More than 1,400 sixth-graders had their strength tested with a hand-grip exercise. Overall, the stronger kids appeared to have a lower risk for heart disease and diabetes, according to the study, which was published online March 31 in the journal Pediatrics.
Greater strength was associated with lower levels of "bad" LDL cholesterol and triglycerides (another blood fat), and higher levels of "good" HDL cholesterol. The stronger kids also had slightly lower blood pressure and blood sugar.
What's more, the benefits were specifically linked to muscle strength. It wasn't just a matter of strong kids being thinner or more physically active, said researcher Paul Gordon, a professor of health, human performance and recreation at Baylor University.
"Even when you factor in these other things, that association with strength is still there," Gordon said.
The finding doesn't prove that stronger muscles directly lead to better health, Gordon said. Still, he said, "this sheds light on the fact that strength may be just as important a predictor of kids' [health] as aerobic fitness."
A pediatrician who reviewed the study agreed. "This suggests kids need a strong heart and strong muscles," said Dr. Bradley Berg, medical director of pediatrics at Scott & White Healthcare in Round Rock, Texas.
"I think the take-home is, kids should be well-rounded in their activities," Berg said. "Not just cardiovascular, and not just strengthening."
That doesn't mean your child needs to pump iron, Berg and Gordon said. Younger kids should lift only light weights, if any, until their bones are mature, Berg said.
Whatever their age, kids can increase their strength with any activity that involves "lifting their body weight against gravity," Gordon said. That can mean climbing -- on an indoor climbing wall or old-fashioned monkey bars on the playground -- or doing push-ups, for example.
Dr. Timothy Church, a professor of preventive medicine at Pennington Biomedical Research Center in Baton Rouge, La., said the activity a child participates in doesn't need to be weight-lifting. "It could be yoga," he said.
Church has studied the health effects of different forms of exercise in adults. He said the combination of strength training and aerobic exercise -- like brisk walking -- works better than either alone when it comes to cutting blood-sugar levels and excess weight.
"There's no reason to believe that wouldn't be true in kids too," he said.
The new findings are based on more than 1,400 students, aged 10 to 12, at 17 Michigan schools. Based on their hand-grip test, the children were divided into three groups of low, moderate and high strength.
Among boys, the third with the greatest strength had LDL levels that were 10 points lower, on average, than the third with the lowest strength. The same was true for girls. When it came to triglycerides, stronger kids had levels that were 20 to 30 points lower.
As for blood pressure and blood sugar, the differences were less dramatic. The high-strength group had levels that were a few points lower, on average, than the low-strength group.
Why would strength affect something like cholesterol? "Muscle is different, metabolically, than fat or bone," Berg said. "Muscle is active. It burns a lot of calories."
It makes sense, he said, that stronger muscles would influence blood fats and sugar.
And although an 11-year-old's cholesterol or blood pressure might not be a health threat now, the concern is that those patterns will continue into adulthood, Gordon said. "Kids with risk factors tend to become young adults with risk factors," he said.
Church said the findings offer one more reason to make sure kids have well-rounded, quality physical education in school. "This is an example of why PE is so important," he said.
The U.S. Centers for Disease Control and Prevention has exercise recommendations for kids.