Be careful of your knees, girls.

A new medical study has found a rise in injuries to one of the knee's four major connective tissues, or ligaments. Known as the anterior cruciate ligament (ACL), this tissue runs diagonally across the knees. It helps hold the bones of your leg together. Serious damage occurs when the ACL is stretched too far or torn.

Girls, it turns out, are more likely than boys to get this injury.

The new study compared girls and boys playing the same sport. It found that girls are 2.5 to 6.2 times more likely to have an ACL injury.

About 70 percent of these injuries happen when there is no contact with another player at all.

"Usually, you feel a pop in the knee and then see swelling," says Randall Schultz, a doctor who works on bones and joints. "Sometimes it will hurt - other times, the pain might not be that bad."

"It's not something you have to go to the emergency room for," he says. "However, you should see a doctor the next day or after the weekend is over to see what's going on and what needs to be done. In the meantime, put ice on it and use crutches to try to stay off it," Schultz says.

Jump. Turn. Stop. Kick. Pain.

Sports where kids stop suddenly and turn a different direction usually are the cause. Such sports include soccer, gymnastics, volleyball and basketball. Boys also saw problems with football which, unlike the others, is a contact sport. Doctors see injuries arising from all of these sports, along with cheerleading and dance.

The risk goes up for girls once they hit age 12 or 13 and for boys around age 14 or 15. Why? Puberty. The teen years are a time when kids grow faster, and their bodies can't always keep up. This is especially true of the leg's tibia and femur bones, the two bones on the front of the leg.

For girls, hormones also make the ligaments looser. Because their muscles are growing in size and strength, teen boys are more able to handle the quick bone growth.

An ACL Injury Can Sideline An Athlete

The number of athletic kids with ACL injuries is on the rise because of a change in how they play sports: More kids these days are focusing on one sport intensely all year, with few breaks. In the past, kids played a variety of sports with time off in between seasons.

The treatment now is also different. It used to be that you would simply keep the knee from moving, but now ACL tears require an operation to replace the ligament. After the operation, athletes must wait six months before returning to sports.

Even with treatment, an ACL injury can have long-lasting effects, however. Athletes who had an ACL injury are 10 times more likely to develop arthritis in their knee than those who never had one.

"I have daughters that play sports - I'm worried about it," Schultz says.

An earlier study looked at athletes older than 18 with an ACL injury. It found that around half were never able to play as well as they had before their injury.

Once you have an ACL tear, you are 15 percent more likely to have another.

Choices In Sports Training

So, what does all this research mean for parents, and for their daughters (and sons)?

Consider choosing sports programs with an athletic trainer who is leading kids through knee-saving exercises.

Physical therapist Bethany Thoresen says prevention is all about strengthening the knees. "It's about doing exercises that are going to strengthen the hips and leg muscles. Pilates is great for that," she says.

Thoresen also recommends that kids who are active in sports see a physical therapist or trainer before an injury happens. The professional will look at how kids jump and and at how much control they have over their muscles. It's good to be able to identify areas of weakness. Trainers or therapists will then show kids how to do exercises to strengthen those weak areas.

While you can download exercise videos, don't leave it at that. Thoresen added: To prevent injury, a professional should check to make sure the exercises are being done properly.

Parents also should get realistic about how much sports their kids are doing. They should have days off and should be doing different activities. If a child is doing the same sport all year, she is overusing some muscles and not strengthening others.

"It's too much," Thoresen says. "They are not ready for it."