Effect of Time and Frequency Windows in Analyzing the Muscle Typology of Adductor Pollicis and Triceps Brachii Muscles Using Surface EMG Signals and Smoothed Pseudo-Wigner-Ville Distribution

The selection of time and frequency windows is having a significant impact on the time-frequency (TF) resolution and interference suppression of smoothed pseudo-Wigner-Ville distribution (SPWVD). In this study, the effect of time and frequency windows in demonstrating the fiber type differences among adductor pollicis (AP) and triceps brachii (TB) muscles is investigated using surface electromyography signals and SPWVD-derived features. The first and last two seconds of the pre-processed signals are considered non-fatigue (NF) and fatigue (FT) segments, respectively. The median frequency (MDF) and mean power of the TF spectrum (MPS) are extracted from the obtained spectra. The statistical significance of SPWVD-derived features in differentiating the signals of AP and TB during NF and FT states are evaluated for different time and frequency windows, using the Mann-Whitney test. Results indicate that the frequency-based feature showed greater statistical significance compared to the energy-based feature for all the different time and frequency window lengths taken. MDF is found to decrease for both muscles, whereas MPS increases for TB and decreases in the case of AP. The highest significant value of 5.39E-27 is observed for MDF with time and frequency window length of 201 and 2001, respectively, for differentiating the signals during the fatigue state. Moreover, all the extracted features showed high statistical significance with p<0.001 in discriminating the signals irrespective of the various window lengths considered in this study. This reflects the effectiveness of SPWVD-derived features in highlighting the fiber type variations in muscles. The proposed study can be used to investigate the fiber type proportion in various skeletal muscles under different myoneural conditions.