Effects on human motor strategies of physical interaction with a force-controlled wrist rehabilitation robot

In this work we analyze the effects of a force control scheme, implemented on a robot for wrist rehabilitation (InMotion3 by Interactive Motion Inc. [1]) to reduce the perceived mechanical impedance, on human motor strategies during pointing tasks. Previous studies showed that mechanical impedance of the robot interferes with natural motor strategies [2]. Three healthy subjects performed pointing tasks in three different operative ways: i) with a lightweight hand-held device (capable of measuring wrist rotations without introducing negligible loading effects); ii) with the InMotion3 controlled using the default control with the impedance parameters set to zero; iii) with the same robot, controlled using a direct force control scheme, which reduces the perceived mechanical impedance. In order to study the effects of the force control on the intrinsic kinematic constraints, we assessed wrist rotations with the same protocol we already used to study wrist motor strategies during redundant pointing tasks. The thickness of Donders surfaces (see [3] and [4] for details) indicates how much a soft constraint (such as Donders Law) applies to wrist kinematics during pointing tasks. The effects of force control can be primarily found in the variability of the fitted surfaces as well as in their thickness values. In particular, thickness values, which for the uncontrolled robot are one order of magnitude smaller than the physiological values (suggesting that the constraint has a mechanical origin rather than a neural one), return back into the physiological range in the case of the force-controlled robot.