Multi-channel intramuscular and surface EMG decomposition by convolutive blind source separation

TLDR

Motor unit behavior is traditionally studied by recording EMG signals and using decomposition algorithms to separate individual motor unit action potentials from multi‑unit recordings. This study introduces a general framework for decomposing multi‑channel intramuscular and surface EMG signals and validates it experimentally. The framework uses convolutive blind source separation with sphering and iterative source extraction, tested on intramuscular recordings from thin‑film electrodes on the abductor digiti minimi and tibialis anterior and on high‑density surface EMG grids on the first dorsal interosseous, with validation against manual decomposition and a two‑source method up to 90 % MVC. The method identified an average of 14 ± 7 common sources with 92.8 ± 3.2 % discharge‑timing agreement, and a decomposability index of 16.0 ± 2.2 versus 15.0 ± 3.0 for manual decomposition, demonstrating reliable decomposition of multi‑channel EMG for large motor‑unit populations.

Abstract

The study of motor unit behavior has been classically performed by selective recording systems of muscle electrical activity (EMG signals) and decomposition algorithms able to discriminate between individual motor unit action potentials from multi-unit signals. In this study, we provide a general framework for the decomposition of multi-channel intramuscular and surface EMG signals and we extensively validate this approach with experimental recordings.First, we describe the conditions under which the assumptions of the convolutive blind separation model are satisfied. Second, we propose an approach of convolutive sphering of the observations followed by an iterative extraction of the sources. This approach is then validated using intramuscular signals recorded by novel multi-channel thin-film electrodes on the Abductor Digiti Minimi of the hand and Tibilias Anterior muscles, as well as on high-density surface EMG signals recorded by electrode grids on the First Dorsal Interosseous muscle. The validation was based on the comparison with the gold standard of manual decomposition (for intramuscular recordings) and on the two-source method (for comparison of intramuscular and surface EMG recordings) for the three human muscles and contraction forces of up to 90% MVC.The average number of common sources identified for the validation was 14 ± 7 (averaged across all trials and subjects and all comparisons), with a rate of agreement in their discharge timings of 92.8 ± 3.2%. The average Decomposability Index, calculated on the automatic decomposed signals, was 16.0 ± 2.2 (7.3-44.1). For comparison, the same index calculated on the manual decomposed signals was 15.0 ± 3.0 (6.3-76.6).These results show that the method provides a solid framework for the decomposition of multi-channel invasive and non-invasive EMG signals that allows the study of the behavior of a large number of concurrently active motor units.