Between 2008 and 2010, researchers found, U.S. high school athletes suffered concussions at a rate of 2.5 for every 10,000 times they hit the playing field, for practice or competition.

Nearly half -- 47 percent -- happened in football. But girls' soccer and basketball, and boys' wrestling, ice hockey and lacrosse were among the other sports with a risk of head injury.

Just over eight percent of all concussions happened in girls' soccer, while girls' basketball and boys' wrestling accounted for almost six percent each.

Boys' ice hockey accounted for fewer total concussions, but it beat all other sports when it came to proportion: Of all injuries to boys in ice hockey, 22 percent were concussions.

Most of the head injuries happened when players collided with each other. But even some kids in non-contact sports -- like softball, gymnastics, cheerleading and swimming -- suffered blows to the head.

"They're not just occurring in full-contact sports," said lead researcher Natalie McIlvain, of the Research Institute at Nationwide Children's Hospital in Columbus, Ohio.

The findings are not meant to alarm parents, she and her colleagues say, since the benefits of sports far outweigh the risk of concussion.

But parents, coaches and kids alike should be aware of the risk, and the symptoms of concussion, said Christy Collins, a senior research associate at Nationwide Children's.

"It's very important for kids to be involved in sports," Collins said. "But at the same time we have to keep them safe."

The findings, reported in the American Journal of Sports Medicine, come at a time of increasing concern about concussions in young athletes.

Estimates of the number of Americans suffering sports-related concussions have been climbing in recent years. That's partly because more people are playing contact sports, young athletes are training more aggressively at an earlier age, and doctors are more aggressive about diagnosing concussions.

A recent study found that in 2008, there were five concussions for every 10,000 U.S. high school athletes who hit the playing field. That was up from just about one per 10,000 a decade earlier.

Last year, the American Academy of Pediatrics (AAP) recommended that kids with concussions get cleared by a doctor before they can get back on the playing field.

But that also means coaches, parents and kids need to know the signs of a concussion, McIlvain and Collins said.

Concussion symptoms include headache, dizziness, nausea, ringing in the ears, fatigue and confusion -- though these problems may not become noticeable until hours after the jolt to the head. And contrary to popular belief, concussions usually do not involve loss of consciousness.

Among the findings in the new study was a higher concussion risk among girls than boys.

In "gender-comparable" sports, girls had a 70 percent higher concussion rate than boys.

It's not clear why. But, Collins said, it may have to do with girls' lesser neck strength. Concussions happen when an impact is strong enough to jar the brain within the skull; it's thought that greater neck strength might offer some protection -- though that's still under study.

Once a concussion is recognized, the AAP says that kids should rest, both physically and mentally. Schoolwork and other mental tasks, even video game playing, can make symptoms worse -- so athletes with a concussion may need a break from non-physical activities as well.

The biggest concern is that injured kids not return to the game until their symptoms have fully passed.

If an athlete suffers another head injury before the initial concussion has healed, there's a risk of "second-impact syndrome" -- which can have severe consequences, including bleeding inside the skull, brain swelling and death.

"The real danger," Collins said, "is returning to play too soon."

SOURCE: bit.ly/z2ITzw American Journal of Sports Medicine, online January 27, 2012.

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