## 4 Discussion  
In 274 trials (91.3% of total trials), the volunteers could adjust the pinch force in proportion to the changes in pulling force. However, the remaining trials were excluded prior to analysis due to improper adjustment of pinch force to the pulling force. This is considered as human errors such that \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variables** & **Trial 1** & **Trial 2** & **Trial 3** & **Average** \\ \hline Pinch force (N) & 14.61 & 17.81 & 15.39 & 15.93 \\ Pulling force (N) & 7.29 & 8.61 & 7.05 & 7.65 \\ Static COF & 0.498 & 0.483 & 0.458 & 0.48 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measurements recorded from the right hand of a volunteer. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variable** & **Mean** & **Standard Deviation** & **Range** \\ \hline Pinch force (N) & Right hand & 12.17 & 3.02 & 5.36–18.48 \\ Left hand & 11.67 & 2.82 & 6.79–17.67 \\ Pulling force (N) & Right hand & 6.25 & 2.19 & 2.37–10.77 \\ Left hand & 5.92 & 1.86 & 1.88–10.67 \\ Static COF & Right hand & 0.518 & 0.146 & 0.27–0.85 \\ Left hand & 0.517 & 0.145 & 0.23–0.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of pinch force, pulling force, and static COF values at slip onset for 50 volunteers. Figure 13: Distribution of: (**a**) pinch force; (**b**) pulling force; and (**c**) static COF for 50 volunteers. the volunteers might not follow the experiment's procedures and instructions probably due to delay in reaction, missing the instructions, or absence of visual feedback. According to the UE-FMA protocol, the subject should resist a gentle pull to avoid slip. This means that the pincer object slips when the subject is no longer able to exert enough pinch force. In this study, it was initially hypothesized that the slip onset was expected at the maximum pinch force just before the point where the pincer object totally slipped. Interestingly, it was found that the slip onset occurred much earlier and was subjected to a sub-maximal pinch force. This result opens up the possibility of using the slip onset as a measurement of pinch improvement such that the patient would develop a stronger pinch force over rehabilitation time at the slip onset. Furthermore, the continuous changes of slip onset would improve the responsiveness of the pinch assessment such that the scoring is not subjected only to score 1 and 2 as in the current UE-FMA scoring criteria. The pinch-force relationship was found to be linear prior to the slip onset as the static COF between human skin and copper alloy was constant. However, there was some variability in pinch and pulling force, as indicated by peaks approximately at 7.5, 12, and 14.5 N, as depicted in Figure 12. The highest variability was observed at 7.5 N pinch force at the loading phase in which the linear electric actuator started to pull. This is consistent with previous studies stated that the subjects generally show stronger pinch forces at the loading phase [59]. Besides, the pinch force rate (N/s) reached the highest at the loading phase when the volunteer was subjected to unpredictable loading [27]. The study conducted by [PERSON] et al. [60] showed that pinch force variability is lower when visual feedback is provided to the subject. In addition, the pinch force control is greatly influenced by the safety margin factor during the unpredictable loading [61]. The safety margin is an additional amount of pinch force applied to guard against slip, which is different from volunteer to volunteer [62]. The other two peaks at 12 and 14.5 N can be interpreted such that the volunteer should adapt his pinch force against the continuous automatic pull without visual feedback; hence, the safety margin is continuously changed to adapt the continuous increase in pulling force. Therefore, it is possible that the volunteer may unconsciously fail to adapt his pinch force to the increase in pulling force at some events.