No one wants to hear the distinctive pop of an ACL tearing or rupturing, which typically means the player's season is at an end. The ACL, or anterior cruciate ligament, is located inside the knee and provides stability during movements such as cutting, turning, twisting and jumping. Deanna Brinks, M.D., a physical medicine and rehabilitation and sports medicine physician at the Mayo Clinic Health System in Mankato, explains prevention and treatment.