Independent control of joint stiffness in the framework of the equilibrium-point hypothesis |
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Authors: | Mark L Latash |
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Institution: | (1) Department of Physiology and Department of Physical Medicine and Rehabilitation, Rush Medical College, 1753 W. Congress Parkway, 60612 Chicago, IL, USA |
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Abstract: | In the framework of the equilibrium-point hypothesis, virtual trajectories and joint stiffness patterns have been reconstructed
during two motor tasks practiced against a constant bias torque. One task required a voluntary increase in joint stiffness
while preserving the original joint position. The other task involved fast elbow flexions over 36°. Joint stiffness gradually
subsided after the termination of fast movements. In both tasks, the external torque could slowly and unexpectedly change.
The subjects were required not to change their motor commands if the torque changed, i.e. “to do the same no matter what the
motor did”. In both tasks, changes in joint stiffness were accompanied by unchanged virtual trajectories that were also independent
of the absolute value of the bias torque. By contrast, the intercept of the joint compliant characteristic with the angle
axis,r(t)-function, has demonstrated a clear dependence upon both the level of coactivation and external load. We assume that a template
virtual trajectory is generated at a certain level of the motor hierarchy and is later scaled taking into account some commonly
changing dynamic factors of the movement execution, for example, external load. The scaling leads to the generation of commands
to the segmental structures that can be expressed, according to the equilibrium-point hypothesis, as changes in the thresholds
of the tonic stretch reflex for corresponding muscles. |
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Keywords: | |
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