If videogames like "Madden NFL" didn't exist, 12-year-old Tom might go outside and toss around a real football - and he'd have a better chance of sprinting for a touchdown without getting winded.
Too much small-screen recreation could undermine physical fitness, Australian researchers have found, in a new study that looks at how e-mail and text messaging, TV, videogames and net surfing affect aerobic endurance in adolescents.
Two hours of daily screen time appears to be the "cut point" above which kids are significantly less likely to be fit, found researchers led by Louise Hardy, Ph.D., at the New South Wales Centre of Overweight and Obesity at the University of Sydney.
"The effect was consistently stronger among all girls compared with boys," Hardy said. "The longer girls spent on screen recreation the less fit they were, and the evidence of this effect increases with age among girls."
Older boys were less affected, no matter how long they spent on screen recreation.
The study doesn't confirm cause and effect: It might be that small screens do lure kids away from active play, or it could be that fit kids are less likely to plop down in front of a screen in the first place.
Past studies have looked at the effects of TV viewing on fitness, but not other forms of electronics that can turn kids into couch potatoes. The new study appears in the February 2009 issue of the American Journal of Preventive Medicine .
The research team surveyed 2,750 New South Wales students on their physical activity and screen use. They also put students through the "pacer" test, a shuttle run that physical education teachers use to measure fitness.
"Among younger boys in grades six and eight, fitness levels were lower as their screen time increased, but this was not apparent among older boys," Hardy said. She added that although the10th-grade boys reported the most screen time - some as much as 10 hours a day - and finished 20 percent fewer laps than other boys in their age group, that result was not statistically significant.
"We think these boys, who would be 15 to16 years old, probably have developed enough muscle mass which allows them to sit and be fit," Hardy said.
Guidelines from American Academy of Pediatrics and Healthy People 2010 call for kids to keep viewing to a daily maximum of two hours.
American kids are more sedentary than their Australian peers, said James Sallis, Ph.D., a physical activity researcher at San Diego State University: "I've seen six to six and a half hours per day of recreational screen time in the U.S. Based on that, we might expect to find that the fitness situation is worse here."
Still, the lure of the sedentary is strong, Sallis acknowledges: "There's a lot of money involved in getting kids glued to the screen. It's like candy to a baby: lots of color, motion and flash to get them mesmerized."
Sallis said that parents have become more reluctant to encourage their kids to play outdoors due to fears of traffic risks and "stranger danger" - although the risk of such abduction is minimal. "As a consequence, parents increase the risk of low fitness and obesity, which is extremely common."
Hardy said the take-home message is to encourage "all kids to exchange some screen time for active time," and for parents to "keep kids screen time under two hours a day, and aim for 'no-screen' days."