WEDNESDAY, Jan. 7 (HealthDay News) -- Teenage girls who spend more than two hours of "screen time" a day watching TV, surfing the Web or text-messaging are less likely to be physically fit, a new Australian study finds.

Interestingly, boys who were part of the same study were more likely to be able to "sit and be fit," said lead author Louise Hardy, a postdoctoral fellow at the New South Wales Centre for Overweight and Obesity at the University of Sydney.

Boys, particularly older teenage boys, may be less affected by the time they spend watching TV, playing computer games, and other small-screen activities because their growth spurts have led to sufficient muscle mass to maintain fitness and still engage in a large amount of sedentary behavior, the study suggested.

Boys are also more likely to play sports as well as computer games, said Dr. Goutham Rao, clinical director of the Weight Management and Wellness Center at the Children's Hospital of Pittsburgh. An adolescent boy who's on the swim team, for instance, may also spend a lot of time playing a video game like "World of Warcraft," he said.

"There is a culture of physical fitness among boys, and there also is a culture of sedentary activity," he added.

From his experience, Rao said, "there is only a small subset of teenage girls who engage in physical activity regularly." He added that he finds that adolescent girls at his center are more interested in small-screen communication devices, such as texting, than they are in video games.

The Australian researchers said their study may be the first to associate "sedentariness" -- a measure of cardio-respiratory fitness -- with the widely accepted American Academy of Pediatrics' guidelines that children from 2 to 18 years old not spend more than two hours a day on small-screen recreation.

"This is important because ensuring that one has good cardio-respiratory fitness reduces the risk of developing cardiovascular disease," Hardy said. For the study, the researchers measured cardio-respiratory fitness based on the number of laps run at a set pace.

The study, which relied on data collected in 2004, defined small screen activity as watching TV, videos and recreational computer use. The data came from a survey of a representative sample of 2,750 Australian students in sixth, eighth, and 10th grades.

More current data might be even more striking because of the growth of new screen technologies such as XBoxes, PSPs, and Wiis, Hardy suggested. The impact of new small-screen technology may show up in a repeat of the research scheduled for 2010, she said.

The findings will be published in the February issue of the American Journal of Preventive Medicine.

The researchers said the study was limited in its ability to make a cause-and-effect relationship between more small-screen time and less physical activity because other factors might be involved.

The question of whether less physically fit girls are more likely to engage in sedentary activities is one that Rao said he'd like to see pursued in future research.

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