## 2 Methods
### Data Acquisition  
A total of 12 subjects were recruited for the experiment, including 10 able-bodied subjects and 2 transhumeral amputees. Subjects gave consent to participate in the experiment, which was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The able-bodied subjects were adults with no musculoskeletal disorders or other serious health problems and who have good physical functioning. The eligibility criteria for the amputee subjects were preservation of the major muscles of the upper arm after the cutoff, good control in moving the phantom arm as well as repeatable phantom arm movements with a minimum amplitude of greater than 20 degrees, and absence of phantom arm pain. Based the joint project of the National Key Research and Development Program of China, the experiment has been approved by the Ethics Committee of Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences (YSB-2022-Y07026). Participants provided written informed consent to participate in this study. Information on the able-bodied and amputee subjects is shown in Tables 1 and 2, respectively. Eight sEMG electrodes were arranged equidistant and parallel from each other around the upper arms of both types of subjects to ensure that the electrodes covered the major muscles of the subjects' biceps, triceps, and brachialis muscles while capturing as many valid sEMG signals as possible [23; 24]. The skin on the upper arm of the subjects was treated with hair clipping, removal of dead skin and alcohol cleansing to ensure the stable acquisition of the surface sEMG signals. The sEMG signals were acquired using a Noraxon sEMG signal acquisition device with a sampling frequency of 1500 Hz. During signal acquisition, subjects were asked to sit in front of the table and place their upper arms at the most comfortable positions on the support platform by adjusting the seat height and the tilt angle of the support platform. To prevent signal recordings from being affected by tabletop compression, the sensors located in a slot in the center of the support plate do not touch the support plate and remain suspended. Changes in arm position can also lead to significant differences in sEMG signals, affecting recognition performance. Three arm elevation positions were selected to complete sEMG acquisition \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Gender** & **Age** & **Dominant Hand** & **Upper Arm Length (cm)** \\ \hline S1 & female & 21 & right & 35 \\ S2 & male & 21 & right & 33 \\ S3 & male & 22 & right & 36 \\ S4 & female & 22 & right & 30 \\ S5 & female & 22 & right & 28 \\ S6 & male & 25 & right & 34 \\ S7 & male & 24 & right & 35 \\ S8 & female & 23 & right & 31 \\ S9 & male & 24 & right & 29 \\ S10 & male & 22 & right & 32 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Information on the able-bodied subjects. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**No.** & **Age** & **Dominant Hand** & **Amputation Side** & **Amputation Years** & **Amputation Reason** & **Amputation Length (cm)** & **Complete Upper Arm Length (cm)** \\ \hline TH1 & 41 & right & left & 9 & industrial injury & 23 cm & 31 cm \\ TH2 & 39 & right & left & 5 & industrial injury & 25 cm & 33 cm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Information on the amputee subjects. experiments, as shown in Figure 1. Within these three arm elevation positions, subjects are able to carry out most of the activities required for daily life, including the following:  
* P1: upper arm parallel to the horizontal plane;
* P2: elevated upper arm to 45\({}^{\circ}\) from the horizontal plane;
* P3: elevated upper arm to 90\({}^{\circ}\) from the horizontal plane. A video of the movements is displayed on the computer screen directly in front of the subject as a guide, and the subjects are asked to imitate the movement in the video synchronously with the guidance.