When it comes to knee injuries, none are more common than ones related to the anterior cruciate ligament, commonly called the ACL. This major band of supportive tissue in the knee is injured so frequently that research available at the National Library of Medicine shows that nearly 1 out of every 3,500 people experience a tear, sprain or break in the area at least once a year, causing some 400,000 ACL surgeries to take place annually in the United States.

But ACL injuries can occur for a wide variety of reasons, making it helpful to know what's behind the most common ones. 

Where is your ACL? 

The ACL is one of four main ligaments within the knee, and the one that provides the most "stability to the knee joint," says Dr. Anikar Chhabra, an orthopedic surgeon, sports medicine specialist, and chair of the division of sports medicine for Mayo Clinic in Arizona. He explains that the ligament is composed of two areas known as the anteromedial bundle, "which controls anterior and posterior stability," and the posterolateral bundle, "which is important for rotational stability."

The ACL is located inside the knee joint, and connects the femur - called the thigh bone, to the tibia - called the shin bone. The patella - called the kneecap, is located in front of the ACL and helps protect it and the other ligaments that run through the center of the knee. 

Why are ACL injuries so common?

Injuries to the ACL happen for many reasons but are especially common in sports that involve a sudden change of direction such as with soccer, gymnastics, basketball, or football. ACL injuries can also occur as a result of car or bike accidents and in sports and activities where one experiences a direct impact to the knee such as a football player being tackled "low and from behind," or from an athlete crashing into a stable object, "such as a skier’s knee running against a tree," says Dr. Loren Fishman, a professor of physical medicine and rehabilitation at Columbia University,

ACL injuries can also occur when "jumping and landing while a knee is still extended or from twisting your knee while your foot is still on the ground," explains Paul Schroeder, a physical therapist and spokesperson for the American Physical Therapy Association. 

Beyond sports-induced injuries, Chhabra says that other issues can contribute to an ACL tear such as malalignment problems or offsetting notch sizes of the kneecap, excess weight constantly bearing down on the area, hormonal changes, or from someone having ligamentous laxity. 

Women are also known to have "up to an eight times increased risk of sustaining an ACL injury compared to males," he adds. This is primarily because of women having less protective muscle mass around the knee and because women's joints typically have more looseness and greater range of motion than the joints of men. 

How can ACL injuries be prevented?

While it isn't possible to completely avoid an ACL injury, there are ways to minimize risk. Fishman recommends avoiding movements or activities that can cause any sudden stress to the knee and he suggests keeping one's hamstrings strong, "which tend to keep the tibia back, assisting the ACL with its work."

Schroeder similarly recommends strengthening the tibia but adds that it's also important to strengthen all other muscles within one's hips, legs and knees. Exercises that can help with targeting these muscles include leg lifts, lunges, squats, lateral walks, mountain climbers, and Pilates and yoga movements and positions.

Other suggestions to lower sports-related ACL injuries include "learning and executing proper form and techniques," offers Chhabra, wearing appropriate shoes that "provide good support and help maintain knee stability," and avoiding sports specialization, "as this can lead to overtraining, overuse, and muscle and joint fatigue." 

How can I tell if I tore my ACL?

If you feel pain in your knee, lose range of motion, feel unstable, hear a popping noise, experience swelling or difficulty walking, or have any other reason to suspect an ACL injury, "it's important to see a sports medicine or orthopedic specialist," Chhabra recommends. He says such providers can examine the area and use X-rays and other equipment to diagnose any problems. 

"Once an ACL tear has been discovered, you would work with a surgeon and physical therapist to determine if surgery is necessary, and, if so, what that recovery will look like," says Schroeder. "Whether you have surgery or not, your physical therapist will work with you to restore your muscle strength and your gait and balance, so you can return to activity once healed."