Girls playing sports should be careful with their knees.

A new study shows there is reason for worry: More and more kids are seriously injuring a part of their knee. The part that's getting hurt is known as the anterior cruciate ligament (ACL).

The ACL is not a bone, but a ligament - something a bit like a rubber band. It helps hold the bones of your leg together. If the ACL is stretched too far or torn, serious problems result.

Girls have more reason to worry than boys, though. It turns out they're getting hurt more often.

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

You Don't Have To Tackle To Get Hurt

Most injuries happen without kids even touching another player.

Usually, you feel a pop in the knee and then see swelling, says doctor Randall Schultz. 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 soon afterward. In the meantime, put ice on it and use crutches to try to stay off it.

Sports where kids stop suddenly and turn in a different direction usually are the cause. Such sports include soccer, gymnastics, volleyball and basketball. Boys also saw problems with football which is a contact sport. Along with those sports, doctors also see injuries arising from cheerleading and dance.

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

With girls, the ligaments become looser during the teen years. With teen boys, the muscles generally become larger. This makes them more able to handle the quick growth of their bones.

An ACL Injury Can Stick Around A Long Time

The number of kids with ACL injuries is on the rise because of a change in how kids play sports: These days, more kids are focusing on one sport all year, with few breaks. Most kids used to play several different sports. They also took time off in between seasons.

An ACL injury can have long-lasting effects. Athletes who've had one are very likely to develop arthritis in their knee. In fact, they are 10 times more likely than athletes 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 long-lasting problems were common. About half the athletes were never able to play as well as they had before their injury.

Once you have an ACL injury, you also are more likely to have another.

Knee Exercises Are Important

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

Here's one good idea: Look for a coach who has kids do knee-saving exercises.

Bethany Thoresen is an exercise expert. She says the best way to prevent an ACL injury is to strengthen the knees. It's also important to strengthen the hips and leg muscles.

Parents also should get realistic about how much sports their children are doing. Kids should have days off and they should be playing different sports. 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."