## 3 Results  
### Subjects and Experimental Setup  
We chose five healthy adults with a mean age of 24 \(\pm\) 3 years who participated in our experiment. All subjects were given adequate information about the purpose and procedure of the study and each subject's informed consent was obtained before participation. This experiment was approved by the Medical Ethics Committee of Hefei Institutes of Physical Sciences, Chinese Academy of Sciences, Hefei, China. The physical parameters of each subject are shown in the Table 1.  
The interaction force is estimated while the elbow joint is performing isometric contraction. As shown in Figure 8A, considering the physiological characteristics of human hand movement and the corresponding major muscles during contraction, the most common flexion and extension movements of hand movement were selected in this experiment. The three main muscles involved in the exercise were chosen: biceps brachii, brachialis, brachioradialis. The subjects were asked to sit in a chair and the right elbow joint was fixed at a different angle (\(30^{\circ},60^{\circ},90^{\circ},120^{\circ}\)) through the joint retainer and positioned on the test desk for isometric contraction. Three sEMG sensors were placed at each end of the belly of the muscle (an area without muscle activity was selected as a reference electrode to exclude voltages not generated by the human body autonomously). The sEMG sensor is a double conduction muscle electrical module that consists of analog circuit acquisition and digital signal filter processing. The sampling frequency was set to 1000 Hz. The six-axis force sensor independently designed by the Institute of Intelligent Machines (IIM), Chinese Academy of Sciences (CAS) was utilized for our experiments. The prototype and internal structure of a six-axis force sensor are shown in Figure 8B,C. We only selected the Z axis to measure the interaction force at the end of the upper limb. The parameters of force sensor and sEMG sensor are shown in Table 2. We can compare the estimated force and actual force in real-time through the monitor. The process of our experiment is \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Subject** & **Gender** & **Age** & **Mass (kg)** & **Height (cm)** \\ \hline A1 & Male & 27 & 77 & 176 \\ A2 & Male & 26 & 70 & 180 \\ A3 & Male & 22 & 80 & 170 \\ A4 & Female & 22 & 52 & 165 \\ A5 & Female & 23 & 48 & 161 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The physical parameters of each subject. described in detail as follows: The five subjects performed three repeated rounds of the experiment in sequence, and each round was completed in three stages. 1. Complete 100% MVC (Maximum Voluntary Contraction) isometric contraction task within 1 s; 2. Complete 100% MVC isometric contraction task within 2 s; 3. Complete the 100% MVC isometric contraction task within 3 s. The experiment procedures for each subject is shown in Table 3. Each subject was required to get sufficient rest after each contraction task before performing the next one. However, due to the different physical conditions of the subjects, the length of rest required was different. There is no exact way to get the amount of rest one needs. In order to ensure the consistency of the experiment, in this experiment we stipulate a 10-s rest after the completion of each contraction task, which is long enough. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Force Sensor** & **sEMG Sensor** \\ \hline Model & CAS-6F/MS-SM & EDK0056 \\ Power supply voltage & DC(9V) & DC(5V) \\ Temperature range & \(-30\)\({}^{\circ}\)C\(-70\)\({}^{\circ}\)C & \(-20\)\({}^{\circ}\)C\(-60\)\({}^{\circ}\)C \\ Output Signal & Analog Signal & Analog Signal \\ Maximum of output & 1000 N & 4.5 V \\ Communication & RS-485 & Bluetooth 4.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of force sensor and sEMG sensor. Figure 8: (**A**) System layout in the experiment.