A turning, off-balance skier leaning backwards is a recipe for knee disaster, according to a new report from a panel of sports medicine and skiing experts.

The group picked apart videos of 20 World Cup skiers who had suffered an anterior cruciate ligament (ACL) tear - an injury that wipes the skier out for a year and can have health consequences down the road.

“Prior to this study we really had no idea how these potentially devastating injuries, the ACL injuries, actually happen in World Cup skiers,” said Dr. Roald Bahr, one of the report’s authors from the Norwegian School of Sport Sciences in Oslo, Norway.

“Most people probably think that they happen as the skier is crashing, tumbling down the course,” he told Reuters Health. “As the study really shows, the ligament is torn while the skier is still skiing, and then they crash.”

Coming to that conclusion wasn’t necessarily enjoyable. The experts each had to review videos of the run that led to the injury on their own to determine the exact moment of the ACL tear, then came to a consensus. After that, they recorded all the details of the moment of injury: the skier’s behavior and situation, as well as the angles of joints and positions of limbs.

When you do that 20 times over, Bahr said, “the patterns are remarkably consistent. Once you’ve spotted what actually happens, it becomes very, very clear that there’s a consistent pattern here, the same thing happening over and over again.”

Here’s the stickiest situation: a skier is trying to make a turn on the course, but leaning too far backwards and inwards into the turn while off-balance. That causes his outer ski to lift off the snow. When the skier tries to reach out with his leg to get the ski back on the ground, the very back of the ski hits the snow, pulling the leg with it and rotating the lower leg.

The force on the knee caused by that rotation is too much for the ACL - one of four ligaments connecting the thighbone to the shinbone - to take.

ACL tears also happened when skiers were forced into a split, when their inner ski hooked on to a gate during a turn, and when they landed on the back of their skis after a jump.

Bahr and his colleagues observed the injuries in skiers in the downhill, slalom, giant slalom, and super-G events.

Bahr said that wet and slushy snow, more common late in the ski season, makes it more likely for the back of the ski to catch on the snow, upping a skier’s chance of ACL injury.

And although those injuries often heal up within a year, he explained that people who have suffered ACL tears have a much higher risk of getting arthritis in the future, regardless of the treatment they get.

The findings of the new report, published in the American Journal of Sports Medicine, may have implications for preventing future ACL injuries.

“Perhaps it is possible to train the skiers to recognize that position and bail out in time,” Bahr said.

Although the study only included world-class athletes, the same injuries happen to recreational skiers, he added.

“A lot of recreational skiers believe that the equipment they rent in the ski rental…will protect them from having injuries if they fall,” he said. “But the truth is, although it does protect against other types of injuries, it doesn’t protect the knee from a ligament injury.”

The message is to “learn how to fall and weight the downhill ski, even when you are afraid,” Dr. Michael Tuggy, a physician at Swedish Medical Center in Seattle and a former ski patroller told Reuters Health in an email.

“The natural tendency is to fall back into the hill, off the downhill ski, which is the perfect setup for an ACL tear.”

SOURCE: American Journal of Sports Medicine, online April 22, 2011.