Quantitative and geometric motor unit analysis using magnetomyography

ABSTRACT Objective Magnetomyography (MMG) is currently a rather unexplored neurophysiological modality and it is not known to which extent the number of motor units have an influence on the amplitude and the direction of the MMG-signal. Methods A simultaneous invasive electromyography (iEMG), surface EMG (sEMG) and MMG using optically pumped magnetometer (OPM-MMG) of the right abductor digiti minimi muscle (ADM) of two healthy participants was recorded during a stepwise increasing electrical stimulation of the ADM innervating ulnar nerve. Then, the number of electrically evoked motor units was estimated (MUNE), the magnetic field vectors were reconstructed and aligned to the muscular anatomy. In addition, a finite element simulation of the ADM muscle was performed and compared to the experimental data. Results The more motor units were activated by increasing electrical stimulation, the stronger the MMG signal became, which was the same for iEMG&sEMG (r>0.96). The finite element simulation showed the same relation between the magnetic and electric signal. Further, based on the simulation the number of activated muscular fibers and neuromuscular units could be estimated the ratio of signal to fibers determined. In addition, the precise vector direction of the magnetomyography (MMG) signal can reliably be recorded following the electric stimulation of the ulnar nerve and followed the muscle fiber direction. Conclusion The MMG signal can be used to determine the amount of activated motor units, but also analysis of the magnetic field vector corresponds to the muscle fiber direction, offering a functional as well as structural characterization of muscles. The modelling and simulation is especially helpful to understand the magnetic muscular signal in detail. Significance Next to establishing MUNE in MMG, our results provide the first quantitative comparison between MMG vs. iEMG&sEMG and highlight the possibilities of the vector component analysis in MMG. Highlights - Comparative study of MMG, iEMG&sEMG using electrically induced activation of motor units. - MUNE in MMG is possible and is potentially superior to surface EMG. - The vector components of the MMG-signal correspond to the muscle fiber direction of the muscle. - Finite element simulation of the muscular magnetic and electric signal