Motor Unit Number Estimation Using Magnetomyography
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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 30–65 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.
Significance
These findings demonstrate that OPM-MMG enables contactless motor unit number estimation.
