Contactless measurement of muscle fiber conduction velocity - a novel approach using optically pumped magnetometers.

Muscle fiber conduction velocity (MFCV) describes the speed at which electrical activity propagates along muscle fibers and is typically assessed using invasive or surface electromyography (EMG). Because electrical currents generate magnetic fields, their propagation velocity can also be measured biomagnetically using magnetomyography (MMG), offering the advantage of a contactless approach. To test this hypothesis, we recorded MMG signals from the right biceps brachii muscle of 24 healthy subjects (12 male, 12 female) using a linear array of seven optically pumped magnetometers (OPMs). Subjects maintained their force for 30 seconds at 20%, 40%, and 60% of their maximum voluntary contraction (MVC). In 20 subjects, propagation of MMG signals was observable, enabling us to localize the innervation zone. We then estimated the MFCV for each condition by cross-correlating double-differentiated MMG signals. To validate our results, we examined whether our MFCV estimations increased with higher force levels, a well-documented characteristic of the neuromuscular system. The median MFCV increased with force significantly (p = 0.007), with median values of 3.2 m/s at 20%, 3.8 m/s at 40%, and 4.4 m/s at 60% across all 20 subjects. Given the exploratory and pioneering nature of measuring magnetic MFCV in MMG using OPMs for the first time, we have demonstrated not only that MFCV can be measured without contact but also that the localization of the innervation zone is possible. This study paves the way for further application and development of quantum sensors for contactless clinical neurophysiology. .