The ACL consists of individual fiber bundles, named for their insertion points on the tibial footprint. The larger anteromedial (AM) bundle inserts anteromedially on the tibial side and originates more proximally on the femoral side than the posterolateral (PL) bundle, which is posterolateral relative to AM bundle.
Studies indicate that the AM bundle tightens in flexion while the PL bundle relaxes; in extension, the PL bundle tightens while the AM bundle relaxes. What is unusual about the workings of the two bundles is the way their insertion points on the femur relate to each other in flexion and extension. In extension, their insertion points are vertical to each other, but with the knee in 90 degrees flexion, the insertion points are horizontal to each other. Likewise, the bundles themselves shift from parallel orientation in extension to crossed orientation in flexion.