## 1 Introduction  
Motocross is a sport activity during which athletes must ride over rough terrain that presents bumps, turns, and muddy floors. Races last for about 40 min [1]. The greatest number of injuries is reported when performing turns, bumps, and during the qualification phase. The injury rate during qualification is 122.2 per 1000 activity hours [2]. The same study reports that the overall injury rate for motorcycle competitions (22.4 per 1000 activity hours) is 3.6 times higher than that of players in Major League soccer.  
The American Motorcyclist Association (AMA) attempts to mitigate these risks by applying a set of rules annually; indeed, the rules dictate mandatory protective equipment. However, the mandatory protective gear does not include protections against the frequently encountered ligamentous knee injuries [3].  
Musculoskeletal injuries are involved in most accidents; this is particularly true for the upper and lower limbs. Specifically, 42% of total ligament injuries are knee injuries [3]. Some of the most diffused injuries involve anterior cruciate ligament (ACL) ruptures (42.7% of knee injuries), followed by menisci damages (20.22%) and medial collateral ligament (MCL) ruptures (14.61%) [4]. The use of knee brackets has been associated with a decrease in the incidence of motorcross-related soft tissue knee injuries [4]. The main purpose of this study was to create a new FEM model to evaluate the protective effect of a commercial motorcross bracket, in particular its behaviour when hyperextension occurs. To create this model, reverse engineering based on a 3D scanning of the components was used and the issues correlated were fixed.  
In the literature, there are several studies that deal with the main causes of injuries and their related factors. [PERSON] et al. [5] conducted a deep study on ACL injuries; they evaluated different factors that are involved in ACL injury during jump landing in no-contact conditions (no external forces involved), i.e., only foot-ground reaction and segmental gravitoinertial force were considered. The effects of the impact load, quadriceps force and knee flexion angle were studied, employing instrumented cadaveric knees. They reported that the relative strain increases in the anterior cruciate ligament were proportional to the increase in quadriceps force and knee flexion angle [5]. Another study realized by [PERSON] et al. [6] analysed the same load condition, including the hip joint. They implemented a simulator for impact landings that admitted the application of muscle forces and different conditions of the hip joint that can be either free to rotate or fixed at a given flexion angle. The result showed that elevated levels of quadriceps force prevented injury even under restricted hip flexion angles, therefore, the most important factors that influence injury prevention are the flexion of the hip and the force of the quadriceps [6].  
Additional factors that can be considered for injury prevention include the joint moments that are present during sport activities. A study reported that valgus moment plays a crucial factor on ACL strain, as reported by the authors during a simulation conducted to investigate the influence of isolated valgus moment on the ACL strain during a single leg landing using a 3D model with human loading data. During the simulation, the authors set the application of a valgus moment starting from 0 Nm to 51 Nm; the results showed a non-linear relationship between ACL strain and the magnitude of valgus moment [7].  
It can be envisaged that there will be necessary further numerical and experimental research related to knee biomechanics. Finite element simulations are especially necessary due to the lack of large experimental datasets (with some exceptions, e.g., [8]), in particular for sport lesions, where the knowledge is even more limited [9].