Muscle fatigue revisited - Insights from optically pumped magnetometers

Muscle fatigue is well characterized electromyographically, nevertheless only information about summed potential differences is detectable. In contrast, recently developed quantum sensors optically pumped magnetometers (OPMs) offer the advantage of recording both the electrical current propagation in the muscle as well as its geometry, by measuring the magnetic field generated by the muscular action potentials. Magnetomyographic investigation of muscle fatigue is still lacking and it is an open question whether fatigue is characterized similarly in magnetomyography (MMG) compared to electromyography (EMG). Herein, we investigated the muscle fatigue during a 3×1-min strong isometric contraction of the rectus femoris muscle of 12 healthy subjects using simultaneous EMG-MMG (4-channel surface EMG and 4 OPM along the rectus femoris muscle). Both EMG and MMG showed the characteristic frequency decrease in the signal magnitude during isometric contraction, which is typical for muscle fatigue. In addition, it was shown that the main part of this frequency decrease seems to occur in the circular component of the magnetic field around the muscle fibers and less longitudinally along the muscle fibers. Overall, these results show not only that magnetomyography is capable of reproducing the electromyographic standards in identifying muscular fatigue, but it also adds relevant information about the spatial characterization of the signal. Therefore, OPM-MMG offers new insights for the study of muscular activity and might serve as a new, supplementary neurophysiological method.