## 2 Methods
### Measurement Module  
The sensing module for signals of muscle force and angle was conducted by two sets of EMG sensors and nine-axis sensors, which then transmitted the collected data to the back-end computer directly or via Bluetooth communication through a microcontroller for signal recording and analysis in Figure 1. The signal processing module mainly processed by EMG signal, and the collected data were processed by software programs written from the LabVIEW (National Instruments, Austin, TX, USA) for digital filtering, full-wave rectification, and normalization, which would be used for subsequent analysis. The data analysis and evaluation were used to interpret the required index parameters of the signal analysis, and then presented in a graphical user interface (GUI) to provide feedback for doctors and subjects and to help understand the current state.  
The system used an Arduino UNO (HAOYU Electronics, Shenzhen, China) development board as a microcontroller unit (MCU). This platform adopted the ATmega series, which is equipped with a multi-channel Analog-to-Digital Converter (ADC) with a 10-bit resolution conversion voltage value, combined with the EMG signal sensing module to convert the voltage, displayed with 0-1023 integer. After referring to the sampling frequency of EMG myoelectric signal frequency band that concentrated at 20-400 Hz [20], the sampling frequency of this study was set to 1 kHz in order to meet the data acquisition requirements.  
The EMG signal was composed of two sets of SHIELD-EKG-EMG (Olimex Ltd., Plovdiv, Bulgaria) which could measure electromyography (EMG) and electrocardiogram (ECG) signal along with an expansion module compatible with the Arduino UNO development board.  
Up to 6 groups could be stacked for 6-channel reception to measure myoelectric signals or electrocardiograms during movement. The locations of the study were VMO and VL to monitor and record the EMG signal between them, and the SHIELD-EKG-EMG-PRO was used to connect the SHIELD-EKG-EMG module.  
In this study, the myoelectric signals of the VL and on quadriceps femoris muscle were also taken. The position of the electrodes was placed according to the sensing points described in [PERSON] and Blumenstein [21], which were mainly for the detection of the medial and lateral femoral muscles. Considering the difference in signal, each electrode was fixed in place on the front end of the VMO and VL muscle lines as the sensing point placement position that could be clearly identified with the naked eye or palpation. If the front end portion failed to be seen from the naked eye during the placement process, the VMO sensing point would then be placed at about 6 cm away from the patella where the inner side of the patella and the femur was intersected at 40\({}^{\circ}\), and the VL sensing point would be placed at about 9 cm away from the patella where the outer side of the patella and the femur was intersected at 30\({}^{\circ}\), while the reference electrode patch was attached to the tibia of the calf [22].  
In order to match the EMG measurement on user's leg movement, the system was equipped with GY-85 (HAOYU Electronics, Shenzhen, China) of nine-axis acceleration Inertial Measurement Units (IMU), which was fixed on the flat upper part of the femur. This has the ability to calculate angle and acceleration as well as to simultaneously record the disturbance of the patella and myoelectric signals.