The position of the ACL in the knee is shown in the figure to the right. Its primary function is to prevent the tibia from sliding too far anteriorly on the tibia. ACL rupture renders the knee functionally unstable, and stability is restored by either reconstruction and rehabilitation, and sometimes rehabilitation alone. Either way, many months of rehabilitation are required for a successful return to sports.

Probably the most serious issue regarding the ACL-deficient knee is the tendency toward early onset of arthritis and progressive damage to the meniscus cartilage. The meniscus can be damaged at the same time as the original ACL injury, and it can also be repetitively damaged from instability episodes. The problem is that, lacking an ACL, the posterior horn of the medial meniscus becomes wedged in the joint as the tibia slides forward. Since the meniscus is such a vital structure for cushioning impact within the knee and promoting stability, its preservation is of utmost importance.

ACL rupture is usually dramatic. In a questionnaire of 90 athletes who had sustained ACL injuries, only three were able to keep playing immediately after injury. Unlike many other sports injuries, which usually involve contact with another player, most ACL injuries occur without direct impact to the knee. In video analysis of college players incurring non-contact ACL ruptures, the men were injured while landing from a jump, whereas about half the women were injured while landing from a jump, and the other half while stopping suddenly. Thus, risky maneuvers in basketball would include rebounding, turnovers, and when a player has to abruptly change her path, and those in soccer would include abrupt change of direction and stopping. Analysis of how male and female athletes perform these maneuvers has given us clues as to why there is a higher rate of ACL rupture in female athletes.

Girls and women tend to land from a jump, change direction and come to a stop from running while keeping the knee relatively straight, or extended, whereas males tend to have the knee flexed. In addition, there has been the surprising finding that athletically trained females are quadriceps dominant. This was illustrated in an experiment in which muscle reaction time for different leg muscle groups was measured in response to the application of an anterior translation motion applied to the tibia on the femur. In other words, the tibia is abruptly pulled forward at the knee, and the leg muscles’ reaction to this is measured. In athletically-trained women, the first muscle to generate peak force was the quadriceps group (anterior thigh), whereas in untrained females and all males, it was the hamstrings. Somehow, in the process of athletic training, female athletes change from being hamstring dominant to being quad dominant.

This has tremendous significance for the ACL, since anterior tibial translation (sliding the tibia forward) stresses the ACL. The hamstrings are positioned to counteract this motion, so they are normally protective for the ACL. Without proper firing of the hamstrings, ACL rupture becomes more likely. An additional surprising finding is that a person’s own quadriceps muscle group has the ability to rupture the ACL. As the knee moves into full extension, quadriceps contraction causes an anterior pull on the tibia, due to the angle formed by the patellar tendon (see figure below).

The scenario for a non-contact ACL rupture in a female athlete is therefore as follows: landing from a jump, changing direction or stopping suddenly occurs with too much knee extension and too little hamstring involvement and too little involvement of the hip muscles. There may also be a rotational force on the tibia, such as occurs when landing off balance. Quadriceps contraction exerts an excessive load on the ACL, causing its sudden failure.

Prevention strategies have focused on improving the biomechanics in female athletes. This includes learning to land, pivot and stop in more of a crouched posture, stopping in three steps instead of one, rounding out turns instead of cutting abruptly, as well as improving the strength of hip stabilizing muscles. One might expect that simply strengthening the hamstrings would be protective, but it is not. The problem is not that they are weak, but that their timing of contraction is incorrect.

Using such prevention techniques, some programs have been able to reduce the frequency of female ACL injuries to that of the males. This should be the goal of any such program, to correct the mechanical problems that are responsible for the gender difference in this serious injury.

Contact the Midwest Institute of Sports Medicine to learn more about prevention programs.