Cardiorespiratory fitness influences health in youth and adulthood, said lead study author Eliane Peterhans, a sports sciences researcher at the University of Konstanz in Germany. "It is very important to understand how adolescents behave because then you have a chance to correct unhealthy behaviors," she added.

The study was part of a large German study and included 1,328 teens. Researchers used bicycles to assess participants' cardiorespiratory fitness and gathered information about their and their families' health behaviors.

Peterhans and her colleagues found that having two parents with normal weight positively predicted cardiorespiratory fitness in both boys and girls. Since body weight is related to nutrition, having normal weight parents may reflect an overall healthier family environment, the authors suggested. However, having parents who were physically active did not influence teen fitness, suggesting that teens may not regard their parents as role models for fitness.

Having a normal body weight and health behaviors such as going to the gym, engaging in leisure time physical activity, and riding a bike to school most strongly predicted cardiorespiratory fitness for both boys and girls, but this association was generally stronger for boys.

Boys who spent less than two hours per day in front of the television or computer were more likely to be fit than boys who reported more screen time. Screen time did not have a significant effect on girls' fitness, although only 40 percent of girls spent more than 2 hours per day in front of a screen compared to 65 percent of boys.

Overall, family health behaviors were less related to girls' cardiorespiratory fitness than boys' fitness. "We need more research in girls, especially," Peterhans said. "For example, maybe peer behavior is a more important influence on girls' cardiorespiratory fitness than boys."

Aaron Carrel, M.D., medical director at the University of Wisconsin Health's Pediatric Fitness Clinic commented, "I thought that was an important finding…that the same things that work for boys may not work for girls. If we are trying to intervene and create much healthier environments -- whether it's in schools or communities -- the more we understand what impacts boys and girls differently, the more we can successfully design better interventions."