Introduction: The EMG signal is the electrical manifestation of motor unit (MU) recruitment processes underlying the contractile dynamics of muscle fibers. The analysis methodology frequently carried out includes a preprocessing stage based on artifact removal and stationarity testing, as well as a feature extraction and interpretation stage. Generally, stationarity criteria are difficult to meet when EMG signals are evoked by momentary activations (bursting activity). Thus, the study and/or characterization of contractile patterns evoked in free-moving protocols require particular treatments.
Methods: Here, we propose a new approach for quantitatively measuring stationarity using the mean, variance, and autocovariance test (MVA test) and Permutation Entropy for measuring the uncertainty degree. This methodology was applied to EMG signals obtained from a Parkinson's disease (PD) lesion model to longitudinally study the muscle function alterations.
Results and Discussion: The MVA test was compared with the classic Reverse Arrangement test (RA-test). The RA test indicated that EMG signals become more stationary over post-injury time. However, the MVA test revealed that the temporal structure of EMG around the maximum recruitment zone of motor units presents incremental non-stationary characteristics (in variance and autocovariance) over post-injury time. Likewise, it was observed that the initial phase of motor recruitment in the biceps femoris (BF) muscle (around the onset) presents a high non-stationary component, which increases over post-injury time. Permutation entropy measures throughout the contractile dynamics of the BF muscle revealed that the uncertainty degree decreases in the initial phase of contraction as the animal's post-injury time increases.
Conclusions: The analysis proposed allowed for a longitudinal characterization of muscle function alterations in an animal model of PD in terms of the stationarity properties of EMG signals. Furthermore, it was observed that permutation entropy could serve as a robust biomarker for quantifying neuromuscular remodeling caused by PD progression.
