In this paper, we investigated a relationship between human-arm mechanical impedance and task performance when subjects increase co-contraction level voluntarily. Subjects were asked to perform point-to-point movements in the horizontal plane, and hand trajectory and surface electromyographic (EMG) activity were recorded. Stiffness that estimated from index of muscle co-contraction around the joint (IMCJ) was calculated by surface EMG signal. When required high accuracy by reduce target size, stiffness level increased. When co-contraction level increased, EMG, torque and position deviation increased. But, end-point error did not increase and was largest for the lowest co-contraction level. The increasing EMG deviation suggests that noise increase with muscle activation. However, the change in impedance reduces the effect of this noise on the final task error.
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