Effects of Impedance Reduction of a Robot for Wrist Rehabilitation on Human Motor Strategies in Healthy Subjects during Pointing Tasks

Studies on human motor control demonstrated the existence of simplifying strategies (namely`Donders' law') adopted to deal with kinematically redundant motor tasks. In recent research weshowed that Donders' law also holds for human wrist during pointing tasks, and that it is heavilyperturbed when interacting with a highly back-drivable state-of-the-art rehabilitation robot. Wehypothesized that this depends on the excessive mechanical impedance of the Pronation/Supination(PS) joint of the robot and in this work we analyzed the eects of its reduction. To this end wedeployed a basic force control scheme, which minimizes human-robot interaction force. This resultedin a 70% reduction of the inertia in PS joint and in decrease of 81% and 78% of the interactiontorques during 1-DOF and 3-DOFs tasks. To assess the eects on human motor strategies, pointingtasks were performed by three subjects with a lightweight handheld device, interacting with therobot using its standard PD control (setting impedance to zero) and with the force-controlled robot.We quantied Donders' law as 2-dimensional surfaces in the 3-dimensional conguration space ofrotations. Results revealed that the subject-specic features of Donders' surfaces reappeared afterthe reduction of robot impedance obtained via the force control.