Intuitive, multidimensional motor unit control after paralysing spinal cord injury using non-invasive recordings

Spinal cord injury (SCI) results in profound motor impairment for approximately 20 million people worldwide. Regaining hand use is one of their highest priorities. Interestingly, even severely affected individuals can still generate some level of muscle activity (EMG) in their paralysed muscles when attempting to perform a movement. Here we asked whether participants with no hand function due to a cervical SCI could still use their spared inputs to volitionally control the activity of motor units from paralysed muscles, which we detected using high-density surface EMG recordings. Participants could readily control those motor units to accurately perform three different one-dimensional tracking tasks that imposed different control requirements, with their performance plateauing after a few days of practice. Moreover, this accurate control readily transferred to a two-dimensional cursor navigation task and allowed participants to play a series of one-dimensional ("Pong"), and two-dimensional ("Snake") games, as well as navigate a virtual wheelchair using three independent control signals. This robust performance was achieved despite the presence of spasticity, and even in one participant who had a motor complete injury. These findings demonstrate the promise of non-invasive motor unit recordings for restoring hand function, as well as a potential tool for neurorehabilitation after paralysis.