## 2 Materials and Methods
### Volunteers Recruitment and Experimental Protocol  
Fifty right-handed healthy male students participated. All volunteers had no prior history of upper limb injury and passed the health status questionnaire (SF-36). To avoid bias in the force measurements, the study was conducted on a single-gender because males and females have different pinch strength [56]. Furthermore, the study excluded construction workers and professional sportsmen as they have stronger pinch force than the average male population [57]. The mean age, height, and weight for volunteers were 21.86 \(\pm\) 1.41 years, 170.41 \(\pm\) 7.03 cm, and 69.49 \(\pm\) 17.9 kg, respectively. Ethical approval (Reference number: JKEUPM-2017-248) for this study was obtained from The Ethics Committee for Research Involving Human Subjects of Universiti Putra Malaysia.  
In this study, the improved wrist/hand UE-FMA protocol by [PERSON] et al. [24] was adopted. The volunteers were instructed to sit at 90\({}^{\circ}\) on a chair, shoulder in the neutral position, elbow at 90\({}^{\circ}\)  
Figure 8: Displacement–voltage relationships of three times calibration.  
Figure 9: Block diagram of the data acquisition process.  
and forearm in resting position. A bedside table was positioned to support the volunteer's arm and elbow, as shown in Figure 10a. The volunteers were also instructed to clean and dry their hands before commencing the experiment to avoid any influence of moisture through sweat. Then, they were instructed to hold the pincer object comfortably using their thumb and index fingertips, as in Figure 10b. Once the linear actuator started to pull the pincer object, the volunteers attempted to resist this pulling as much as possible. Since no visual feedback was provided, the volunteer had to react and adjust accordingly to the perceived amount of pulling force. To achieve pinch stability, the pinch force had to be adjusted in proportion to the changes in the pulling force. The linear actuator stopped exerting pulling force the moment the pincer object had totally slipped away from the volunteer's fingers, and this was indicated by a 20-mm displacement slip. Figure 2 (Appendix A) demonstrates the flowchart to execute the experiment. The data collection involved both right and left hands. The duration of one trial began when the linear actuator received the control signal from the Arduino(r) Due, and ended the instant the pincer object slipped totally away. The analysis of the recorded data showed the pinch and pulling force values at the slip onset. The slip onset was defined as the moment when the pincer object started to shift with respect to the starting position, indicating that the object slipped away from the volunteer's fingers. During the trial, one set of measurements, which included the pinch force, pulling force, and displacement, were recorded, resulting in 300 datasets (50 volunteers \(\times\) 2 hands \(\times\) 3 trials per hand). To reduce the effect of the physical and mental fatigue, an adequate resting period of 3 min was given between each trial. [PERSON]'s alpha (\(\alpha\)) was used to measure the reliability among the three trials of the hand, which as computed using the following formula [58]:  
\[\alpha\;=\;\frac{N.\overline{c}}{v+(N-1).\overline{c}} \tag{3}\]  
where \(N\) is the number of trials, \(\overline{c}\) is the average inter-item covariance among the trials, and \(v\) is the average variance.