Reduced Effort Does Not Imply Slacking: Responsiveness to Error Increases With Robotic Assistance

In both neurorehabilitation and functional augmentation, the patient or the user's muscular effort diminishes when the movement of their limb is supported by a robot. Is this relaxation a result of “slacking” by letting the robot take-over the movement, resulting in less responsiveness in the task? To address this question, we tested subjects who controlled a virtual cursor isometrically to track a moving target without and with different assistants. We measured the force applied by the subject as a metric for effort and estimated their control gain as the metric for responsiveness in the task. Although subjects applied less force with position assistance, the norm of the control gain increased with all assistants, i.e., they applied proportionately larger forces for the same difference between the cursor and the target states. Furthermore, assisting velocity errors improved baseline performance without reducing effort. Though all assistants improved task performance, the control gain adapted differently to position and velocity assistance. Position assistance was exploited to accurately track the target, whereas velocity assistance was treated as a disturbance, and was effectively nullified as it prevented submovements that minimized positional error. Our results show that robotic assistance increases task responsiveness in healthy individuals and that assisting velocity errors could boost patient performance without reducing their motor effort.