# Effect of Muscle-Specific Fatigue on the Risk of Anterior Cruciate Ligament Injury in Females
## 4 Discussion  
The study aimed to evaluate the effect of muscle-specific fatigue of the QF and HA on the factors related to ACL injuries such as ACL load, knee angle, force, and moment during the DDM task. Therefore, we hypothesize that the factors affecting ACL injury after QF fatigue will decrease compared to before QF fatigue and vice-versa in the case of HA fatigue. It was confirmed that QF is the major muscle causing ACL injury since ACL load increased significantly after QF fatigue. However, the ACL load after HA fatigue did not significantly decrease, which leads us to speculate that HA might not have a preventive mechanism against ACL injury. According to [PERSON], et al. [48], the ACL load varies according to the knee angle, with the highest load being observed at 30\({}^{\circ}\) starting from a fully extended knee at 0\({}^{\circ}\). Also, it was reported that the ACL load increases as the valgus and internal rotation of the knee joints increases. Particularly, studies conducted on women showed that the knee joints appear to be abducted up to 20\({}^{\circ}\) in DDM and landing movements [5,18]. There was a significant reduction in the maximum flexion knee\({}_{\text{angle}}\) and knee valgus\({}_{\text{angle}}\) after HA fatigue in this study. An in-vivo study reported that modification to the tibiofemoral kinematics and its moment arm affects the activation of the hamstrings [49]. Additionally, in a closed kinematic chain, the decrease in the knee flexion angle also decreases the hip flexion angle. Since HA has a more significant mechanical advantage at the hip joint than at the knee joint, HA fatigue would decrease the hip and knee flexion angles [50]. This would result in the reduction of the muscle activation requirement on the HA muscle group. Further, a smaller moment arm of the HA at the knee would reduce torque, and the HA muscle group would not produce enough ACL shear forces at the knee joint to prevent ACL injury. Hence, it is considered that a direct relationship of change in knee kinematics, particularly during a dynamic activity such as DDM, may not be enough to confirm its effect on ACL load from this study. During DDM, the shear force acting forward from the proximal direction of the tibia has been reported to cause ACL injury if the force exceeds 2000 N or more [26]. Numerous studies on the relationship between the contraction of QF and ACL load have reported that a strong contraction of QF in an extended knee position can increase the ACL load and the anterior tibial shear [26,48] whereas, co-contraction of the HA was reported to suppress the increase in ACL load by offsetting the anterior tibial shear force [24, 26, 51]. However, the results of this study did not show statistically significant differences in the maximum knee\({}_{\text{shear}}\) in all three conditions (post-HA, post-QF, and control). Also, the maximum knee shear force was significantly lower after HA fatigue when comparing differences between pre and post DDM. Consequently, the mechanism by which HA reduces ACL load needs to be reconsidered since there was a reduction in knee\({}_{\text{shear}}\) after the fatiguing protocol in this study which contradicts the findings of previous studies; co-contraction between HA and QF results in a reduction of the knee\({}_{\text{shear}}\)[24, 25, 26, 28]. In order to evaluate the cause of ACL injuries, the moment acting on the knee joint must be considered [18, 52]. [PERSON] et al. reported that the risk of ACL injury increases with an increase in the knee valgus angle, medial rotation angle, and adduction moment after fatigue during a squat jump [2]. Another study evaluating the effect of muscular fatigue on different movements by [PERSON] et al. reported a low activation of the QF and HA muscles with a gradual decrease in the activity of HA during DDM [23]. The study suggested that this reduction in HA activity after acute fatigue may increase the possibility of ACL injury during sporting situations.