VISCO-ELASTICITY CHANGE IN HUMAN MUSCLE

Abstract

This paper describes the importance of an adjustable impedance control for contact task robots from the vantage point of human arm control. We know that the human arm operates especially well in contact tasks. The reason is most probably that the joint impedance of the human arm is adjusted directly by muscle viscous friction control and elasticity control. Incidentally, elasticity change is obvious though viscous friction change is not yet obvious in continuous motion pertaining to in vivo muscle. To estimate the muscle viscous friction change and elasticity change in a contact task, we carried out some visual target tracking experiments on the human wrist joint with an elastic motional load. In the experiments, the target moves at a constant speed and the subject tracks a target against the elastic torque of the load. As a result of the experiments, we found that oscillation frequency of the tracking motion of the human wrist joint shifts continuously to a higher region that corresponds with loaded torque increase. This frequency shift suggests that the human arm neuromuscular system changes its viscous frictional coefficient and/or elasticity coefficient continuously by muscle activity change. However, it is not obvious which affects the frequency shift more, the viscous friction change or the elasticity change. Therefore, we carried out computer simulations of dynamic motion on the neuromuscular system model with elastic load. In the model, active muscle was represented by a force generating unit, parallel elastic and viscous frictional components, and a serial elastic component. The coefficients of these components were supposed to change with muscle activity. The simulations were made in three different conditions. In the first condition, the muscle elastic coefficients and viscous frictional coefficient were changed according to the elastic torque of the load. In the second, the elastic coefficients were changed but the viscous coefficient was not changed, and in the third, the viscous coefficient was changed but the elastic coefficients were not changed. Our results showed that the frequency shift depends mainly on viscous frictional coefficient change. Needless to say, the muscle elastic coefficients have to change according to the loaded torque change. Because of the correspondence of the experimental results with the simulations, we can say that the human arm adjusts its joint impedance continuously by muscle viscous friction and elasticity control. We can also say that the compliant and stable robot performing contact task is best controlled by adjusting joint impedance continuously with actuator torque.