"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.