Perceptual and Motor Effects of Muscle Co-activation in a Force Production Task.

We tested several predictions of the theory of motor control with spatial referent coordinates related to effects of muscle coactivation on force production and perception. In particular, we predicted that subjects would produce unintentional force increase by finger flexors and be unaware of this force increase. Healthy subjects performed steady force production task in isometric conditions with visual feedback on the force level. They coactivated muscles of the arm trying to keep the force constant in the absence of visual feedback. This led to a consistent force increase not perceived by the subjects as reflected by their verbal reports. In contrast, when asked to match the force with the contralateral hand, adequate force matching was observed. Using the "inverse piano" apparatus confirmed no change in the referent coordinate of the fingers and an increase in its apparent stiffness. The results confirm the earlier hypothesis on the reciprocal command being hierarchically higher than the coactivation command. The observations suggest that verbal reports and force matching use different neural mechanisms of force perception; the former are dominated by sense of effort, which reflects primarily the magnitude of the reciprocal command. There were only minor differences between the dominant and non-dominant hands, likely reflecting the faster unintentional drifts of control variables in the dominant hand.