Have you ever heard of the phantom foot? No, it’s not a new superhero movie coming out this summer. It’s not even the punch line of a Rex Ryan joke. It’s a concept that might save you from an ACL injury the next time you’re skiing.

Tears of the anterior cruciate ligament (ACL) are among the most feared sporting injuries because of their nasty combination of frequency and severity. The ACL is a tiny ligament, not much bigger than your index finger. Despite its diminutive size, it has an essential role in holding your knee together. It is the most important connector between the thigh and leg, and the ACL allows us to use our knees to bend, twist and change directions by stabilizing the joint.

This little ligament is vulnerable, too. Tears of the ACL are among the most common knee injuries, especially in sports that involve a lot of rapid direction changes like skiing. In fact, 20,000 skiers tear their ACLs each year.

Once the ACL is injured, it does not heal on its own. Surgery is required to replace it. In fact, this is the injury that sidelined Tom Brady and Wes Welker of the New England Patriots in recent seasons. An ACL injury involves a difficult recovery that typically takes about six months.

So what does this have to do with staying safe on the slopes? Well, researchers led by Dr. Robert Johnson at the University of Vermont studied hundreds of ACL injuries and discovered some interesting similarities. It turns out a consistent mechanism causes skiers’ ACLs to be injured.

Dr. Johnson calls this mechanism the phantom foot. By recognizing the components of the phantom foot, skiers can decrease their chances of tearing their ACLs.

The ACL is often injured by forces acting on it from the tail of the ski and the stiff boot top. These components together act like a “phantom foot”, levering the knee into a position of injury. They twist and bend the knee in ways that put the ACL at risk of tearing.

The most dangerous situations occur when a skier is leaning too far back. This most commonly happens after a skier has fallen backward or when they are trying to get up while still moving after having fallen to the rear.

The skier’s hips fall below the knees and the uphill ski becomes unweighted. All of the weight is transferred onto the tail of the downhill ski. Often the ACL gives way at this point.

Dr. Johnson found he could significantly reduce ACL injuries by training skiers to fall forward, to keep their hands over their skis, their arms forward and their feet together. He advised skiers to fall in a controlled way, bailing out to the front rather than fighting to stay upright.

In fact, ski areas that trained their patrollers according to Dr. Johnson’s program decreased their ACL injury rates by around 50 percent.

Paying attention to some common bad habits in skiing technique can also help prevent the phantom foot. Keeping your uphill arm to the rear, letting your hips fall below the knees and skiing off balance leaning backward all are common bad habits. Correcting them can further decrease your chances of an ACL injury if you fall.

It’s not necessary to take Dr. Johnson’s entire course in order to protect your ACL. Just think about the phantom foot next time you’re on the slopes.

Strive for more equal weight distribution across both skis. Practice falling forward rather than backward. And when falling is inevitable, don’t fight it. Let yourself fall in a controlled, safer way.

Dr. James Glazer is a sports medicine physician for Coastal Orthopedics and Sports Medicine in Freeport. He serves as a consultant for the Portland Pirates and the U.S. ski team.