Ergonomic design of a wrist exoskeleton and its effects on natural motor strategies during redundant tasks

This work investigates how to design a comfortable wrist exoskeleton which complies with the natural coordination mechanisms in the redundant wrist. Human sensorimotor control is known to impose intrinsic kinematic constraints to solve redundant motor tasks. To this end, the effect of an exoskeleton on natural motor strategies was assessed during pointing tasks performed with the wrist. The exoskeleton was designed based on the kinematic model of one specific subject. Then wrist orientation was observed during pointing tasks with the exoskeleton in the following conditions: i) optimal alignment between human and exoskeleton joints; ii) varying degrees of misalignment between human and exoskeleton joints; iii) optimal alignment while the PS axis was locked (i.e. no redundancy). The results exhibited a modification of the natural coordination mechanisms characterized by a subject-specific Koenderink shape index. Kruskal-Wallis pairwise analyses demonstrated a significant variation between test conditions indicating a change of intrinsic constraints with misalignment and locked PS. The assessment methodologies presented in this paper can be used to test for ergonomic constraints and can guide the design of robotic systems performing kinematically redundant tasks.