Chae, W-S. Bilateral Comparison of Effective Moment Arms of the Quadriceps Force on Unilateral ACL-Reconstructed Individuals. Korean Journal of Sport Biomechanics, Vol. 13, No. 1, pp. 13-21. The purpose of this study is ⒜ to estimate effective moment arms of quadriceps forces and ⒝ to compare the d_e between the ACL-reconstructed and uninjured knees the same individual. One female (20 yrs old, 2 yrs post-op, hamstring tendon autograft) and two males (22 yrs old, 2 yrs post-op; 28 yrs old, 4 yrs post-op; patellar tendon autografts for both). Sagittal view radiographs were obtained for 6-7 different angles (range 5˚-110˚) from each knee. The d_e was determined by the method of Chow et al.(1999a). The results showed that the maximum d_e values ranged from 4.61 to 5.59 cm and 4.59 to 4.89 cm for the ACL-reconstructed and uninjured knees, respectively. The maximum d_e occurred between 35˚ and 50˚ and 20˚ and 50˚ for the ACL-reconstructed and uninjured knees, respectively. The maximum d_e values ranged from 4.12 to 4.35 cm and 3.12 to 3.63 cm for the ACl-reconstructed and uninjured knees, respectively. The effective moment arm of the knee extensor affects the loads on knee ligaments during knee-extension exercises. Because apparent differences in the moment arm of the quadriceps in different participants, it is very important to use personalized knee joint geometry for the computation of knee joint force. In the present study, no noticeable bilateral difference was found in the male subjects. However, apparent bilateral differences in d_e were observed in the female subject. This suggests that the effects of ACL reconstruction surgery on patellar mechanism deserve further investigation.