Detecting single motor-unit activity in magnetomyography

Objective.The study of motor unit (MU) discharge patterns is critical for understanding the mechanisms underlying human motor behavior. Intramuscular electromyography (iEMG) allows direct study of MU activity but is invasive. Surface electromyography offers a non-invasive alternative, but with lower spatial resolution. Recent advances in optically pumped magnetometers (OPMs) have sparked interest in the magnetic counterpart of EMG, magnetomyography (MMG), as an additional non-contact modality to study the neuromuscular system. However, it remains unclear whether MMG signals recorded with superconducting quantum interference devices (SQUIDs) or OPMs can be used to directly detect individual MUs.Approach.We addressed this question in a proof-of-principle study in which we recorded MMG signals from the abductor digiti minimi muscle using SQUIDs and OPMs. Critically, we simultaneously recorded iEMG from the same muscle as the ground truth to cross-validate the findings from the non-invasive measurements.Main results.We found that invasively recorded MUs can be detected in simultaneously recorded SQUID and OPM MMG signals. Furthermore, we found that individual MUs can be extracted directly from SQUID and OPM MMG and, importantly, validated this finding using the simultaneous iEMG recordings. These results provide converging evidence that individual MU activity is accessible using non-contact MMG.Significance.We demonstrate for the first time that individual MU activity is observable in MMG, paving the way for future research on MU decomposition using MMG. Our findings highlight the potential of MMG as a non-contact modality to study muscle activity in health and disease.