# Finite Element Analysis of a Novel Approach for Knee and Ankle Protection during Landing
## 1 Introduction  
During a jump landing, the lower extremities are most vulnerable to injury resulting from the excessive vertical ground reaction force (vGRF) [1]. Although the vGRFs experienced by the musculoskeletal system are determined by various biomechanical factors such as jumping height, landing technique, wind speed, and ground stiffness [2], they are always several times the body weight (BW) [3, 4]; in a 1.2 m half-squat landing, it can be as much as 14.7 times the BW [5]. In essence, the high impact forces are transmitted upward from distal to proximal, and in this process, multi-joint motion, joint muscles, and articular soft tissues are required to absorb the impact energy. It is, thus, not surprising that the ankle, foot, and knee are the body parts at most risk of injury during landing [6], because the ankle and foot, as the largest weight-bearing parts in the human body [7], are the first to be struck, and the knee exhibits a vital physiological structure of energy absorption, especially the meniscus, articular cartilage, and ligaments, as well as a larger joint range of motion (ROM). In this regard, some studies reported that the most common knee injuries were intra-articular fractures, anterior cruciate ligament (ACL) rupture, and meniscus damage in a half-squat posture [2, 8]. Additionally, for the ankle and foot, it was found that the impact energy would be more likely to induce ankle strains, as well as calcaneus and talus fractures [9, 10]. As stated above, during drop landing, the mechanism of injury and the injury pattern have been investigated sufficiently, and scientists have drawn similar conclusions. To protect lower-extremity joints from injuries during landings, varieties of prophylactic knee and ankle braces emerged over time. There were several studies finding that prophylactic knee and ankle braces could reduce the stress or strain of the main energy-absorbing parts (i.e., ligaments, cartilages, load-bearing bones, and soft tissues) in the joints, thus, playing a role in protection [11; 12]. Although the material and structure of protective braces are different, they work on the same principle, which is to reduce injury by restricting the motion of joints. For example, during a drop landing, wearing elastic, semi-rigid, reaction web, and silicon web knee braces could decrease the angular displacement of the knee on coronal planes [1; 8; 13]. Furthermore, during a single-leg landing, common ankle prophylactics, such as lace-up braces and tapes, could reduce the ROM of ankle plantar and dorsi-flexion by about 10\({}^{\circ}\)[14; 15]. However, the above joint braces that restrain joint ROM may have detrimental effects on the users and impair joint function, decrease performance [16], and lead to functional joint instability [17]. Meanwhile, knee braces or ankle braces were individually worn for protecting knees or ankles, respectively, in previous studies [1; 8; 13; 18]. Therefore, a set of braces, which can provide double-joint protection for the knee and ankle, is necessary. The aim of this study is to propose a novel approach to provide knee and ankle protection for anyone with impact resistance demands, such as soldiers and athletes, during drop landing and to evaluate its protective performance by using finite element (FE) analysis. As shown in Figure 1, the novel protective approach is based on a feline-inspired exoskeleton brace we developed previously [19; 20]. Although we received good application results, the biomechanical responses of the wearer's lower limbs, especially the load-bearing bones, cartilages, and menisci, with the use of the brace were not clear. We hypothesized that the semi-rigid support at the ankle joint could share the high impact force, transmit upward from distal to proximal through the rigid connecting rod, and reduce the risk of ankle inversion/eversion without affecting the ROM of ankle plantar and dorsi-flexion.