The ACL is the ligament that connects the femur (thigh bone) and tibia (shin bone) inside the knee joint. Ligaments are tough, non-stretchable fibers that hold bones together. The ACL, along with the posterior cruciate ligament (PCL), medial collateral ligament (MCL), and menisci (cartilages), helps keep the knee stable and helps protect the knee from shifting, rotating, and hyperextending during running, jumping, or landing.