Computer games, television, a lack of school recess and cuts to physical education classes may all help explain why children aren't getting as much physical activity as they used to.
But now a new study offers insight into exactly how sedentary American youth has become. And the numbers do not paint a pretty picture, particularly as boys and girls enter the post-puberty years.
In the study, published in the July 16 issue of the Journal of the American Medical Association, researchers measured activity levels in 1,032 kids between the ages of 9 through 15 and compared the levels with health experts' recommendations that children engage in at least 60 minutes of moderate to vigorous activity every day.
What they found was that 9-year-old girls and boys did a great job at meeting -- and exceeding -- these exercise guidelines. They got about three hours of moderate to vigorous physical activity a day on both weekdays and weekends.
But as youngsters entered their tweens and early teens, they became increasingly less active.
"About the time of puberty, activity levels start to decline, especially in girls," said Dr. Philip Nader, a lead author of the study and a professor emeritus in the department of pediatrics at the University of California at San Diego. He wrote "You Can Lose Your Baby Fat."
Nader and his colleagues found that young women dropped below the recommended 60 minutes of exercise a day threshold around age 13, while in boys the drop occurred later at 14.7 years old. By age 15, teens exercised for 49 minutes on weekdays and for 35 minutes on Saturday or Sunday.
Although teenagers have more free time on weekends, they don't appear to spend it in physical activity. But what was most surprising to Nader was the sharp rate of decline in movement among the youngsters.
"It was quite significant," he says. "This [bodes] poorly for the direction of society and for our lifelong health.
"Activity is a major determinant of energy balance, and any decline in caloric expenditure doesn't help when it comes to childhood obesity."
Although other studies have tried to measure how much children are moving throughout the day, most of this data has come from self-reported questionnaires.
Nader's research was among the first to take advantage of a movement-counting device known as an accelerometer. This new technology allowed the scientists to capture activity levels from a fairly large group of participants in a consistent way over a six-year period.
The children were asked to wear the accelerometer -- a small rectangular digital device strapped around the waist -- for one week. Data were collected at four different points in time: at age 9, 11, 12 and 15. This mini-computer would start counting when a child was moving forward, and provided a moment-by-moment account of physical activity. A child doing fast and vigorous movement recorded more counts.
The accelerometer could not count water-based activities, such as swimming or bathing, and couldn't be worn during contact sports. Nader didn't believe this missing data would underestimate the youth's activity levels, since he said most children don't participate in water or contact sports every day.
The new method of using the accelerometer to gain objective data is garnering praise from exercise specialists.
"The study is excellent overall and is a major contribution to the existing evidence of a dramatic decline in activity in children and adolescence," said Steven Blair, a professor in the department of exercise science at the Arnold School of Public Health at the University of South Carolina at Columbia.
But Blair said we don't yet know whether this substantial drop in activity is a normal phenomenon seen in all people as they get older or whether it's a red flag, noting, "We don't know yet how to interpret the decline."
And then there's the question of what to do about the findings.
"Schools must become more involved by establishing physical fitness standards and opportunities for all children in an educational and nonthreatening way," said Michael Dupper, an assistant professor in the department of health, exercise science and leisure management at the University of Mississippi.
Parents can also do their part, he added, by turning off the computer and TV, and creating an environment in which children want to make healthy lifestyle choices. And they can also serve as positive role models for fitting activity into a busy day.
Although Nader's research looked at school-age children, he said he saw the opportunity to encourage an enjoyment of physical activity and play in day care so it gets ingrained at an early age.
His advice to children and adults about exercise? "You literally need to find something that moves you," he said.