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The Journal of Neuroscience, July 18, 2007, 27(29):7705-7716; doi:10.1523/JNEUROSCI.0968-07.2007

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Behavioral/Systems/Cognitive
Endpoint Stiffness of the Arm Is Directionally Tuned to Instability in the Environment

David W. Franklin,^1,2 Gary Liaw,² Theodore E. Milner,³ Rieko Osu,^1,2 Etienne Burdet,⁴ and Mitsuo Kawato²

¹National Institute of Information and Communications Technology, Keihanna Science City, Kyoto 619-0288, Japan, ²Advanced Telecommunications Research Institute, Computational Neuroscience Laboratories, Keihanna Science City, Kyoto 619-0288, Japan, ³School of Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6, and ⁴Department of Bioengineering, Imperial College London, London SW7 2AZ, United Kingdom

Correspondence should be addressed to David W. Franklin at his present address: Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK. Email: dwf25{at}cam.ac.uk

It has been shown that humans are able to selectively control the endpoint impedance of their arms when moving in an unstable environment. However, directional instability was only examined for the case in which the main contribution was from coactivation of biarticular muscles. The goal of this study was to examine whether, in general, the CNS activates the sets of muscles that contribute to selective control of impedance in particular directions. Subjects performed reaching movements in three differently oriented unstable environments generated by a robotic manipulandum. After subjects had learned to make relatively straight reaching movements in the unstable force field, the endpoint stiffness of the limb was measured at the midpoint of the movements. For each force field, the endpoint stiffness increased in a specific direction, whereas there was little change in stiffness in the orthogonal direction. The increase in stiffness was oriented along the direction of instability in the environment, which caused the major axis of the stiffness ellipse to rotate toward the instability in the environment. This study confirms that the CNS is able to control the endpoint impedance of the limbs and selectively adapt it to the environment. Furthermore, it supports the idea that the CNS incorporates an impedance controller that acts to ensure stability, reduce movement variability, and reduce metabolic cost.

Key words: motor control; motor learning; stiffness; impedance control; stability; muscle cocontraction; EMG

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Received Aug. 23, 2006; revised May 30, 2007; accepted June 3, 2007.

Correspondence should be addressed to David W. Franklin at his present address: Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK. Email: dwf25{at}cam.ac.uk

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