High-density magnetomyography with optical magnetometers reveals spatiotemporal patterns of evoked muscle activity

Magnetomyography (MMG) holds great potential as a contactless alternative to electromyography (EMG). However, the spatiotemporal dynamics of the myomagnetic field in vivo remain largely unknown, limiting signal interpretability and hence the use of MMG. Here, the myomagnetic field of the abductor digiti minimi muscle is measured in vivo at 69-by-2 effective positions by scanning a grid of six triaxial optically pumped magnetometers (OPMs) along the muscle, while reproducible muscle contractions are evoked electrically. In this way, the compound muscle action field (CMAF) is measured with high density (1 mm sampling steps) and its evolution is referenced to the underlying anatomy based on 3D scans and magnetic resonance images. A model is used to simulate the expected signals. It suggests that the clinically relevant innervation zone (IZ) manifests as a zero-crossing of the azimuthal CMAF component. Based on this, contactless IZ localization is demonstrated in three subjects by projecting the zero-crossing onto the skin. The estimated IZ localizations deviate by 1.7 mm to 4.1 mm from estimates with simultaneously recorded 64-channel surface EMG. With the first millimeter-scale spatiotemporal mapping of magnetic muscle activity in vivo, this study paves the way for the use of MMG in biomedical applications.