  While electromyography (EMG) and magnetomyography (MMG) are both methods to
measure the electrical activity of skeletal muscles, no systematic comparison
between both signals exists. Within this work, we propose a systemic in silico
model for EMG and MMG and test the hypothesis that MMG surpasses EMG in terms
of spatial selectivity. The results show that MMG provides a slightly better
spatial selectivity than EMG when recorded directly on the muscle surface.
However, there is a remarkable difference in spatial selectivity for
non-invasive surface measurements. The spatial selectivity of the MMG
components aligned with the muscle fibres and normal to the body surface
outperforms the spatial selectivity of surface EMG. Particularly, for the MMG's
normal-to-the-surface component the influence of subcutaneous fat is minimal.
Further, for the first time, we analyse the contribution of different
structural components, i.e., muscle fibres from different motor units and the
extracellular space, to the measurable biomagnetic field. Notably, the
simulations show that the normal-to-the-surface MMG component, the contribution
from volume currents in the extracellular space and in surrounding inactive
tissues is negligible. Further, our model predicts a surprisingly high
contribution of the passive muscle fibres to the observable magnetic field.
