## 2 Materials and Methods
### Design of the Joint Restrictor  
To achieve the constraint condition of ab/d joints, a joint restrictor was developed, as shown in Figure 2a. It consisted of up to 4 finger rings with a thickness of 0.5 mm connected by connecting strips. There were evenly distributed small holes on the strips to adjust the distance between the rings. The size of each part of the restrictor is customized by the hand size of each subject. \(L1\), \(L2\), and \(L3\) are the distances between adjacent fingers. \(D1\), \(D2\), \(D3\), and \(D4\) are the diameters of the index finger, middle finger, ring finger, and little finger, respectively. The rings of the index finger, middle finger, and ring finger are aligned in the vertical direction while the ring of the little finger is lower than them. It is an anatomical design, and the distance between the ring of the little finger and other rings in  
Figure 1: (**a**) A skeletal joint model of the human hand; (**b**) the grasp type used: precision disk [30].  
the vertical direction is \(H\). The restrictor worn by each subject was customized according to the measurement of their hands. The hand size of each subject is shown in Table 1.  
There are three types of restrictors according to the four types of constraints of ab/d DOF (Figure 2b): The first constraint can limit the ab/d motions of all four fingers; the second constraint can limit the ab/d motions of the index finger, middle finger, and ring finger; and the third constraint can limit the ab/d motions of the index finger and middle finger. By wearing it (Figure 2c), the ab/d DOF of the corresponding finger was locked, but all of the fingers could still flex to grasp. It should be noted that the restrictor also limits the separate bending of the fingers, leading to a coupled bending motion. Since the grasp type used in this study is _precision disk_, which does not require the fingers to bend alone, the coupling of bending is acceptable. In addition, during the experiments, the objects can still produce a small range of abduction movements of no more than 5 degrees due to material deformation after wearing the restrictor. It is small compared with normal abduction, which is acceptable. The restrictor was made of 3D printing resin material with a very light weight, and the heaviest one was 19 g.  
\begin{table}
\begin{tabular}{l c c c c c c c c c c c} \hline \hline  & **Age** & **Gender** & **L1** & **L2** & **L3** & **D1** & **D2** & **D3** & **D4** & **H** & \
\begin{tabular}{c} **Hand** \\ **Length** \\ \end{tabular} &
\begin{tabular}{c} **Hand** \\ **Strength** \\ \end{tabular} \\ \hline
**Subject 1 (S1)** & 24 & M & 22.3 & 20.4 & 20.1 & 19.2 & 18.3 & 18.3 & 17.2 & 8.2 & 188 & 48.33 \\
**Subject 2 (S2)** & 26 & M & 20.8 & 20.6 & 20.9 & 19.1 & 18.4 & 18.4 & 16.5 & 7.4 & 187 & 65.42 \\
**Subject 3 (S3)** & 21 & M & 21.5 & 20.0 & 20.5 & 17.5 & 17.9 & 17.2 & 15.7 & 8.2 & 170 & 73.28 \\
**Subject 4 (S4)** & 29 & M & 22.5 & 21.4 & 20.0 & 19.3 & 19.4 & 18.9 & 18.0 & 9.8 & 162 & 62.0 \\
**Subject 5 (S5)** & 30 & F & 20.0 & 19.6 & 19.5 & 17.8 & 17.6 & 16.4 & 14.2 & 5.6 & 156 & 43.8 \\
**Subject 6 (S6)** & 38 & F & 22.5 & 20.5 & 19.3 & 17.8 & 17.5 & 16.5 & 15.2 & 6.4 & 164 & 61.63 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters of the hands of the subjects.  
Figure 2: The ab/d DOFs restrictor. (**a**) Parameter definition, (**b**) three types of restrictors, (**c**) a human hand wearing the restrictor.