Whether children can and should participate in strength training has been a contentious issue. But new research suggests it is safe and may encourage young people to be more active in their everyday lives.

Researchers randomly assigned one group of 10- to 14-year-olds to strength train twice a week and others to go to their typical gym classes.

After a few months, kids who did squats, crunches and bench presses were stronger than their classmates. And boys who did strength training had upped their weekly exercise by 10 percent.

"The initial idea was that training increases children's motivation to be physically active," said Dr. Udo Meinhardt. He led the study at the PEZZ Center for Pediatric Endocrinology in Zurich, Switzerland.

He said the program was both simple and cheap to put in place.

"Our message would clearly be that yes, strength training should be a part of the gym class setting," Meinhardt told Reuters Health.

His team's study included 102 children from fifth- and seventh-grade classes. Kids who were randomly picked for strength training lifted twice a week for 45 minutes when they would otherwise have gym class.

There were no injuries during the study period. Both boys and girls improved their leg and arm strength after 19 weeks of training compared to those who went to gym class.

The youths wore devices to measure how much they moved over a week-long period before the program and again just after it ended.

Boys burned more energy after completing the program. The 10-percent difference is equivalent to an extra weekly 28-mile bike ride, the researchers write in Pediatrics.

But there was no difference in how much exercise girls got before and after strength training. Researchers said it's possible the effect was different because more girls had already entered puberty - so hormone changes may have played a role in their activity levels.

A few months after the training program ended, boys' activity levels had gone back to normal as well. That suggests young people need to keep lifting to see long-term benefits, Meinhardt said.

"I believe that exercise honestly makes you feel better … and maybe that's a part of it," Dr. Cordelia Carter said of the benefits among boys.

"That the stronger your muscles are, the more fun it is to use them. And once you feel that good post-exercise, you might want to continue to do it," she told Reuters Health.

Carter is a pediatric orthopedist at the Yale School of Medicine in New Haven, Connecticut. She has studied strength training among kids but wasn't involved in the new research.

She said one of the most important findings from this study and others is that strength training is safe for young people, with proper technique and supervision.

In this study, weights were gradually introduced once children had learned the correct technique.

"It should almost never be about how much you can lift," Carter said.

"Teaching strength training and teaching it safely in a supervised setting is important and I think it absolutely should be incorporated into our regular physical education," she added.

Carter said parents who feel they can properly supervise their kids and teach good technique can encourage strength training at home. Or they can hire a trainer or coach for a couple of sessions to make sure children learn the right way to train.