# Effect of Muscle-Specific Fatigue on the Risk of Anterior Cruciate Ligament Injury in Females
## 4 Discussion
In this study, there was a reduction in the knee joint's maximum extension and adduction moment after QF fatigue. In contrast, no significant difference in knee moment was observed after HA fatigue. Hamstring activation has been known to provide a posterior force to protect the ACL damage when landing with a larger knee flexion angle [24]. However, when the hamstring is fatigued after repeated knee extension and acts as an antagonist, a decrease in the resultant joint moment has been reported [53]. This reduced activation of the hamstring with knee moment is considered to have a reduced effect on ACL loading. The results of these studies show similarities to this study, which did not show a substantial change in knee moment after hamstring fatigue. The ACL load calculated during the DDM task in previous studies was limited to a 1-bundle ACL model [30, 44, 45]. Although another preceding study has used a two-bundle ACL model, the simulation was limited to evaluate the knee flexion and extension during gait and could be hardly used for an actual sporting condition [54]. Therefore, in this study, the load of aACL and pACL was calculated by inserting a two-bundle ACL model into the musculoskeletal modeling system, which was used to analyze directional diversion and landing maneuver [21, 45, 54]. In this study, maximum aACL and pACL force after QF and HA fatigue did not show significant changes. Although a significant reduction in the mean aACL after QF fatigue at the landing phase was observed, no significant changes in components of ACL force after HA fatigue were observed. Thus, although the hypothesis that the reduction in the ability of the neuromuscular system to produce force due to fatigue in QF could reduce ACL loads was indirectly proven, the hypothesis that fatigue of the HA muscle group would increase the ACL load could not be accepted. Therefore, the results of this study should be interpreted with caution, while more research is needed to reach certain conclusions. Nevertheless, we suggest coaches develop training methods that include strategies to reduce the eccentric contraction of quadriceps through knee flexion when performing movements with a sudden change in direction, such as DDM. To our knowledge, no prior studies have analyzed the effect of muscle-specific fatigue of QF and HA muscle groups and its relation to the ACL injury on a dynamic motion such as DDM. Nevertheless, gait after QF fatigue has been reported to increase the knee external rotation angle and abduction moment [55]. Another study conducted on the effect of fatigue on lower extremities muscles during squat jump reported an increase in the knee joint's external and internal rotation angles and the adduction moment, which could increase the risk of ACL injury [2]. Although there are differences in the kinematics of the movement performed, these changes may attribute to the decrease in the neuromuscular ability of QF after fatigue, thereby reducing the factors related to increased ACL load such as extension moment and adduction moment of the knee joint. There are three primary limitations associated with this investigation. First, the fatigue protocol utilized for the thigh muscles around the femur in this study involved only flexion/extension of the knee joint. Therefore, the relationship of factors associated with the ACL injury after fatiguing the rotational component of the femur (adduction, valgus, internal/external rotation) could not be identified. Further, future research can consider using the fatiguing protocol with the rotational treatment to observe its effect on the factors of an ACL injury. Secondly, we were not able to measure the muscle activity of QF and HA during DDM. The EMG sensor had to be attached before the fatigue condition, or else the delay in attachment could show a sign of recovery. However, as the isokinetic strength measurement system was used to induce fatigue in this study, the sensor could not be attached because subjects complained of pain at the sensor attachment site when strong pressure was applied.