This lecture delves into the application of knee joint kinematics to ACL reconstruction, focusing on gait analysis before and after surgery. It covers the anatomy of the knee, ACL injuries, surgical procedures, and the use of magnetic and inertial measurement systems for gait analysis. The presentation includes the comparison of knee angles in healthy subjects and patients post-surgery, emphasizing the importance of restoring knee laxity. Various protocols for gait analysis in lab and field settings are discussed, along with the measurement of knee angles during gait cycles pre and post ACL reconstruction.