Here’s a good reason to encourage your teenager to play more than one sport: New research finds kids who concentrate on only one sport may be at risk for stress fractures, tendinitis and knee injuries.

“It’s wonderful for a child to love a sport and to want to engage in it, but we must keep in mind the number of hours spent playing,” said study author Alison Field, a professor of epidemiology and pediatrics at Brown University. “They add up pretty quickly.”

The findings are a result of following more than 10,000 older children throughout the United States. The bottom line is that kids who do the most hours of intense activity per week, and that happens to be those focusing on a single sport, are the most likely to be injured.

Field hopes that coaches, parents and doctors urge children to engage in less intense, less specialized training.

The best advice is that kids should spend only a moderate amount of time in vigorous physical activity. If they have to specialize, they should replace some training with different types of exercise, such as yoga and conditioning, she suggested.

The risk for injury differed for girls and boys. For girls, no sport stood out as being extra risky. Specializing, however, increased girls’ risk of injury by about 30%.

Specialization did not significantly increase boys’ risk of injury, but baseball, gymnastics or cheerleading did increase the risk, the findings showed.

“There’s been a lot of concern about females having a higher risk of certain injuries,” Field said in a university news release. “The question is: Is that risk highest just as they’re going through their pubertal growth spurt, and then does it come back down a bit? And then we need to talk to coaches and trainers and say, ‘What can we do to mitigate that risk?'”

The report was published online recently in the Orthopaedic Journal of Sports Medicine.

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For more on children and sports, visit Nationwide Children’s Hospital.