Background and purpose: The imbalance ratio of co-contractions of the distal knee muscles causes more anterior shear force and valgus of the knee. Knee valgus is one of the risk factors for noncontact anterior cruciate ligament injury. Therefore, the present research aims to assess the co-contraction of the distal knee muscles in the female athletes at risk of ACL injury. Method: This research is a descriptive comparative study. The population includes the female athletes of 16-26 years old in club teams. Based on the inclusion and exclusion criteria, the samples were selected in a purposive manner and assigned to the healthy group (including 27 people) and the ligament dominance group (27 people). The electromyography activity of the tibialis anterior, peroneus longus, medial and lateral gastrocnemius muscles was recorded during drop landing from a 30-cm-high stair. Based on the specified formula, muscle co-contraction was calculated 100 ms before the foot-ground contact. Data were analyzed by Multivariate Analysis of Variance (MANOVA) at the significance level of (P≤0.05). Findings: The findings suggest that there is a significant difference between the two groups of female athletes with and without ligament dominance in terms of leg muscle co-contraction (P≤0.05). Compared to the healthy control, the co-contraction activity of tibialis anterior with lateral gastrocnemius and medial gastrocnemius with lateral gastrocnemius were lower in the ligament dominance group 100 ms before the foot-ground contact. However, the co-contraction activity of tibialis anterior with peroneus longus muscles and tibialis anterior with medial gastrocnemius in the ligament dominance group were higher than the healthy group. Nevertheless, there was no significant difference between the two groups in terms of the co-contraction of the tibialis anterior with medial gastrocnemius muscles (P≥0.05). Conclusion: Based on the present findings, it can be concluded that paying attention to leg m
