stiffness Elbow Human 


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Copyright information

© Springer-Verlag 1992

Authors and Affiliations

  • D. J. Bennett
    • 1
  • J. M. Hollerbach
    • 2
  • Y. Xu
    • 2
  • I. W. Hunter
    • 2
  1. 1.Department of Brain and Cognitive SciencesMassachusetts Institute of TechnologyCambridgeUSA
  2. 2.Biomedical Engineering DepartmentMcGill UniversityMontrealCanada

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