## 4 Discussion  
The present study compares three dimensional LSMs and the relative contribution of hip, knee, and ankle joint moments to the prevention of limb collapse between males and females. The LSM model allows for a unique sex comparison with the lower extremity represented as a dynamic three component structure. Significant sex differences were noted as females performed the landing task differently than males. Sex differences were most evident at IC in all planes in the most extended 0-25\({}^{\circ}\) condition; no significant differences were noted in the most flexed landing of 50-75\({}^{\circ}\). The differences may therefore be relevant to injuries that occur near full knee extension, such as the non-contact mechanism of ACL injury [1; 4; 28; 34]. Female LSMs preventing frontal plane collapse were significantly less supportive than male LSMs at IC. These findings are consistent with the link between higher incidence of female ACL injury and frontal plane hip and knee moments [3; 28]. In the sagittal plane, females landed with net extensor LSMs at IC, while males utilized flexor LSMs (which subsequently shifted to extensor by 50 ms to generate support), suggesting that females have a more rigid lower limb than males at the point of IC. Transverse plane LSMs supported against internal rotation of the tibia relative to the femur (as defined in the present study), for both sexes at IC. LSM differences suggest that overall female joint moment synergies are different from males at IC for the same deceleration task. The 0-25\({}^{\circ}\) condition of this study correlates to the near full knee extension positioning defined within the non-contact valgus collapse ACL injury mechanism [2; 3; 10]. The valgus collapse mechanism is thought to occur, due in part to frontal plane hip adductor moments, instigating increased knee valgus angles and/or knee adductor moments [2; 3; 10; 28]. Hip abductor moments are thought to mitigate the non-contact mechanism as they yield femoral abduction, which serves to limit medial knee joint opening [3; 24; 28]. Proving our hypothesis, males exhibited greater LSM resistance to frontal plane medial collapse at IC through the generation of supportive hip abductor, knee adductor, and ankle adductor moments, while females produced only supportive knee and ankle moments (Figure 3, Tables 1 and 2). Females produced unsupportive hip adductor moments (Table 2). These findings indicate that females landed with less frontal plane limb support due to the hip contributing unsupportive hip adductor moments at IC. This sex difference may predispose females to increased valgus knee loading compared to males, regardless of support contributedat the knee and ankle. Furthermore, this pattern is in keeping with previously defined patterns of increased female hip adductor utilization at landing [3, 28]. It should be noted that the diminished support against frontal plane collapse in females was limited to IC, by 50 ms female frontal plane LSMs were significantly greater than males (Table 2). Therefore, the timing of utilized hip moments would seem to be a critical factor in the potential injury process. Sagittal plane injury theories are founded on the quadriceps overload premise, implying that damage to structures, such as the ACL, occurs due to excessive knee extensor moments [45, 47, 48]. In the present study, knee extensor moments increased as knee flexion increased at landing (Tables 2 and 4). If viewed from the perspective of quadriceps overload, this finding implies a greater risk of injury with deeper knee flexion, which is contrary to non-contact ACL injury mechanisms, which are noted to occur with the knee near full extension [2, 3, 10]. The sagittal plane results identify sex differences that do not support the quadriceps overload theory but rather an injury mechanism of multiple joint and plane involvement. The impact of sagittal plane hip and ankle moments on sagittal plane models of injury has received minimal consideration.