## 4 Results
### Perceived Cylinder Lifting Difficulty  
As mentioned in the latter part of Section 3.5.2, each participant was asked to state their perceived ease and difficulty when grasping and lifting the 1000 g cylinders. As shown in Table 1, each person perceives the difficulty of lifting the heaviest cylinder differently. The first and second participants stated that two cylinders were perceived as difficult, while others mentioned only one cylinder. An interesting finding was that all participants recognized the first or second cylinder as the most difficult. The first cylinder had the largest diameter, while the fifth cylinder had the smallest diameter. On the other hand, the easiest cylinder varied between the third, fourth, and fifth. The information in the table suggests that participants primarily experienced difficulty when lifting the heaviest cylinder in either an almost entirely open or closed hand posture, as represented by the first and fifth cylinders, respectively. To fully understand the association between the perceived cylinder difficulty level and the behavior of the muscle, a comparison of muscle information at these two difficulty levels is necessary. Figure 16 shows the comparison of the average of the RMS of EMG for the first finger condition in perceived easy and difficult cylinders for each participant. The bar in green represents perceived easy cylinders, while the bar in red represents perceived difficult cylinders.  
Figure 15: Time series comparison between ground truth and estimation of RMS of EMG.  
Figure 16 shows that most participants have a higher average RMS for the perceived easy cylinder than for the difficult cylinder, which is represented by participants 1, 2, 3, 4, 8, 9, and 10. On the other hand, participants 5, 6, and 7 have a higher RMS of EMG on the perceived difficult cylinder. An interesting result occurred with participants 1 and 2, where both cylinders perceived as difficult had a smaller RMS of EMG than the cylinder perceived as easy. Some participants showed a clear gap in the power of the RMS of EMG between perceived easy and difficult cylinders, such as participants 2, 3, 4, 7, 8, and 10. The remaining participants showed a marginal gap in perceived difficulty, especially in the first participant for the first and fifth cylinders. Subsequently, an assessment of the perceived difficulty levels associated with the estimation outcomes was conducted. This assessment aimed to further analyze the model's performance with actual ground truth data. Figure 17 depicts the average estimated root mean square (RMS) of electromyography (EMG) from the random forest (RF) model under the first finger condition for each participant. Consistent with Figure 16, among the estimation results, seven participants reported a higher average RMS for the perceived easy cylinder compared to the perceived difficult one. However, only four participants exhibited a linear correlation between the ground truth and the  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Participant**} & \multicolumn{2}{c}{**Perceived Cylinder Difficulty Level**} \\ \cline{2-3}  & **Easy** & **Hard** \\ \hline
2 & 5 & 1 \& 6 \\ \hline
3 & 4 & 6 \\ \hline
4 & 5 & 6 \\ \hline
6 & 5 & 1 \\ \hline
7 & 5 & 6 \\ \hline
8 & 3 & 6 \\ \hline
9 & 4 & 1 \\ \hline
10 & 5 & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Perceived heaviest cylinder’s lifting difficulty.  
Figure 16: Average ground truth’s RMS of EMG across eight channels of the first finger condition in perceived easy and difficult cylinders.  
estimated results. The first participant demonstrated nearly identical results for both the ground truth and the estimations. On the other hand, participants 4, 5, 6, 7, and 9 exhibited an inverse relationship.