If your child isn't physically active, he or she is more likely to have emotional and behavioral problems, according to a study published in the October issue of the journal of the American College of Sports Medicine.

The study, by Finnish researchers, was of 7,000 15- and 16-year-old boys and girls, who answered questions on a survey about their level of physical activity and their mental and emotional states.

Boys who reported less than an hour of vigorous physical activity a week were more likely to be depressed and withdrawn than those who exercised regularly.

Girls who exercised rarely also were more likely to be depressed, the Finnish researchers said. In addition, inactive girls were more likely to report rule-breaking behaviors.

The study "specified a group we don't have much information about," said Dr. Glen Getz, a child and adolescent psychologist who heads the KidsShape program at Allegheny General Hospital. But he wasn't surprised by the results.

"They were looking at adolescents, but the same is true for small children, and for adults," Dr. Getz said.

Research she's conducted supports the findings of the Finnish study, said Dr. Aimee Kimball, the director of mental training at UPMC's Center for Sports Medicine.

"I just completed a study of student athletes across Pennsylvania," she said. "The majority of people in the study thought sports was a good way to keep kids out of trouble."

There are three reasons why exercise is good for the mental and emotional as well as the physical health of young people, Dr. Getz said.

"Neurophysiologically, we know that exercise is linked to serotonin release and endorphins, which give you a natural high," he said. "That in itself will decrease emotional and behavioral problems."

"Exercise leads to higher self esteem," Dr. Getz said. "That will obviously improve mood."

Finally, he said, "just being part of a group, a feeling of belonging, helps kids feel less depressed and sad."

Exercise isn't just good for children when they are children, Dr. Kimball said.

"Developing healthy life habits when you're young produces lifetime benefits, not just physical but emotional," she said.

How much exercise is enough? According to the guidelines established by the American College of Sports Medicine and the American Heart Association, children under the age of 18 should be active for at least an hour a day.