Reliability of Spatiotemporal Neuromuscular Activity Patterns in Magnetomyography Across Force Levels

BackgroundMagnetomyography (MMG) using an optically pumped magnetometer (OPM) provides a contactless and non-invasive approach to assess neuromuscular activity. However, given the limited studies using optically pumped magnetometer magnetomyography (OPM-MMG) and that those primarily used a single OPM, it remains to be characterized how stable individual neuromuscular activity patterns are over time and across different force levels when using multiple OPMs, specifically an array-based MMG. Methods12 healthy subjects performed ramped isometric contractions at 10, 20, 40, 60, and 80% maximal voluntary contraction (MVC), while a 2x3 OPM array recorded MMG signals from the tibialis anterior (TA). For the test-retest reliability, ramps at 20% and 60% MVC were each repeated once. For each subject, Spearmans rank correlation was computed across all OPM sensors between force conditions to assess consistency of feature rankings, and within-subject spatial repeatability between repeated 20% and 60% MVC ramps was quantified using ICC (3,1) (two-way mixed-effects, single-measure, absolute agreement). ResultsIn 9 of 12 subjects, Spearmans rank analyses showed generally high correlations across force levels ({rho} = 0.37 to 0.91, p<0.05), whereas in the remaining 3 subjects, spatial patterns were less stable ({rho} = -0.73 to 0.58, p<0.05). ICC (3,1) between repeated ramps indicated high within-subject spatial repeatability of the array pattern in 9 out of 12 subjects, with ICC > 0.75 at 20% MVC (5 out of 9 subjects) and 60% MVC (6 out of 9 subjects), respectively. In contrast, the remaining 3 subjects showed lower ICCs (20% MVC: -0.28 to 0.51; 60% MVC: 0.22 to 0.80). ConclusionArray-based OPM-MMG shows high within-subject stability of spatial patterns across force levels and strong test-retest repeatability in the majority of subjects (9 out of 12), supporting its use for characterizing force-dependent neuromuscular activity while acknowledging inter-subject variability.