Haptic Rendering Modulates Task Performance, Physical Effort and Movement Strategy during Robot-Assisted Training

Research on neurorehabilitation has emphasized that somatosensory information about the interaction with the environment during physical training is crucial to provoke brain plasticity. Despite this, only a small number of robotic devices provide haptic rendering of the virtual environment during neurorehabilitation exercises, the majority with simple structures. However, to provide realistic haptic rendering while supporting neurological patients to perform motor tasks, a transparent robot with several degrees of freedom is needed. In this study, we employed Disturbance Observers to achieve high transparency and fine haptic capabilities on the six DoF exoskeleton ARMin. We incorporated arm weight compensation to reduce the excessive physical effort required to move against gravity, promoting movement performance and directing the participants’ effort to the interaction with the environment. The effect of haptic rendering and its interaction with arm weight compensation were evaluated with six healthy participants. The task consisted of inverting a virtual pendulum and keeping it inverted. We found that haptic rendering of the pendulum dynamics affects the movement strategy the participants follow, i.e., they covered a significantly larger workspace with the end-effector at a significantly higher speed, and required moderate physical effort. The inclusion of arm weight support increased task performance and reduced participants’ effort, while it did not change the movement strategy. Our results suggest that haptic rendering, together with arm weight support, are potential interventions to enhance neurorehabilitation due to the added somatosensory information during motor training.