Training improves direct motor unit control by an individual with spinal cord injury

Spinal cord injury (SCI) has no cure and has left millions of people worldwide with paralysis. Recent studies have shown that despite paralysis, residual muscle activity can often be detected while performing intended movements. This provides an opportunity to identify residual motor unit (MU) action potentials and use them as control signals. In this case study, we explored whether a tetraplegic individual with cervical SCI could improve their ability to control the activity of individual motor units during a four-week, five-day experimental study. Each day, we recorded residual muscle electromyography (EMG) while the participant attempted a selected finger movement. From these recordings, we extracted three control signals (classic EMG amplitude, the discharge rate of a single MU, and the cumulative spike train (CST) of all identified MUs). The participant used sequentially these control signals to track three target profiles (trapezoidal, sinusoidal, and pseudo-sinusoidal) in a biofeedback paradigm. The results showed that a number of spared motor units could be identified and used for proportional control of cursor position. Compared to classic EMG amplitude control, superior performance was achieved with the MU and CST control signals. Moreover, we noted improvement in MU and CST control performance during several days of training, which was not apparent for EMG amplitude control. Our results suggest that MU-based control has potential for neural interfacing in individuals with SCI.