## 4 Discussion  
The FEM model of a lower limb represents a useful tool for evaluating the protective effect of a commercial bracket. The model can serve as an investigative tool to shed light on the mechanical response of the lower limb under crash conditions in motocross. Yet, to date, no FEM model has been employed to assess lower limb injuries in motocross. The models can be used to evaluate the improvement in bracket design, human safety protection, and lower limb limits under crashes in motocross or other sports. To create the FEM model of the bracket, we used the reverse engineering method based on 3D scanning. Nowadays, 3D scanners have a wide range of applications in modern manufacturing and quality control [17]. The 3D scanning of the bracket components creates point clouds or an STL file that need to be applied to FEM (Ansys 2020) software. Figure 10: Pressure distribution in the connection component (femoral side) of the limb for the FE model (**a**) with and (**b**) without bracket. The geometrical anatomy of the lower limb was based on MRI images that reflected the exact parts of the lower limb. The constitutive laws of the materials were set according to the mechanical properties from previous work from the literature, to ensure the reliability of the newly created model. This work proposed a new methodology to evaluate the effect of the impact on the cruciate ligaments. A unique component between the tibia and femur was adopted to represent all internal components of the knee joint (cartilage, meniscus, synovial fluid, ACL, and PCL). However, the method here proposed simplified the complex mechanical behaviour of all the soft tissue inside the joint, and future improvements are needed. To evaluate the protective effect of the bracket on the knee joint, two points that represent the bone insertion of the ACL were considered. By comparing the results of the distance between the ACL insertion points during the impact in the axis X, Y, and Z for the two simulations, it is important to note that all the distances in the three directions for the simulation with the bracket are lower compared to the results of the simulation without the bracket (Figure 6). By evaluating the distance between the ACL insertion points in the Y direction, which represent the plane where the hyperextension could occur, the maximum distance shows a decrease of 2% (time 0.40 m/s) and 8% (time 0.95 m/s) for the first and the second peak, respectively, compared to the result without the bracket (Figure 6). However, evaluating the same relative difference at other timesteps, this can assume even larger values, reaching a maximum of 37% at \(t=0.55\) m/s. These results point out the protective effects of the bracket, which acts as a constraining and stabilising force. In the other two directions, the distance between the two points is smaller compared to the results without the bracket during the impact, especially in the X direction where the three peaks show significant reductions of 25.8%, 18.9% and 38.5%, respectively (time 0.35, 0.65 and 0.95 m/s) (Figure 6). This result could represent a realistic effect of the bracket, which is fixed to the leg and could reduce the possible movement of the bones. Furthermore, considering the total displacement between the two points, a significant reduction of 17.6% and 21.5% is noticeable for the two peaks, respectively (Figure 6e,f). In addition, evaluating the singular displacement of the two in the tibia and femur, it has been observed that the displacement along the X, Y, and Z axes decrease with the application of the bracket. Considering the point that represents the bone insertion of the ACL on the tibia, decreases of 75.88%, 16.28%, and 40.70% along the X, Y, and Z axes, respectively, are shown in the model with the bracket. Whereas, considering the bone insertion of the ACL on the femur, there are decreases of 18.36%, 44.91%, and 35.77% along the X, Y, and Z axes, respectively.