Real-time distance monitoring in magnetomyography

<i>Objective.</i>Magnetomyography (MMG) using optically pumped magnetometers (OPM) offers a contactless, non-invasive approach to assess muscle activity. However, fluctuations in the sensor-to-source distance during MMG recordings pose a significant challenge to accurate signal interpretation since amplitude decays with distance. No established method exists for MMG to continuously monitor sensor-to-source distance changes in real-time.<i>Approach.</i>This study presents a new non-magnetic, cost-effective solution using a digital fiber optic sensor to continuously measure the distance between an OPM and the subject's skin. Following sensor calibration, distance measurements were recorded during an isometric muscle fatigue task in five healthy participants to assess whether MMG amplitude changes were due to physiological effects or variations in sensor-to-source distance. Alongside OPM-MMG and distance tracking, electromyography (EMG), the neurophysiological gold standard, was simultaneously recorded.<i>Main results.</i>We found significant changes in MMG-RMS and MMG-MDF during muscle fatigue that were not merely explained by changes in sensor-to-source distance. Furthermore, we found substantial correlations between OPM-MMG and EMG that were strongest for small sensor-to-source distance (<i>r</i>= 0.91).<i>Significance.</i>Fiber optic sensors offer non-magnetic, precise, real-time monitoring of the distance between the OPM and the skin, making it ideal for MMG applications to account for distance-related variability during measurements. Our results suggest that changes in MMG-RMS and MMG-MDF during muscle fatigue reflect genuine physiological effects rather than distance confounds.