## 4 Discussion  
Many researchers have endeavoured to satisfy the assumptions of stationarity in EMG signals in order to adopt TD features in pattern recognition. Stationarity tests were conducted to evaluate the data of EMG signals for various types of contractions. According to [PERSON] et al., the EMG signals recorded from upper limb during ramp and step isometric contractions are relatively stable, as there were no identifiable trends in the mean for 16 segments [21]. The researchers found that EMG signals were stationary in most cases--92.2% within 512 ms window size. As mentioned, the assessment of stationarity depends on the window size. [PERSON] and [PERSON] showed that the stationarity level of the EMG signals increase when the window size increases with a relatively low stationarity with window sizes of 250 and 500 ms in the RA test in their study [33]. Contrary to [PERSON] et al. [24], the stationarity increased only from 128 ms to 375 ms window size with an average 94.29%. Even though the peak of the EMG stationarity level was found at the 512-samples window depending on one subject, on average, there existed a small decrement and increment between 512 ms and 1024 ms window size. Meanwhile, the stationarity of EMG was at the highest with 250 ms window size and lowest at 1000 ms window size in the MRA test compared to 500 and 2000 ms [25]. Nonetheless, most of the available studies concentrated on isometric contraction. During isotonic contraction, the EMG signal's stationarity can be affected by the location of an electrode, changes in muscle length, and muscle forces. It was reported that a suitable window size for isotonic contractions lies within the 125 ms window size range with approximately 90% stationarity [26]. However, if the threshold was set at 80% stationarity, a suitable window size was determined to be 250 ms or lesser. As a result, EMG signals in this study for the first sub-segment were stationary within 100 ms and 200 ms window sizes compared to 500 and 1000 ms, as the curve of the measured value (\(A\) laid between the upper and lower boundaries based on both RA and MRA tests). This proved that approximately 87-89% of EMG signals were stationary based on both tests. However, the highest percentage on average was gained by a 200 ms window size for both tests. It can be concluded that 200 ms is suitable to analyse EMG signals. The comparison of suitable window size with previous literature on the different types of contractions and stationary tests is depicted in Table 1. Referring to Table 1, the performances of both RA and MRA in signal stationarity are still in the midst of an investigation. Therefore, this study compared the performances of RA and MRA tests for isotonic contraction. Based on Figure 6, there was little difference in 100 window size for both tests, but the same cannot be said for 500 and 1000 ms. There was a small decrement of percentage (0.35%) in MRA test for 100 ms, but a higher increment in the range of 1.8-3.6% was observed for window sizes 500 and 1000 ms. Contrary to the 200 ms window size, the percentages remained the same for both tests. Thus, it can be concluded that the MRA test showed better performance than the RA test, as stated by [PERSON] et al. [26]. Though the validity of the statement was true for 500 and 1000 ms window sizes, the same cannot be concluded for 100 ms window size. The findings of this study affirmed the results reported by [PERSON] and [PERSON] [34] that the performance of TD features in detecting the stages of contraction for upper limb movement is comparable to the classification methods of time-frequency features in isometric contraction [11]. The results of this study are also in line with the research done by [PERSON] et al., as the analysis of TD features within 200 ms window size was the most appropriate technique for classifying the four ankle joint movements in comparison to 50, 100, 150, and 250 ms, with classification accuracy of 97.32% [35].