Institute of Human Anatomy on Instagram: "THE ACL: Your Knee's Front-Line Defense Against Disaster This is what your ACL (anterior cruciate ligament) is actually doing inside your knee joint - and why tearing it is such a big deal. PRIMARY JOB: Stop Forward Slide The ACL prevents "anterior translation" - fancy terms for stopping your tibia (shin bone) from sliding too far forward relative to your femur. Watch how it tightens when we pull the tibia forward on this real cadaver dissection. SECONDA