"What shocked me was the sharp decline in activity," Nader said. The study monitored the habits of the same children from different parts of the country, with different family incomes and different races.
At 9 years old, the kids got about three hours of moderate to vigorous physical activity a day from activities like bike riding, tag or basketball.
However, by the time the kids turned 15, their activity had dropped down to about 49 minutes a day. On the weekends, it was even worse -- kids got about 35 minutes a day.
Researchers estimated that kids' physical activity declined about 40 minutes per day, each year until age 15, when activity fell far below the recommended level.
The Department of Health and Human Service's Dietary Guidelines for Americans suggest that children and adolescents get a minimum of 60 minutes of physical activity, seven days a week.
The study reiterates the American Academy of Pediatrics' recommendation issued on July 7, that some children, as young as 8, be put on cholesterol medicine and receive lipid screening to help fight heart disease.
What makes this study so compelling is how the kids' activities were tracked.
For a full week when the kids were 9, 11, 13, and 15 years old, they wore a computerized device on their belt, called an accelerometer, which measured their movement. The tool gave researchers an accurate view of how kids move and exercise.
The study also found that activity declined for all children across the board, regardless of where they lived, what their family income was or their race. This overarching trend of increasing inactivity points to the raised risk of obesity and heart disease facing America's children.
Nader's findings underscore changes in how children socialize in a digital age. Children spend more time at the computer and television screen, playing video games, than on the street shooting hoops.
"Here's the first time that we have a large group of kids, and it's the same kids from age 9 to 15," Nader said. "So, we are really seeing a culture and a societal decline in activity with age."
The cultural decline in activity that Nader's study identified has much wider implications. The eventual cost is more kids growing into unhealthy adults.