# Classification and Regression of Muscle Neural Signals on Human Lower Extremities via BP_AdaBoost
## 2 Materials and Methods  
### Muscle Selection of Lower Extremity  
Knee joint is the main joint involved in human walking. As the main load-bearing structure of human movement, it is an important source of gait characteristics [15]. The movement of joints depends on the traction of muscles, and the process of walking is the angle change of hip and knee joints in the sagittal plane. According to the principles of human anatomy and the laws of human kinematics, the muscles related to the angle changes of the hip and knee joint include quadriceps femoris and biceps femoris (gastrocnemius and other auxiliary muscles are not included) [16; 17]. Therefore, this article uses OpenSim(a musculoskeletal analysis software) to simulate the changes of muscle fiber length of quadriceps femoris and biceps femoris during a gait cycle. The selected human model gait 2392_Simbody.osim contains 19 bones and 92 muscles of lower limb and trunk models. The results are shown in Figure 1, which simulates the work of 4 muscles in a gait cycle, with an increase in length indicating muscle relaxation and a decrease in length indicating muscle contraction. This indicates that the changes of muscle length of vastus lateralis, rectus femoris, vastus medialis, and vastus intermedius, which make up the quadriceps femoris, are consistent during walking, and they all increase first to reach the first peak and then decrease (the increase in muscle length indicates that the muscle is in a relaxed state, whereas the change of muscle length indicates that the muscle is in a contracted state). Then, another peak appeared at 63% of the gait cycle, but the peak was small. The length change of rectus femoris (red curve) is basically the same as that of the other three muscles of quadriceps femoris before 60% of the gait cycle, but the muscle is in a state of continuous relaxation at 60%-100%. The biceps femoris is located at the back of the thigh and participates in contraction when bending the knee, which is basically opposite to the role of the quadriceps femoris. Therefore, the length change of the biceps femoris (black curve) is also opposite to that of the quadriceps femoris. Now, this article further quantitatively analyzes the correlation of the above 5 muscles. This study uses the correlation analysis module of statistical analysis in software SPSS. Pearson parameter in SPSS program was selected as correlation coefficient, and two tailed NOSIG was used for significance test. The results are shown in Table 1, in which p value shows significance index, and \(p>0.05\) indicates that the two variables are uncorrelated and not significant; \(0.01<p<0.05\) was significant, whereas \(p<0.01\) was extremely significant. R is the correlation coefficient, negative R value indicates negative correlation, and positive R value indicates positive correlation. It illustrates that the significant P values of the 5 muscles are less than 0.01, indicating that there is a very significant change in muscle length. The R value between biceps femoris and quadriceps femoris is negative, the R value between biceps femoris and rectus femoris is the largest, which is \(-0.932\), whereas the R value between lateral, medial, and rectus femoris of quadriceps femoris is 1. This shows that the changes of muscle length are consistent. According to the relevant knowledge of human lower limb medicine, joint movement depends on muscle contraction, and muscle contraction depends on muscle EMG signal feedback [17]. Therefore, studying the correlation between muscle and joint angle can determine the muscles related to knee movement. However, multi-dimensional EMG signals will lead to information redundancy and increase the burden of classifiers based on the relevant literature of pattern recognition [18]. Simultaneously, its surface EMG signal cannot be measured effectively because the femoral  
Figure 1: Curve of thigh muscle length in a single gait cycle. intermediate muscle belongs to deep muscle [17].