# Analyzing the Biomechanical Characteristics of Ski Jumping Take-Off Phase Based on CFD
## 4 Discussion
### Aerodynamic Analysis during the Take-Off Phase
#### 4.2.2 Aerodynamic Characteristics at the End Moment of the Take-Off Phase  
The body posture of ski jumpers during the EMT significantly influences their ability to swiftly transition into the early flight phase and achieve an optimal lift-to-drag ratio. This, in turn, provides more sufficient flight time for a stable flight. Different body postures during the EMT have varying effects on the amount of drag encountered by the athlete and the lift generated. [PERSON] et al. found that hip angle and body-ski angle play a dominant role in aerodynamic performance, with the ski and torso contributing the most to aerodynamics and torque [13]. [PERSON] et al. suggested that athletes achieve the optimal lift-to-drag ratio of approximately 1.5 when the angle between their body and the ski board is 15\({}^{\circ}\) during the flight phase [29]. As shown in Table 4, there are differences in the air drag experienced by various body parts between Chinese and Northwest (NW) elite athletes after take-off. The coordination characteristics and kinematic data during the athletes' take-off reveal that Chinese athletes have a relatively small proportion of time in which the knee joint dominates during take-off. This results in insufficient push-off and extension of the knee joint, preventing it from reaching the optimal angle. Additionally, the strong dominance of the ankle joint during take-off causes an excessively large ankle angle after take-off, resulting in inadequate forward leaning of the body. [PERSON] and [PERSON] found that maintaining stability in the ankle joint during take-off allows the athlete's body to lean forward, gaining forward angular momentum [24]. Compared to NW elite athletes, Chinese athletes tend to have a larger ankle angle and a smaller knee angle, leading to inadequate forward leaning of the body and a larger windward area. Calculations of the air drag experienced by athletes at the EMT (Table 4) reveal that Chinese athletes experience higher air drag in various body segments than NW elite athletes, with significantly higher drag in the bilateral lower legs and torso. This is primarily due to the greater ankle angle during take-off, which results in less forward leaning and higher drag. Our analysis of the drag and lift forces during take-off, with respect to the lift-to-drag ratio generated by different postures at the EMT, shows that NW elite athletes, with smaller ankle angles and fully extended knee angles, achieve better forward leaning, less air drag, and higher lift force compared to Chinese athletes. Consequently, NW elite athletes achieve a higher lift-to-drag ratio (Table 4). Research by [PERSON] suggests that disadvantages during the take-off phase are difficult to compensate for in the subsequent flight phase, implying that the body posture after take-off plays a crucial role in the stability and distance of the flight [4]. [PERSON]'s study on the early flight phase of ski jumpers showed that achieving an optimal lift-to-drag ratio significantly impacts the flight distance [30]. Therefore, NW elite athletes' body posture at the EMT enables them to enter the stable flight phase faster, providing a longer flight time. In contrast, the kinematic characteristics of Chinese athletes result in higher air drag during take-off and subsequent flight adjustments, potentially leading to an unstable flight and a higher probability of injury.