ABC News' Ann Reynolds reports:

Angela Owens said she was "shocked" when 9-year-old daughter Isabella blew out her knee while coming down from the high-bar during gymnastics practice.

Isabella spent a year in physical therapy after undergoing ACL surgery.

"It was something not even on my radar," Owens said. "This is what happens to football players. … I thought she had a sore knee. I wouldn't have even anticipated this was something that could have happened."

The anterior cruciate ligament (ACL) tear is an injury that professional athletes fear. It was the reason behind Lindsey Vonn's absence from the Sochi Olympics, and it cost New England Patriots quarterback Tom Brady most of the 2008 season.

RELATED: Tom Brady sheds light on rise in ACL tears in children.

Doctors say they are seeing more torn ACL cases in elementary schools, from children playing tackle football to doing gymnastics, and the numbers are so large that pediatric orthopedic surgeons like Dr. Daniel Green at New York's Hospital for Special Surgery are calling it an epidemic.

"We recently looked at the database in New York and found a five- to six-fold increase over the last 20 years of ACL surgeries in children," Green said.

The ACL, the main stabilizing ligament of the knee, is not strong enough to take the strain of repeated movements by itself; the whole knee must be strong.

Children who play the same sport repeatedly - doing just a few moves over and over again - are at risk and girls even more so. They are six times more likely to tear their ACL than boys, according to the PEP Program: Prevent Injury and Enhance Performance, which has developed exercise routines to lower the risk.

New data says there's a simple solution that reduces the risks: warm-up exercises to strengthen the muscles around the knee, no matter what sport.

They involve jumping, stretching and flexing, simulating the movements that children used to do on playgrounds and in backyards.

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Isabella, now 10, has been working with a physical therapist at the Hospital for Special Surgery.

"The physical therapists that I've had. … They noticed when I landed, instead of … using my hips, I used my knees so they gave me some exercises so I wouldn't do that," she said.

The exercises take about 15 to 20 minutes and can be done before practice, according to the PEP program, which was developed by the Santa Monica Sports Medicine Research Foundation, and the FIFA 11+ warm-up program.

Such warm-ups, according to one study, reduce childhood ACL tears by 74 percent. Now the goal is persuading coaches across the United States to adopt them.