Animation 9: Model of active flexion – extension when the weight of the leg is balanced by tension in the patellar tendon
In extension, the tibia is pulled anteriorly relative to the femur,
the ACL is stretched, and the PCL is slack. As the quadriceps is
released, the knee flexes and the ACL slackens progressively, and the
meniscal bearing moves closer to its position in the unloaded state.
At 57°, all fibres of both ligaments are slack. At larger flexion
angles, the fibres of the PCL tighten slightly, and the bearing moves
forwards on the tibia. .The total excursion of the meniscal bearing on
the tibia is reduced compared with passive flexion.