Resistance training is good for kids, but needs to be supervised and tailored for them by a qualified professional, according to a consensus statement by 20 doctors and fitness experts.

The statement appears in the British Journal of Sports Medicine and is endorsed by the American Academy of Pediatrics, the International Federation of Sports Medicine and the U.S. National Collegiate Athletic Association (NCAA).

Resistance training - which can include light free weights, resistance bands or medicine balls, as well as push-ups and other body-weight-bearing exercises - can benefit young bodies and prepare them for active lives, the authors point out.

"You can look at the data from the Centers for Disease Control, kids are becoming less and less fit all the time," said Dr. Tim Hewett, director of the Sports Medicine Biodynamics Center at the University of Cincinnati, who was not involved in the statement. "There are lots of problems related to our current lifestyles."

Lack of strength and overall fitness, cardiovascular and musculoskeletal strength all contribute to poor health for kids, and resistance training can be a great option to help, but moderation is key, he said.

Though many parents fear their children will be injured during exercise or will damage their growing skeletons, childhood is actually the best time to use weight-bearing activity to build bone mass and kids are less likely to suffer sprains than adults, according to the statement.

Programs designed to promote conditioning and correct form for specific sports can also reduce the risk of injuries by 50 percent in kids and teens, the authors write.

"Clearly, participation in physical activity should not begin with competitive sport but should evolve out of preparatory fitness conditioning that is sensibly progressed over time," they write.

But these activities need to be supervised by "qualified professionals," like trainers or coaches, with strength and conditioning certifications.

These leaders need to know how to effectively communicate with kids and teens, as well as what exercises are best for kids and teens, and "how much is too much."

Position statements in support of resistance training like this one make it easier to inform parents and coaches who may still be hesitant, said Jeff Martin, who co-founded CrossFit Kids, a branch of the popular and sometimes controversial training program geared to kids from preschool age and up.

"With respect to resistance training, parents and coaches should always keep in mind that children are not simply miniature adults," Martin said.

Children, especially the youngest ones, benefit most from doing many reps of little or no weight, he said.

Hewett, a former power lifter, agreed with that premise, but expressed concern that CrossFit in particular may be too intense for kids.

"We have to get our kids more active but we can't overdo it," he said.

Kids need to have fun to be engaged, and can vary widely in their ability to pay attention and focus during sets of many reps of the same movement. When they drop weights, he said, they could injure themselves.

Martin recommends that preschool kids never use weights, and should focus instead on fundamental movements like squats.

The American Academy of Pediatrics recommends that eight year olds can start using light weights, but children mature at vastly different rates, Martin says, and some five year olds in his program start using very light objects or dumbbells.

"The key points are moderation, supervision and qualified professionals," Hewett said.

He and Martin agree that "qualified professionals" need more than just certifications, they need experience working with kids so they can assess a child's developmental and emotional maturity, since that can often be an important factor in how much is too much for each child.

"Parents should watch their children's trainers and see how the children react to them. Do they relate well and vice versa?" said Martin. "All the qualifications in the world will not help someone who cannot speak the language of the children or teens in their care."