## 3 Experiment Protocol
### Experiment Procedure  
Eight (4 male and 4 female) healthy adult subjects without any previous history of elbow injury volunteered for this study. The mean (\(\pm\)standard deviation) age, height and mass of the subjects were 26.2 (\(\pm\)3.6) years, 170.2 (\(\pm\)9.1) cm and 65.8 (\(\pm\)13.4) kg, respectively. All subjects were right-hand dominant, and had good health during the experiment. Within 24 h before the experiment, the subjects did not carry out any vigorous exercise. All subjects had previously participated in similar motor control studies. Prior to beginning the experimental session, subjects received a written description of the project and signed informed consent forms. Subjects were then asked to perform a series of familiarization trials. Because the EEG signal needs to be acquired at the same time, the subject was restricted from head movement from preparation to exercise, avoiding swallowing and eye blinking, and performing slow eye movement and facial muscle movement.  
Proprioception is the feeling that moving organs, such as muscles, tendons, and joints, appear on their own during exercise or at rest. For example, a person can sense the position of various parts of the body when closing his eyes. Therefore, our experiment required that the subject felt the movement angle of the elbow joint with subjective feeling under the proprioceptive condition, and there was no visual feedback. The subjects were asked to close their eyes slightly and focus on the motion of the elbow joint. The experimental setup of the internal and external attention conditions required significant visual feedback. In the external focus of attention experiment, the subjects were  
Figure 2: The experimental equipment.  
asked to focus on a display screen (1280 \(\times\) 800 px) with a front distance of 1 m. The sEMG signal and joint angle waveforms, which were the effect of motion, were displayed on the screen. In the internal focus of attention experiment, the subjects were asked to visually notice their elbow joint motions. According to different psychological factors and motion directions, data from six experiments were collected, as shown in Table 1. The three psychological factors included external visual focus of attention (EVA), internal visual focus of attention (IVA), and proprioceptive no visual feedback (NVF). The two motion directions included a horizontal direction (HD) and a vertical direction (VD). In all experiments, the subject performed flexion and extension of the right elbow joint. The angle of the elbow joint was the target variable. All experimental exercise times were 5 s. According to the instructions of the experimenter, the subjects performed flexion and extension of the elbow joint as evenly as possible. Subjects were asked to move their elbow from 0\({}^{\circ}\) to 90\({}^{\circ}\) and then return to the initial position, as shown in Figure 3. In the horizontal direction experiment, the subjects performed elbow rotation while maintaining the shoulder joint at 90\({}^{\circ}\). The starting point of motion was the horizontal extension of the subject's arm. In the vertical direction experiment, the initial posture was that the upper arm was perpendicular to the ground. A minimum of 2 min of rest was given between each motion in order to mitigate the accumulation of fatigue. Several non-standard questions were placed in each experiment to ensure the validity of the experiment. The questions centered around the attention of the subject and the performance of the sensory variables. After confirming that the subjects were not affected by other non-predetermined variables, we marked five valid data points for subsequent processing in each subject's effective experiment.