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This Article Full Text Full Text (PDF) All Versions of this Article: 92/5/3097    most recent 00364.2004v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Franklin, D. W. Articles by Milner, T. E. Search for Related Content PubMed PubMed Citation Articles by Franklin, D. W. Articles by Milner, T. E.

Impedance Control Balances Stability With Metabolically Costly Muscle Activation

¹ATR Computational Neuroscience Laboratories and ²ATR Human Information Science Laboratories, Keihanna Science City, Kyoto 619–0288, Japan; and ³School of Kinesiology, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada

Submitted 9 April 2004; accepted in final form 9 June 2004

Humans are able to stabilize their movements in environments with unstable dynamics by selectively modifying arm impedance independently of force and torque. We further investigated adaptation to unstable dynamics to determine whether the CNS maintains a constant overall level of stability as the instability of the environmental dynamics is varied. Subjects performed reaching movements in unstable force fields of varying strength, generated by a robotic manipulator. Although the force fields disrupted the initial movements, subjects were able to adapt to the novel dynamics and learned to produce straight trajectories. After adaptation, the endpoint stiffness of the arm was measured at the midpoint of the movement. The stiffness had been selectively modified in the direction of the instability. The stiffness in the stable direction was relatively unchanged from that measured during movements in a null force field prior to exposure to the unstable force field. This impedance modification was achieved without changes in force and torque. The overall stiffness of the arm and environment in the direction of instability was adapted to the force field strength such that it remained equivalent to that of the null force field. This suggests that the CNS attempts both to maintain a minimum level of stability and minimize energy expenditure.

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