Ligaments are strong bands of tissue that connect one bone to another. The ACL, one of two ligaments that cross in the middle of the knee, connects your thighbone (femur) to your shinbone (tibia) and helps stabilize your knee joint.
Most ACL injuries happen during sports and fitness activities. The ligament may tear when you slow down suddenly to change direction or pivot with your foot firmly planted, twisting or hyperextending your knee.
Landing awkwardly from a jump can also injure your ACL, as can falls during downhill skiing. A football tackle or motor vehicle accident also can cause an ACL injury. However, most ACL injuries occur without such contact.