Abstract Objective: Investigation 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). Methods: Simultaneous 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. Results: MMG derived MUNE (mMUNE) could be assessed contactlessly. mMUNE estimates were on average 40% lower than EMG-derived MUNE (eMUNE), ranging from 30 to 65 for mMUNE versus 69 to101 for eMUNE. Equalizing the EMG SNR (29 to 31 dB) to match the MMG SNR (18 to 27 dB) yielded comparable eMUNE and mMUNE estimates. Peak-to-peak amplitudes of the supramaximal compound motor unit fields ranged from 34 to 73 pT and single motor unit fields ranged from 0.7 to1.4 pT. Significance: These findings demonstrate that OPM MMG enables contactless motor unit number estimation.
Senay, B., Noury, N., Siegel, M., Röhrle, O., Klotz, T., Marquetand, J.
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