Lead researcher Dawn Coe, an assistant professor at the University of Tennessee in Knoxville who conducted the research as a doctoral student in Michigan State University's kinesiology department, said the study was among the first to examine how academic performance relates to all aspects of physical fitness -- including body fat, muscular strength, flexibility and endurance.
"We looked at the full range of what's called health-related fitness," Coe said in a statement. "Kids aren't really fit if they're doing well in just one of those categories."
Coe and colleagues gathered data from 312 students in sixth through eighth grade at a West Michigan school by gauging the kids' fitness with an established program of push-ups, shuttle runs and other exercises.
They compared the fitness scores to students' letter grades throughout the school year in four core classes and their performance on a standardized test.
The study, published in the Journal of Sports Medicine and Physical Fitness, found the fittest students got the highest test scores and the best grades, regardless of gender or whether they'd yet gone through puberty.