## 3 Results
### Changes in sEMG Parameters and Predictors  
Figure 8 showed the five raw and the five logarithmic parameters over the whole endurance time (T\({}_{\text{end}}\)) for the subject whose T\({}_{\text{end}}\) was almost the same as the mean of the endurance times of all subjects. The left column (a) displays the five raw parameters, and the right column (b) displays the five logarithmic ones with respect to time. The ripple period in LFB and logLFB time series in Figure 8a,b is 2.4 s which can be calculated as 50 s divided by 21 ripples, and the same as the repeated period during the isotonic contractions. Figure 6: An example of the sEMG signal during isotonic contraction. Figure 7: Endurance times in descending order. The time series of LFB, DSI, logLFB, and logDSI increased, and those of the other parameters decreased during the endurance contractions, because muscle fatigue is generally accompanied by an increased firing rate of motor unit recruitment and spectral parameters related to spectral compression during static and dynamic contractions [39]. Similar results were reported in previous studies [9; 23; 27]. The CVs of the five raw and the five logarithmic parameters are compared in Table 2 which shows that the CV of the logarithmic parameter is less than that of the raw one. These results revealed that the logarithmic transformation could reduce the large variability in the raw parameter. Figures 9 and 10 with Tables 3 and 4 show the time series of two predictors namely the area ratio and the slope in the raw and the logarithmic parameters as estimated over every period of 10% of T\({}_{\text{end}}\), respectively. In Figure 9 and Table 3, the area ratios in LFB, DSI, logLFB, and logDSI decreased linearly, and those in the others increased linearly, because  
Figure 8: The time series of sEMG parameters during the endurance time (T\({}_{\text{end}}\)) for the subject whose T\({}_{\text{end}}\) was almost the same as the mean for all measured sEMG of all subjects. The left column (**a**): the five raw parameters; the right column (**b**): the five logarithmic parameters; R2: coefficient of determination; CV: coefficient of variation. the definition varied between 0 and 1 for decreasing patterns and is negative for increasing patterns. In contrast, slopes in LFB, DSI, logLFB, and logD SI decayed exponentially, and those in the others rose exponentially as shown in Figure 10 and Table 4. One-way ANOVA was conducted on changes with respect to every period of 10% T\({}_{\text{end}}\) in each parameter. There were significant differences for the two predictors of all parameters (\(p<0.05\)). Thus, these results showed that the time series of the area ratios and the slopes of the raw and the logarithmic parameters varied independently. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline - & **LFB** & **MPF** & **H/L-FB** & **DSI** & **H/L-SM** \\ \hline Raw & 0.27 \(\pm\) 0.51 & 0.19 \(\pm\) 0.11 & 0.20 \(\pm\) 0.15 & 0.26 \(\pm\) 0.24 & 0.20 \(\pm\) 0.18 \\ \hline Logarithm & 0.12 \(\pm\) 0.04 & 0.17 \(\pm\) 0.13 & 0.12 \(\pm\) 0.05 & 0.14 \(\pm\) 0.08 & 0.17 \(\pm\) 0.09 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean and standard deviation of coefficient of variation (CV) of all parameters. Figure 9: The time series of area ratios in the sEMG raw parameters (**a**) and logarithmic ones (**b**) with respect to time as estimated over every 10% of T\({}_{\text{end}}\) (White circle: LFB and logLFB; Black circle: MPF and logMPF; White triangle: H/L-FB and logH/L-FB; Black triangle: DSI and logDSI; Black square: H/L-SM and logH/L-SM). Figure 10: The time series of slopes in the sEMG raw parameters (**a**) and logarithmic ones (**b**) with respect to time as estimated over every 10% of T\({}_{\text{end}}\) (White circle: LFB and logLFB; Black circle: MPF and logMPF; White triangle: H/L-FB and logH/L-FB; Black triangle: DSI and logDSI; Black square: H/L-SM and logH/L-SM). \begin{table}  
\end{table} 
Table 4: Slopes (mean and S.D.)