Human motor compensations for thixotropy‐dependent changes in muscular resting tension after moderate joint movements

Abstract

This study on healthy subjects explores history-dependent changes in the resting tension of relaxed wrist muscles after moderate joint excursions and the motor control consequences of these changes during voluntary wrist joint position maintenance. Integrated surface electromyogram (IEMG) was recorded from wrist extensor/flexor muscles. Angular position and torque were recorded from the wrist joint. Changes in wrist flexor muscle resting tension were sensed by a force transducer pressed against the tendons. Consecutive stepwise changes (7.5 degrees ) in wrist joint position (within the dorsiflexed range) were either imposed on relaxed subjects or actively performed while the subjects under visual guidance tried to mimic the passive movements. In relaxed subjects, passive joint torque resistance at a given steady dorsiflexed position either gradually declined or rose depending on the direction of the previous transition movements. In corresponding voluntary contraction experiments, the IEMG amplitude from position holding wrist extensors was found to vary in a similar way as the passive torque resistance. Further, there was a strong correlation between history-dependent changes in extensor IEMG amplitude and stress alterations exhibited by the relaxed antagonist flexors. The above described, slowly subsiding post-movement mechanical and motor adaptations were accelerated by brief forceful cocontractions of the forearm muscles. Moderate stepwise changes in joint position are sufficient to induce history-dependent after-effects in passive muscular resting tension, after-effects which during voluntary position holding are effectively compensated for by the motor control system.