Spared Motor Neurons Enable Control of a Robotic Sixth Finger for Functional Grasping in Tetraplegia

: Restoring hand function is a top priority in spinal cord injury (SCI), yet current assistive technologies remain limited in usability and lack intuitive control. We developed a non-invasive neuromechatronic interface that translates spared motor neuron activity from individuals with tetraplegia into the motion of a supernumerary robotic sixth finger. Three individuals with chronic, motor complete cervical SCI (C5-C6 level, > 8 years post-injury), unable to voluntarily move their fingers, participated in the study. High-density surface electromyography (HDsEMG) signals were recorded from paralyzed forearm muscles during attempted hand opening and closing. These signals were decomposed in real time into individual motor unit activity, which was then used to proportionally control the robotic finger. Participants' ability to use the robotic finger was evaluated through a series of functional grasping tasks involving objects of daily living. After only a few minutes of training, all participants were able to intuitively modulate the activity of their spared motor units and achieve proportional control of the robotic finger. Tasks successfully completed included those requiring force from the digits, such as removing a spray can lid. Additionally, participants were able to perform some tasks that, for them, would normally require both hands using only one hand with the robotic finger. These findings demonstrate that a non-invasive neural interface leveraging spared motor neurons can enable intuitive, functional grasping in individuals with SCI, without requiring the adoption of new movement strategies.