By Cary Groner
Injuries or ruptures of the anterior cruciate ligament (ACL) of the knee have long plagued athletes involved in sports that require heavy doses of running, jumping and rapid change of direction (such as skiing and soccer). Recently, researchers have begun to clarify the factors that may put athletes at risk for noncontact ACL injuries. Some risk factors can’t be changed easily, if at all (for example, gender). But other risks are modifiable, and clinicians and trainers have designed training programs that significantly reduce ACL injury rates.
Several forces conspire together when someone tears this crucial knee-stabilizing ligament. First, as the athlete steps, cuts direction or lands, the femur moves inward toward the center line of the body — a process known as adduction — while simultaneously rotating inward as well. This tightens the medial collateral ligament (MCL) and puts more stress on the outside of the knee joint. This, in turn, stresses and affects the alignment of the primary lower-leg bone, the tibia. All of these body movements happening at the same time can tear or rupture the ACL.
Women suffer about 38,000 ACL injuries in the United States every year, according to the National Institutes of Health — that makes them about five to eight times more likely to injure or tear their ACLs than men.
The injury happens the same way to both men and women, but several factors make women more vulnerable. First, because of the structure of their hips, women are already predisposed to that knock-kneed position (also called knee valgus) when they land or change direction. Second, women’s ligaments are naturally more lax than men’s, which makes them more prone to overstretching or tearing. Third, women have a narrower femoral notch, meaning there’s less space for the ACL to run through near the bottom of the femur. Fourth, the muscles that stabilize the position of the femur — and subsequently the knee — are located primarily in the hip and trunk, and women tend not to recruit those muscles as efficiently as men do. Other factors, such as the degree of knee extension, play a role as well.
Clinicians have tried a number of ways to reduce the risk of ACL injury, with mixed results. These include:
Bracing doesn’t usually offer much help. For one thing, the brace is typically oriented to the outside of the knee, but the problem with a valgus knee is that the knee is caving inward, and there’s not much a brace can do about that. Braces also have to contend with a lot of muscle and fat above the knee, which tends to make them slip around and prevents them from controlling the knee’s motion. Braces do affect proprioception — they give the wearer a better sense of where the knee joint is in space — and that can build confidence and improve performance to some degree. But their ability to affect the forces that cause ACL injuries is limited.
Foot orthoses show more promise. People who are predisposed to valgus knee also tend to roll their foot inward when it hits the ground, and some research suggests that controlling that foot movement (or pronation) with an in-shoe orthosis can affect the way the entire leg operates. First, it helps prevent the shin bone (tibia) from rotating inward; this, in turn, keeps the knee from moving so far into that valgus position, which decreases the associated stresses on the ACL. One study in female college basketball players found that those who didn’t wear orthoses injured their ACLs seven times more often that those who did. Most clinicians and trainers stress that those who wear orthoses should also do exercises to strengthen and stabilize the legs and feet.
Exercise and Training
Exercise therapy holds the greatest promise for preventing ACL injuries. Research has shown that, in addition to the poor recruitment of hip and trunk muscles, female athletes also tend to rely more on their quadriceps for stability than males do. The ratio of hamstrings-to-quadriceps strength (the H/Q ratio) is an important predictor of ACL injury risk. It’s also fairly easy to fix.
Trainers and physical therapists have developed several regimens that address H/Q ratio as well as the neuromuscular discrepancies between men and women that account for their different recruitment of hip and trunk muscles (for example, the gluteal muscles). This neuromuscular approach includes a variety of jumps, hops and bounding maneuvers; it also includes resistance and speed training, as well as core strengthening and balance work.
The results of studies where these methods were implemented have been impressive. They’ve been shown to lower torque forces at the knee and to decrease knee injury in adolescent female athletes by a factor greater than three. In one case, ACL sprains were 75 percent lower in those who did the exercises versus those who did not.
Men can benefit from these training regimens as well, but because of the biomechanical issues to which women are predisposed, research shows that women have the most to gain. If you’re concerned about the risk of ACL injuries, talk to your doctor, your physical therapist or your trainer. If they’re up to date, they’ll be able to assess your risk and come up with a training program to help you minimize it.
Published May 21, 2012.
Cary Groner is a writer based in the San Francisco Bay Area. His recent novel, Exiles, placed fourth on the Chicago Tribune’s list of the best books of 2011.