Background and purpose: previous studies have investigated the effect of heel height on chronic ankle instability, but few studies have investigated the effect of shoe collar height on muscle co-contraction ratio. The purpose of this study was to investigate the effect of shoe collar height on the co-contraction of lower limb muscles during balance tests by athletes with chronic ankle instability. Methods: 28 male athletes (14 with chronic ankle instability, 14 healthy) participated in this study. The Cumberland questionnaire was used to diagnose chronic ankle instability. The electrical activity of eight muscles of the tibialis anterior, peroneus longus, medial and lateral gastrocnemius, soleus, vastus medialis and lateralis, and biceps femoris in three positions with bare feet, with short and high collar shoes during the leg stance time and Y test. Findings: findings showed that the shoe collar height had a significant effect on the muscle co-contraction ratio during the static balance test (P=0.001). Also, findings showed that during the static balance test, the co-contraction ratio in the condition of bare feet, is more than in short and long collar shoes, and the co-contraction ratio in long collar shoes is more than that of short collar shoes. On the other hand, the results showed that the shoe collar height had no significant effect on the co-contraction ratio of the muscles during the static balance test between the two groups (P=0.57). In addition, the shoe collar height had a significant effect on the muscle co-contraction ratio during the Y balance test (P=0.001). Also, the co-contraction ratio in the lateral-posterior direction was higher than the anterior, medial-posterior direction, and the co-contraction ratio in the anterior direction was higher than the medial-posterior direction. In addition, the shoe collar height had no significant effect on the co-contraction ratio during the performance of the Y balance test between different groups (P=0.31). Conclusion: Although the co-contraction ratio was higher in the condition of long shoe collar during balance tests, and this creates a better condition for the stability of the joint, it is suggested to research the effect of various shoe collar heights to investigate sports performance
