What the ACL is
The ACL is one of four main ligaments in the knee. Ligaments are strong, flexible bands of tissue that connect bones together. The ACL is located deep inside the knee joint and runs diagonally from the top of the shinbone (tibia) to the bottom of the thighbone (femur). It is about the thickness of a pencil and roughly two inches long.
How the ACL works
The ACL works like a rope that holds the knee stable. When you move your leg, the ACL prevents the shinbone from sliding forward in front of the thighbone. It also helps control the rotation of the knee when you twist or turn. Without a healthy ACL, the knee becomes loose and unstable, making it difficult to move with confidence or play sports.
Common ACL injuries
ACL injuries happen most often in sports that involve sudden stops, jumps, or changes in direction, such as basketball, soccer, and football. An ACL tear can occur when the knee is twisted awkwardly or bent backward. A torn ACL causes immediate pain, swelling, and a feeling that the knee is giving out or becoming unstable.
Treatment and recovery
A partially torn or minor ACL injury might heal with rest, ice, compression, and physical therapy. However, a completely torn ACL usually requires surgery to reconstruct it using a graft from another tendon or ligament. After surgery, patients typically need 6 to 12 months of rehabilitation exercises and physical therapy to regain strength and return to normal activities.
Prevention
You can help protect your ACL by maintaining strong leg muscles, using proper techniques during sports, wearing appropriate footwear, and doing warm-up exercises before physical activity. Strengthening your quadriceps, hamstrings, and core muscles helps support the knee and reduces injury risk. 
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