Running around and exercising after school at the Georgia Prevention Institute at the Medical College of Georgia must seem like a welcome break for Imone Hill, a fifth-grader at Tobacco Road Elementary School. With dance classes and church choir practice, she doesn't make it home until 8 p.m. sometimes.

"I kind of feel emotionally better," said Imone, who turned 11 on Wednesday. "I kind of feel calmed down, letting everything go."

It may be more than just stress relief. Researchers at MCG found that overweight sedentary kids who participated in an after-school exercise program were less depressed and had higher self-esteem than those who didn't. More importantly, the research showed a "dose response" effect from the exercise, with 40 minutes of exercise eliciting greater results than 20 minutes.

The study, published online in the Journal of Pediatric Psychology , is the first to show a measurable dose response from exercise on depression, according to the MCG researchers. Catherine Davis, one of the study authors, said it provides strong proof that exercise was the cause of the improvement and not something else.

"If A actually causes B, then more of A should cause more of B," which is what they found, she said.

Those who exercised 40 minutes also had an increase in their sense of their physical appearance and self-worth, the study found. The researchers had included the program with 20 minutes of exercise because it would be easier for schools to implement, said the study's lead author, Karen Petty, a postdoctoral fellow in psychology at MCG.

"And the results there weren't as exciting as we had hoped, but still I think that even with the 40-minute group, programs could certainly be designed around that," she said, "or at least with that in mind, knowing that it is going to help with depressive symptoms and with self-esteem as well."

In fact, the vast majority of the children in the study would be considered psychologically normal and not depressed, Dr. Davis said.

"And yet, when they were exercising, their mood did seem to improve," she said.

The idea would be to use exercise more as a preventive measure to help children avoid developing depression and to improve self-esteem, Dr. Davis said.

Children who are overweight are more prone to depression and poor self-esteem, and those conditions can feed off each other, Dr. Petty said.

"Certainly when you're depressed and you don't feel very good about yourself you don't feel like getting up and exercising or being active," she said. "You kind of sit on the couch, maybe eat a little bit more, tend to eat comfort foods, that kind of thing. That leads to more overweight, more feelings of depression, etc. It certainly goes hand in hand."

The researchers hope their results will help convince school officials and others that adding physical activity programs is worthwhile.

And it could extend beyond that to the community as a whole, they said.

"The main thing that I would like to see is that we as a society take responsibility for making sure that our children are encouraged to engage in a healthy amount of physical activity in an environment that is encouraging to people," Dr. Davis said, "so that they learn to enjoy it and as a result they remain healthy and happy."

"Certainly children are going to be more likely to do things if parents are involved in doing things as well," Dr. Petty said. "It would be nice to see a bigger outreach as far as these things go."