Optimal impedance control for task achievement in the presence of signal-dependent noise

Rieko Osu, Naoki Kamimura, Hiroshi Iwasaki, Eri Nakano, Chris M. Harris, Yasuhiro Wada, Mitsuo Kawato*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

132 Citations (Scopus)


There is an infinity of impedance parameter values, and thus different co-contraction levels, that can produce similar movement kinematics from which the CNS must select one. Although signal-dependent noise (SDN) predicts larger motor-command variability during higher co-contraction, the relationship between impedance and task performance is not theoretically obvious and thus was examined here. Subjects made goal-directed, single-joint elbow movements to either move naturally to different target sizes or voluntarily co-contract at different levels. Stiffness was estimated as the weighted summation of rectified EMG signals through the index of muscle co-contraction around the joint (IMCJ) proposed previously. When subjects made movements to targets of different sizes, IMCJ increased with the accuracy requirements, leading to reduced endpoint deviations. Therefore without the need for great accuracy, subjects accepted worse performance with lower co-contraction. When subjects were asked to increase co-contraction, the variability of EMG and torque both increased, suggesting that noise in the neuromotor command increased with muscle activation. In contrast, the final positional error was smallest for the highest IMCJ level. Although co-contraction increases the motor-command noise, the effect of this noise on the task performance is reduced. Subjects were able to regulate their impedance and control endpoint variance as the task requirements changed, and they did not voluntarily select the high impedance that generated the minimum endpoint error. These data contradict predictions of the SDN-based theory, which postulates minimization of only endpoint variance and thus require its revision.

Original languageEnglish
Pages (from-to)1199-1215
Number of pages17
JournalJournal of Neurophysiology
Issue number2
Publication statusPublished - 2004 Aug
Externally publishedYes

ASJC Scopus subject areas

  • General Neuroscience
  • Physiology


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