Differences in knee valgus loading between sexes have been reported in previous studies focusing on knee joint motion and related muscles during landing 19–21. However, recent studies suggest that ankle-to-knee biomechanical interaction may influence knee frontal plane motion 12,22,23. Thus, the purpose of this study was to determine whether sex differences exist in the association between sagittal ankle control (ankle initial contact angle (ankle ROM) and plantar flexion moment) and knee valgus moment. An analysis of the correlation between sagittal ankle control and the knee valgus moment was performed for both males and females. Only the male subjects in this study showed a significant correlation between ankle sagittal kinematics/kinetics and the knee valgus moment. Although the peak knee valgus was not significantly different between sexes, the findings in this study support our hypothesis.
Regarding the male subjects, it was found that the peak vGRF and peak knee valgus moment were negatively correlated with the ankle plantar flexion angle at initial contact and ROM. This result suggests that in the case of males, it is important to use a large ankle plantarflexion angle at initial contact and/or ROM to effectively absorb the landing impact and reduce the knee valgus loading. Our results agree with previous findings that landing with a large ankle plantar flexion angle at initial contact or ROM reduces landing impact 15,16,18. In addition, the GRF during landing can induce frontal plane loading in the lower extremity joint 24,25. Moreover, Hewett et al. reported that the peak knee valgus moment in females was more positively correlated with the peak landing force compared with flexion and extension moments and valgus moment at the knee, hip, and ankle during landing 26. In this study, females showed a larger plantarflexion angle at initial contact and ROM than males, but there was no significant correlation with peak vGRF. This may imply that landing with a large ankle plantarflexion angle at initial contact or ROM may have played a more important role in reducing the knee valgus loading in males than in females through landing force attenuation. The reason why only males showed a significant relationship cannot be determined through this study, and further study on the mechanism of gender differences in this relationship is warranted.
Our results indicate that more ankle plantar flexion moment was associated with more ankle inversion moment and less knee valgus moment in males, but not in females. One might suppose from these results that females may not be able to generate sufficient ankle inversion moment coupled with plantarflexion moment to counteract knee valgus. Ankle plantar-flexor muscles (gastrocnemius, soleus, and tibialis posterior) act more dominantly during landing than dorsiflexors (tibialis anterior) 27, and previous studies 11,28 have suggested that the triceps muscle force produces the ankle inversion moment in the sitting position. In addition, the tibialis posterior and anterior muscles function as invertors of the foot 29. Thus, ankle muscles related to plantar flexion and dorsiflexion may be accompanied by ankle inversion during landing. With regard to the ankle frontal plane moment at landing, Thompson et al. 30 suggested the possibility of an association between the ankle frontal plane moment and the knee frontal plane moment during double-leg landing. Thompson et al. 30 reported that the intervention group who participated in an ACL injury prevention program showed increased peak ankle eversion moments and peak knee valgus moments during double-leg landing. Excessive ankle eversion has been observed with dynamic knee valgus when non-contact ACL injury occur 6. Joseph et al. 31 suggested that restricting ankle eversion reduces knee valgus motion. Although there was no significant difference in the peak ankle inversion moment between sexes, only males in this study showed a significant correlation between ankle peak plantar flexion and the inversion moment. This result suggests that sex differences in ACL injury risk related to knee valgus loading may be partially explained by differences in coupling between ankle plantarflexion and the inversion moment.
In addition, the positive relationship between ankle plantarflexion moment and the inversion moment, which exists only in males, may suggest that there is a sex difference in the dynamic balance of the entire body during single-leg landing. The ankle plantar flexor muscles play an important role in controlling postural stability during single-leg standing 32. Vieira et al. 11 reported that the peak ankle inversion moment amounted to approximately 13% of the peak ankle plantarflexion moment in the sitting position and suggested that these relationships between ankle plantarflexion and the inversion moment play a role in the mediolateral balance of the entire body. Activation of the ankle muscle (gastrocnemius muscle) with other shank muscles provides stability to the body during single-leg standing 33. It is relatively easier to lose body balance with a single-leg landing than with a double-leg landing because of the shift of the center of mass of the entire body 34; thus, only individual landing legs need to absorb the landing impact by the muscle-tendon unit 35. A video analysis study demonstrated that the loss of body balance during single-leg landing is frequently observed at the incidence of an ACL injury 36. In this study, the peak ankle inversion moment amounted to approximately 21% of the peak ankle plantar flexion moment for males during single-leg landing. Collectively, the greater ankle plantarflexion moment with the inversion moment may reduce the risk of ACL injury by improving balance during single-leg landing in males.
In conclusion, the effects of ankle plantar flexion angle at initial contact, ROM, and ankle plantar flexion moment on the knee valgus moment differed between sexes during single-leg landing. Only males demonstrated that peak knee valgus moment was negatively correlated with the peak ankle plantar flexion moment, ankle plantar flexion angle at initial contact, and ROM. In addition, in males only, the peak ankle plantar flexion moment was positively correlated with the peak ankle inversion moment. These findings suggest that altering ankle landing strategies in the sagittal plane during single-leg landing may reduce the knee valgus moment, which is one of risk factors for ACL injuries, in males only. The current investigation may help explain one of the reasons underlying the phenomenon of sex differences in ACL injury related to the knee valgus moment.