The ACL, or the anterior cruciate ligament is one of four ligaments that help support and stabilize the knee. Without the ligaments, there would be nothing that could help you control the knee’s movements. The ACL is responsible for pulling on the tibia, or shin bone. When you straighten your leg, the ACL pulls on the muscles to stabilize the tibia, preventing it from moving too far. If you have a ruptured or torn ACL, that stability isn’t there and your knee may tend to give way all of a sudden.