Motor Unit Number Estimation in Magnetomyography

ObjectiveInvestigation of the feasibility and characteristics of contactless motor unit number estimation (MUNE) using optically pumped magnetometer-based magnetomyography (OPM-MMG) as compared to surface electromyography (EMG). MethodsSimultaneous electrically evoked OPM-MMG and EMG signals of the abductor digiti minimi muscle (ADM) were measured in three healthy participants. To characterize MUNE across both modalities and account for within-subject physiological variability, 20 repetitions of electrical stimulation of the ulnar nerve at randomized intensities ranging from 5 to 30 mA in 0.1 mA increments were performed, resulting in a total of 5,020 evoked responses per subject. We quantitatively compared of MUNE and evoked EMG/MMG signal characteristics, including signal-to-noise ratio (SNR) and motor unit response amplitudes. SNR was equalized between measurement modalities to estimate the effect of SNR on MUNE. ResultsMMG-derived MUNE (mMUNE) could be assessed contactlessly. mMUNE estimates were on average 40% lower than EMG-derived MUNE (eMUNE), ranging from 30-65 for mMUNE versus 69-101 for eMUNE. Equalizing the EMG SNR (29-31 dB) to match the MMG SNR (18-27 dB) yielded comparable eMUNE and mMUNE estimates. Peak-to-peak amplitudes of the supramaximal compound motor unit fields ranged from 34-73 pT and single motor unit fields ranged from 0.7-1.4 pT. SignificanceThese findings demonstrate that OPM-MMG enables contactless motor unit number estimation.