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 we
showed that Donders' law also holds for human wrist during pointing tasks, and that it is heavily
perturbed when interacting with a highly back-drivable state-of-the-art rehabilitation robot. We
hypothesized that this depends on the excessive mechanical impedance of the Pronation/Supination
(PS) joint of the robot and in this work we analyzed the effects of its reduction. To this end we
deployed a basic force control scheme, which minimizes human-robot interaction force. This resulted
in a 70% reduction of the inertia in PS joint and in decrease of 81% and 78% of the interaction
torques during 1-DOF and 3-DOFs tasks. To assess the effects on human motor strategies, pointing
tasks were performed by three subjects with a lightweight handheld device, interacting with the
robot using its standard PD control (setting impedance to zero) and with the force-controlled robot.
We quantified Donders' law as 2-dimensional surfaces in the 3-dimensional configuration space of
rotations. Results revealed that the subject-specific features of Donders' surfaces reappeared after
the reduction of robot impedance obtained via the force control
