The knee is a joint where three main bones join: the femur, or thigh bone; the tibia, or shin bone; and the patella, or knee cap. Several ligaments attach to the femur and tibia and give the joint strength and stability. One of these, the ACL, is in the center of the knee and limits rotation and the forward movement of the tibia.
The ACL is most often stretched or torn by a sudden twisting motion — when, for example, the feet are planted one way and the knees are turned another. A child can also injure the ACL by quickly changing the direction in which they are moving; by putting the brakes on too quickly when running; or, when landing from a jump. A woman's body structure and hormones cause more force on the ligaments, increasing the likelihood of injury during sports and athletic activities.