Excitability tests using high-density surface-EMG : A novel approach to studying single motor units

Abstract Objective To study excitability of single motor units (MUs) using high-density surface-EMG. Methods Motor unit action potentials (MUAPs) were evoked by submaximal stimulation of the median nerve at the wrist and recorded with a 9 × 14 electrode grid on the skin overlying the thenar muscles. For excitability tests of single MUs, the most optimal specific single-channel surface-EMG signal was selected based on the spatiotemporal profile of single MUs. Results Axonal excitability measures were successfully obtained from 14 single MUs derived from ten healthy subjects. Selecting the optimal single-channel surface-EMG signals minimized interference from other single MUs and improved signal-to-noise ratio. The muscle fiber conduction velocity (MFCV) could also be derived from the unique spatiotemporal profile of single MUs. Conclusion High-density surface-EMG helps to isolate single MUAP responses, making it a suitable technique for assessing excitability in multiple single motor axons per nerve. Significance Our method enables the reliable study of ion-channel dysfunction in single motor axons of nerves without any requirement for specific conditions, such as prominent MU loss or enlarged MUAPs due to collateral sprouting.