Fig. 1

KINARM exoskeleton robot, tasks, and methodology to quantify rigidity. a The KINARM exoskeleton robot and virtual workspace used to quantify subject performance. Data collection is collected on the computer to the left of the KINARM virtual workspace. b Illustrations of the task battery for PD assessment. In the object hit task (top row, left panel) the subjects had to hit the red circles. In the object hit and avoid task (top row, right panel) subjects had to avoid hitting certain shapes (e.g. hit only squares and vertical ovals). The passive stretch task (bottom row, left panel) quantified rigidity by passively moving the arm at the elbow and recording the required torques (e.g. blue arrow for extension). In the unloading task (bottom row, right panel) the subject initially resists a joint torque (red arrow) and then the joint torque is suddenly removed (white X) and the subject has to maintain their arm posture by moving their hand back to the central target as quickly as possible. c A schematic of how the passive stretch task was used to model PD rigidity by moving elbow through a range of velocities (upper panel)