Girls, be careful with your knees.

A new study shows that 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.

It turns out that girls are more likely than boys to suffer this injury.

The new study compared girls and boys playing the same sport. It found that girls are two to six times more likely to hurt their ACL than boys.

Seven out of 10 injuries happen without kids even touching another player.

Knees Are Sensitive

"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 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.

Sports where players 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. 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. 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. Teen boys generally see an increase in the size and strength of their muscles. This makes them more able to handle the quick growth of their bones.

Bouncing Back May Take Awhile

The number of 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 several different sports. They also took time off in between seasons.

The treatment now is also different. It used to be that you would simply keep the knee from moving. Now, if an ACL tears, doctors put in a new ligament. Afterward, the athlete must wait six months before returning to sports.

An ACL injury can have long-lasting effects. Athletes who've had one are very likely to develop arthritis in the knee. Arthritis would make the knee swollen and painful to use. Athletes with an ACL injury are 10 times more likely to develop arthritis than athletes who never had an ACL injury.

"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 again 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.

Be Sure And Warm Up The Muscles

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

One good idea is to look for a coach who does warm-ups with knee-saving exercises.

Bethany Thoresen is an expert when it comes to helpful exercises. 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. They should have days off and 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."