The purpose of this study is (a) to estimate effective moment arms of quadriceps forces and (b) to compare the $d_e$ between the ACL-reconstructed and uninjured knees from 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 $(range;5^{circ}-110^{circ})$ from each knee. The do was determined by the method of Chow et al. (1999a). The results showed that the maximum de values ranged from 4.61 to 5.59cm and 4.59 to 4.89cm for the ACL-reconstructed and uninjured knees, respectively. The maximum $d_e$ occurred between $35^{circ};and;50^{circ};and;20^{circ};and;50^{circ}$ for the ACL-reconstructed and uninjured knees, respectively. The minimum do values ranged from 4.12 to 4.35 cm and 3.12 to 3.63cm 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 de were observed in the female subject. This suggests that the effects of ACL reconstruction surgery on patellar mechanism deserve further investigation.