## 2 Materials and Methods
### FE Modelling from the Geometrical Model
#### 2.3.3 Finite Element Analysis  
The finite element model of the lower limb was created by mixing 3D scanning and 3D reconstruction from MRI images in DICOM format. The bones of the femur and tibia were manually segmented using SLICER 3D [10]. At first, a threshold in the range of the bone pixel intensity was performed, also to isolate the connected pixels of the bone element. After the segmentation, the post processing was performed: the smooth median operation was applied to the mask, and finally the.stl files were exported using a smoothing factor. The same procedure was carried out for the skin. All the internal components of the knee joint (cartilage, meniscus, ACL, and Posterior Cruciate Ligament (PCL)) were considered as a single component, called \"Connection\" and modelled as a homogeneous continuous material. This approach was adopted considering the combined compressive stiffness (CCS) method of the tiboferomal joint for FEM as conveyed by [PERSON] et al. [11]. In the same way, the Compressive Young Modulus for this component was set at 2.40 MPa, again according to [11]. The realization of the described model, that did not include all the soft tissues listed before, is of course a simplification. It did not consider some factors such as tendons, ligaments, and muscles, but it is reasonable. To the best of the authors' knowledge, there are no other similar studies that analyse the protective influence of motorbike braces on knee movements. The present study prefers to focus on considering less but more detailed structures (which will be extended in future studies) to enable a first evaluation of the protective effect of the bracket. Moreover, only linear models were set for all materials, since this study represents the first approach where an all-in-one component to represent the joint was developed and adopted. The bone component and the skin of the limb were imported in STL format on SpaceClaim and aligned with the bracket components. To evaluate the bracket's protective effect on the knee joint, it was decided to conduct two numerical simulations, the first without the bracket and the second with it. The model was used to simulate the condition of landing after a jump. A weight of 80 kg and a jump of 5 m height were imposed. The corresponding impact velocity was obtained considering the hypothesis that a mass (composed of the motorbike \(-110\) kg- and the athlete \(-80\) kg) of 190 kg has a potential energy of 9319.5 J at a height of 5 m. At the impact instant, the potential energy is converted in kinetical energy, obtaining the velocity value of 9.9 m/s. Hence, the load was applied as a step on the resected section of the tibial bone component, and fixed support was applied to the resected section of the femoral component. This indirect approach was preferred as simulations are often known to produce some errors in the estimation of knee joint loading [12]. Numerical implementation was conducted using the Explicit Dynamics on Ansys 2020. The \"end time\" of the simulations was imposed at 0.001 s. This assumption was based on real-life observations of similar incidents during motocross races. It is necessary to consider that the maximum displacement may potentially not reach its peak value at this point in time, but this would mostly depend on case-by-case considerations, while this working hypothesis is required to compare the results with and without the brace under similar conditions. Fixed support was provided to the proximal surfaces of the femur. Furthermore, a displacement was applied to the tibia distal surface with X-direction movement equal to zero (assuming no translation in this direction). The explicit dynamic analysis was chosen based on the study [13], which compares three different analyses--static, dynamic implicit, and dynamic explicit solutions--applied to a knee joint study.