Reliability of spatiotemporal neuromuscular activity patterns in magnetomyography across force levels

Abstract Objective: Magnetomyography (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. Approach: 12 healthy subjects performed ramped isometric contractions at 10, 20, 40, 60, and 80% MVC, while a 2×3 OPM array recorded MMG signals from the tibialis anterior (TA). Test-retest repeatability was selectively validated at 20% and 60% maximal voluntary contraction (MVC), with one repeated ramp performed at each force level. For each subject, Spearman's 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). Main Results: In 9 out of 12 subjects (75.0%; exact 95% CI: 42.8-94.5%), Spearman's rank analyses showed generally high correlations across force levels (ρ = 0.37 to 0.91, p<0.05), whereas in the remaining 3 subjects (25.0%; exact 95% CI: 5.5-57.2%), spatial patterns were less stable (ρ = -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). Significance: Array-based OPM-MMG shows high within-subject stability of spatial patterns across force levels and strong test-retest repeatability (selectively validated at 20% and 60% MVC) in the majority of subjects (9 out of 12), supporting its use for characterizing force-dependent neuromuscular activity while acknowledging inter-subject variability.