# Simulation Analysis of Knee Ligaments in the Landing Phase of Freestyle Skiing Aerial
## 4 Discussion and Implications  
In the balancing phase of landing in freestyle skiing aerial sports, complex internal stresses are formed within the knee joint. In this study, only neutral, forward, and backward landing positions were considered. The principal differences between the three scenarios were position of center of gravity and the magnitude and direction of the reaction force.  
In neutral landing, the reaction force provided by the landing slope was essentially perpendicular to the tibial plateau. Under the combined action of the impact force of falling and the frictional force of the snow, the analysis demonstrated that the tibia moved back and forth slightly within the knee joint. During knee flexion, the front and rear muscles of the thigh worked together to stabilize the knee joint.  
Figure 10: Stress distribution of tibial plateau and meniscus: (**a**) forward landing; (**b**) neutral landing; (**c**) backward landing.  
Figure 9: Distribution and stress–time curve of the ligaments in backward landing.  
According to the results of the simulation, peak stress on the tibial plateau and meniscus were greatest at this point in time for all three landing scenarios, but the relative displacement of the tibia was small. Stress on the cruciate ligaments in the balancing phase in neutral landing was the smallest of all three, and so was the safest form of landing.  
In forward landing, slight backward motion of the tibia occurred within the knee joint and a vertical line through the center of gravity moved forward beyond the tibial plateau. As balancing progressed, the anterior thigh muscle was required to reposition the tibia [24]. According to the results of the simulation, peak stress in the tibial plateau and meniscus were smallest at this moment in all three conditions, with regions of stress concentrated in the anterior horn of the menisci. Because the medial meniscus was compressed by the femoral cartilage and resulted in lateral movement, stress in the middle of the MCL during the balancing phase of forward landing was the highest of all three landing scenarios.  
In backward landing, elastic force of deformation of the skis and the impact force of falling resulted in the tibia exhibiting forward movement of the knee joint. As balancing progressed, the position of the tibia was adjusted towards the neutral position by action of the posterior thigh muscle [25] in a tangential direction, the tibia becoming pushed backwards. According to the results of the simulation, peak stress on the tibial plateau and meniscus was observed in forward and neutral landing, but of all three scenarios, stress on the ACL and PCL was largest in backward landing.  
The results of the simulation in the three scenarios above demonstrate that the maximum value of stress (Table 1) in each ligament was less than the ultimate stress of the ligament. Even so, not all the ligaments are completely safe. Such as if athletes with the accumulated training, degenerative changes would occur within the region of stress concentration in the tibial cartilage and meniscus, in addition to fatigue and damage within the position of stress concentration in the cruciate ligament. If internal or external rotation of the knee joint occurred during landing, the risk of knee cartilage, meniscus or ligament injury would greatly increase. Over a controllable range, whether in forward or backward landing, aerialists require sufficiently strong anterior and posterior thigh muscles to maintain stability in the knee joint. If the strength of the anterior and posterior thigh muscles do not match, the risk of cruciate ligament injury is greatly increased.