Research shows that when it comes to preventing certain knee injuries it may be more helpful to use your brain than your legs.

In a study published in 2009 in the Medicine and Science in Sports and Exercise, researchers at the University of Michigan had people perform squats on just one leg until that leg was shaky and tired.

Researchers find it useful to experimentally wear down leg muscles because most injuries occur when athletes are tired and wobbly.

Then they tested study participant’s reactions to various jumping and movement commands. They discovered that both legs – not just the fatigued one – showed the same diminished responses that had the potential to lead to injury of the anterior cruciate ligament (ACL).

The ACL is one of the four major ligaments that support and stabilize the knee joint. No one is quite sure why, but ACL injuries are on the rise, and they frequently lead to knee osteoarthritis later in life.

“The fact that we didn’t fatigue that leg but it still showed big changes highlighted that the brain was the thing that was getting tired,” says Scott McClean, PhD, the lead researcher and an assistant professor with Michigan’s School of Kinesiology and director of the University's biomechanics laboratory. “It was the brain that got really tired and couldn't make appropriate decisions or send the information down, even to the limb that wasn’t tired.”

McClean says that until now, research and prevention strategies for ACL injuries have focused on strengthening and training the muscles that support the knee joint, often by practicing simplistic movements in a controlled setting.

“We teach them to jump off a box and they get good at that, but it doesn’t replicate the randomness of sport, and our goal is to do that more effectively,” he says.

Michigan researchers say their study shows it would be far more effective to use virtual reality technology to train the brain to react to random, unexpected and realistic athletic scenarios – essentially training the brain and reflexive responses to counter fatigue and therefore ACL injury risk.

“In a virtual environment, you can replicate different scenarios and by doing that, our goal is to teach the brain to develop a strategy to pick up on cues more quickly and respond more effectively,” McClean says. “It’s similar to driving a stick shift car. If you do things over and over and over, all of a sudden it becomes more automated.”

McClean says while it's likely easier for professional athletes to gain access to expensive virtual reality technology now, in time he believes it will be available to the general public because several companies are working on relatively cheap training components.

But William Garrett, MD, an orthopedic surgeon and professor of orthopedics at Duke University Medical Center says he thinks this study may be interesting to athletes, but it would not seem to apply to the majority of ACL injuries he treats.

“Most ACL injuries I see aren't even during fatigue. They are slip-ups or a bad landing or a bad twist or turn. Something that you're not expecting to happen,” he says. “Rarely is someone fully fatigued when this happens.”

Dr. Garrett also says he doesn't think virtual reality training is very practical. “I see little evidence to think that it will (help),” he says. “We don't know that you can change motor control for both knees at the same time and I certainly don’t know that you can do it from virtual reality training."

In a related paper that appears online at the British Journal of Sports Medicine, McClean and his team of researchers tested the single leg landing of 13 men and 13 women, after working their legs to the point of fatigue. While they found that fatigue made both men and women sloppy in their landings, women tended to be most affected by being tired, landing more often at dangerous angles that increased their chances of injury.

Scientists say the effects of fatigue on women may explain why they are two to eight times more likely to tear the ligament than men while playing the same sport.

McClean says these results are startling because the incidence of ACL injuries peaks in teenagers between ages 15 and 18, and people who tear their ACLs often develop knee osteoarthritis within five years of their injury.

“It's a short term problem but long term as well because of the debilitation. That indicates we may have men and women in their forties in the near future needing knee replacements,” he says. “So that's the driving force behind it. That's why we want to stop these [injuries] so much.”