U.S. kids are not only too heavy; they're also out of shape, according to a new study of 5th and 7th grade students in Georgia.

Half didn't reach minimum standards for healthy aerobic fitness, Dr. Kenneth E. Powell, a physician in private practice, and his colleagues found, while nearly a quarter didn't make the grade in terms of muscle fitness, endurance or flexibility.

Powell and his team determined body mass index (BMI) and tested fitness and physical activity levels of 5,248 students from 93 schools across Georgia.

They found that 30 percent of the students were overweight — their BMI was outside the healthy range — and 22 percent didn't get the recommended 60 minutes a day of moderate to vigorous physical activity.

Fifty-two percent of the middle school students failed a test of cardiorespiratory fitness, while 23 percent did not meet minimum standards for two out of four tests of strength, endurance and flexibility, according to a report in the American Journal of Preventive Medicine.

Boys were more likely than girls to be outside the healthy BMI range, and they were also less likely to meet standards for cardiorespiratory fitness. Insufficient physical activity was more common among black and Hispanic students than among white students.

"Although there were some significant differences in test scores between demographic subgroups," Powell and his team write, "a more important finding is that none of the subgroups performed very well on any of the measures."

"These data," the conclude, "are consistent with the suggestion that physical inactivity has led to deficient levels of health-related fitness in more areas than just body composition, and that monitoring all components of health-related fitness would provide helpful information about the health of U.S. children and youth."