Non-Negative Matrix Factorization of Simulated High Density Surface Electromyograms Reflects Both Muscle Excitation and Muscle Shortening

We analyzed muscle excitation estimation systematically by Non-negative matrix factorization (NMF) from surface electromyograms (EMG) during dynamic contractions of biceps brachii (BB) muscles. We used motor unit action potentials (MUAPs) estimated experimentally from surface EMGs during slow dynamic contractions of BB muscles in healthy young males, and convolved them by simulated motor unit firing patterns. Different uncorrelated muscle excitation and muscle shortening profiles were combined when generating the EMG signals from left and right BBs (64 channels per muscle). EMG signals were rectified, low-passed filtered and decomposed by NMF into 2, 3, 4 or 6 components. The identified NMF components demonstrated good separation of left and right BB activity, but relatively large sensitivity of NMF components to muscle shortening, especially at high levels of muscle excitation. When averaged across different numbers of identified NMF components at excitation levels ranging from 40 to 80%, the average correlation coefficient between the NMF components and muscle shortening profiles was 0.45 ± 0.15. At excitation levels between 0 and 40 % these correlations decreased to 0.15 ± 0.09. Therefore, NMF components reflect both muscle excitation and muscle shortening profiles.