## 3 Results
### sEMG
#### 3.2.2 Mean Power Frequency (MPF)  
Utilizing fast Fourier transform, the raw electromyographic signals acquired by the electromyograph were transformed into frequency domain signals. Mean Power Frequency (MPF) was utilized as the primary analysis parameter, with smaller MPF values indicating higher levels of muscle fatigue. Throughout data processing, it was noted that alterations in Mean Power Frequency were relatively minor, posing challenges in accurately comparing parameter variations from the curves. Therefore, mean values were selected for comparative analysis. Detailed parameter comparisons for each muscle are provided in Table 2 (The formula is shown in Appendix A).  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Mean Muscle MPF under**} & \multicolumn{3}{c}{**Mean Muscle MPF under the**} & \multicolumn{3}{c}{**Mean Muscle MPF under**} \\ \multicolumn{3}{c}{**Name of Muscle**} & \multicolumn{3}{c}{**Computer Tasks**} & \multicolumn{3}{c}{**Writing Task**} & \multicolumn{3}{c}{**Ironing Tasks**} \\ \cline{2-10}  & \(\mathbf{0}^{\circ}\) & \(\mathbf{20}^{\circ}\) & \(\mathbf{40}^{\circ}\) & \(\mathbf{0}^{\circ}\) & \(\mathbf{20}^{\circ}\) & \(\mathbf{40}^{\circ}\) & \(\mathbf{0}^{\circ}\) & \(\mathbf{20}^{\circ}\) & \(\mathbf{40}^{\circ}\) \\ \hline \hline \multicolumn{10}{c}{} \\ \hline \hline \multicolumn{10}{c}{} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Changes in the mean value of MPF of each muscle under the three tilt angles of the chair surface (Hz).  
Table 2 illustrates that during computer tasks, the Mean Power Frequency (MPF) values of the left erector spinae muscle are highest at a seat incline angle of 20\({}^{\circ}\) and lowest at 0\({}^{\circ}\). Conversely, the MPF values of the right erector spinae muscle peak at a seat incline angle of 40\({}^{\circ}\) and hit the lowest point at 0\({}^{\circ}\). The MPF values of the left vastus lateralis muscle reach their peak at a seat incline angle of 20\({}^{\circ}\) and bottom out at 0\({}^{\circ}\). Conversely, for the right vastus lateralis muscle, there is little disparity between seat incline angles of 0\({}^{\circ}\) and 20\({}^{\circ}\), both of which are higher than the MPF value at 40\({}^{\circ}\). The MPF values of the left gastrocnemius muscle peak at a seat incline angle of 20\({}^{\circ}\) but show no significant difference compared with those at 0\({}^{\circ}\). Conversely, the MPF values of the right gastrocnemius muscle reach their peak at a seat incline angle of 20\({}^{\circ}\) and hit their lowest point at 40\({}^{\circ}\).  
During writing tasks, the MPF values of the left erector spinae muscle peak at a seat incline angle of 40\({}^{\circ}\) and reach their lowest point at 0\({}^{\circ}\), while for the right erector spinae muscle, they peak at a seat incline angle of 20\({}^{\circ}\) and bottom out at 40\({}^{\circ}\). The MPF values of the left vastus lateralis muscle peak at a seat incline angle of 20\({}^{\circ}\) and are lowest at 40\({}^{\circ}\), whereas those of the right vastus lateralis muscle peak at a seat incline angle of 40\({}^{\circ}\) and are relatively lower at 0\({}^{\circ}\) and 20\({}^{\circ}\). The MPF value of the left gastrocnemius muscle is highest at a seat incline angle of 20\({}^{\circ}\) and relatively lower at 0\({}^{\circ}\). Regarding the right gastrocnemius muscle, the MPF value is highest at a seat incline angle of 40\({}^{\circ}\), with a smaller difference compared with 20\({}^{\circ}\). During the ironing task, the MPF values of the six selected muscles are all higher at a seat incline angle of 0\({}^{\circ}\) compared with incline angles of 20\({}^{\circ}\) and 40\({}^{\circ}\).  
In light of the diverse Integrated Electromyographic (IEMG) signal fluctuations and the comparison of Mean Power Frequency (MPF) mean values across various seat incline angles, we derived favorable seating postures for each muscle across different task scenarios. The more advantageous angle values after comparing the seat tilt angles are listed in Table 3.