Anterior cruciate ligament (ACL) injuries are caused when the knee is straightened beyond its normal limits (hyperextended), twisted, or bent side to side.

Typical situations that can lead to ACL injuries include:

• Changing direction quickly or cutting around an obstacle or another player with one foot solidly planted on the ground. (This can happen in sports that put high demand on the ACL such as basketball, football, soccer, skiing, and gymnastics.)
• Landing after a jump with a sudden slowing down, especially if the leg is straight or slightly bent (such as in basketball).
• Falling off a ladder, stepping off a curb, jumping from a moderate or extreme height, stepping into a hole, or missing a step when walking down a staircase. Injuries like these tend to be caused by stopping suddenly, with the leg straight or slightly bent.

Inactive people and some older adults who have weak leg muscles may injure their knees during normal daily activities. But they usually injure bones, not ligaments.

When contact causes an ACL injury, it can be from playing a sport, from a sudden and severe accident, or from less obvious contact injuries. In football, receiving a clipping contact injury-in which the bent knee is struck from the outside-can cause an ACL injury.