Making sure kids have good muscle fitness might also benefit their school performance, according to a recent U.S. study.

Aerobic fitness has already been linked to better thinking abilities in pre-teen children, but the current study found an independent link between muscle fitness and kids’ performance on memory tests as well as their math and reading skills.

“We’ve seen this relationship for cardiorespiratory fitness many times before,” said senior author Charles H. Hillman of Northeastern University in Boston. “The relationship with strength is novel in children, but based on work with older adults, it was expected.”

For the study, which was funded by Nike, 75 kids aged 9 to 11 years completed an aerobic exercise test at steady speed on a treadmill with gradually increasing incline until they were too out of breath to continue. They completed a similar test of muscular fitness with a battery of upper body, lower body and core exercises using body weight or a medicine ball, including lunges, push-ups and shoulder presses.

The kids did as many repetitions of each exercise as possible within 30 seconds while maintaining proper form.

They also completed computerized tests of working memory, algebra, geometry, reading and writing.

“Aerobic fitness describes the capacity of the lungs to take in and deliver oxygen as well as the heart to effectively distribute oxygen to the body,” Hillman told Reuters Health by email. “Musculoskeletal fitness relates to muscle strength, power and endurance to enable performance in the face of resistance.”

The two go hand in hand, but training one system does not guarantee adaptation to the other system, he said.

After accounting for age, sex, grade, IQ and family education levels, researchers found that kids with better aerobic fitness on the treadmill test also had more accurate responses in the memory test and better performance at algebraic functions.

Kids with more muscular fitness on the body weight and medicine ball tests also did better than others on memory and academic tests, according to the report in Medicine and Science in Sports and Exercise.

“This study shows that during instances that require greater working memory demand, higher fit children (regardless of whether we assessed muscular or aerobic fitness) performed better (i.e., were more accurate in their responses),” Hillman said.

“Working memory refers to the ability to hold and manipulate information for a short period of time to guide cognition or behavior,” he said.

Aerobic fitness also related to mathematics achievement, so there may be an additional benefit of being aerobically fit, he said.

“In this study, we did not measure brain structure or function, so while we can guess, we cannot make statements about the brain,” he said.

Many, if not most, benefits of fitness on thinking and memory are similar in adults, he said.

For now, children should get more than 60 minutes a day of moderate to vigorous physical activity and regular weight bearing exercise, according to current guidelines, he said.

“If it proves possible to enhance children’s memory and academic performance through fitness training, it would provide an attractive alternative to our current intervention strategies for helping struggling students,” said Daniel Belsky, assistant professor of medicine at Duke University School of Medicine in Durham, North Carolina, who was not part of the new study.

“To me, the implication is pretty clear,” Belsky told Reuters Health by email. “We should think about getting kids exercise during the school day as one of the tools we have to enhance their learning.”