## 4 Discussion
### Range of Joint Flexion Angle  
Previous studies have shown that strain in the ACL decreases with increasing knee flexion angle [24; 25]. The studies of [PERSON] et al. [26] and [PERSON] et al. [27] suggested that during the landing phase, when the knee joint approached full extension, it led to a lack of absorption of the GRF, which may be an important component of non  
Figure 5: The fitting result of the stepwise multiple linear regression model 2.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Variables** & **Coeff. (SE)** & **95\% CI** & \(p\) \\ \hline DLJ-KFA & \(0.2618\) (\(6.7806\)) & \(0.1861\)–\(0.3374\) & \(0.0066\) \\ DLJ-PKFM & \(-188.5167\) (\(-9.7477\)) & \(-226.4217\)—\(150.6118\) & \(0.0023\) \\ DLJ-MPA & \(-1.4335\) (\(-5.8184\)) & \(-1.9163\)–\(0.9506\) & \(0.0101\) \\ SLJ-PKFM & \(116.7351\) (\(9.7704\)) & \(93.3179\)–\(140.1523\) & \(0.0023\) \\ SLJ-AP GRF & \(-10.4907\) (\(-4.1361\)) & \(-15.4619\)–\(5.5195\) & \(0.0257\) \\ SLJ-MPA & \(-0.5482\) (\(-4.8501\)) & \(-0.7697\)–\(0.3266\) & \(0.0167\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Stepwise multiple linear regression results of various parameters and injury times (model 2).  
contact ACL injuries. It is reasonable that the range of knee flexion angle is an important predictor of the risk of a knee injury.  
Compared to the single-leg jump, the double-leg jump required a significantly larger range of knee flexion angle. This may be due to the fact that in the double-leg jump, both legs land at the same time, and a larger range of knee flexion angles can be used to absorb the landing impact and maintain balance. In the single-leg jump, only one leg hits the ground, and if a larger range of knee flexion is used to absorb the impact of the landing, more powerful calf muscles are needed to support the body, which will lead to an excessive movement of the center of mass, making it more difficult to maintain balance, and the movement can only be accomplished with a reduced range of knee flexion. Referring to the findings of [PERSON]. [PERSON] et al. [28] and [PERSON] et al. [29], a low flexion angle was the typical knee posture for ACL injuries. [PERSON] et al. [30] also found that the majority of non-contact injuries occurred in the low knee flexion position. Therefore, the double-leg jump with a significantly greater knee flexion angle would be safer due to the fact that the patellofemoral contact area was directly related to the flexion angle, and altering the knee flexion angle affected the incidence and location of impact injuries [31], which in turn reduced the strain of the ACL and decreased the risk of injury. The results of [PERSON] et al. [32] suggested that increasing the knee flexion angle during jump landings may be an effective intervention to improve knee biomechanical risk factors associated with ACL injuries, which was similarly supported by our findings.  
In addition, the range of ankle flexion angle was greater for the double-leg jump, which may be due to the fact that most of the subjects made synergistic movements of the knee and ankle joints and became more accustomed to relying on the knee to enable the body to absorb the landing impact in an optimal manner, thus avoiding excessive stress on the joints. In contrast, less ankle flexion may result in less absorption of the landing impact, leading to more stress being transferred to the knee, increasing the risk of injury [33].  
In summary, when accomplishing movements similar to a single-leg jump, attention should be paid to increasing knee and ankle flexion as much as possible while maintaining balance to absorb the impact of landing and reduce the risk of lower limb musculoskeletal injuries.