Researchers from the National Institute of Child Health and Human Development followed about 1,000 U.S. children at different ages, from 2000 until 2006, discovering a striking dissimilarity in the frequency of activity between age 9 and 15. Whereas 90 percent of children aged 9 do exercises for a couple of hours almost every day, fewer than 3 percent of 15-year-old children do.

Children’s activity was recordered with special gadgets. Standard levels of moderate-to-vigorous activity decrease from three hours a day for 9-year-olds to less than an hour for 15-year-olds. Besides, the research shows the worrying fact that less than a third of youngsters that age get even the minimum advised by the government – that’s equivalent to an hour of moderate-to-vigorous exercise such as cycling, swimming or jogging.

According to the study authors, the decrease creates a touch of concern because sedentariness continues in adulthood, which could put in danger children's' health during their lives. But many people are not aware of the situation’s gravity.  "People don't recognize this as the crisis that it is," said the author who conducted the study, Dr. Philip Nader, a pediatrician and professor emeritus at the University of California, San Diego.

As expected, Dr. Nader said inactivity in teenagers is mostly caused by the popularity of video games, DVDs and Internet use, by "all of the types of things that take children from outside and put them on a couch or in front of a computer." Other contribution factors are schools abandoning gym classes, as well as unhealthy eating patterns.

A sedentary life comes into contact with greater risks for many health problems, such as heart disease, obesity, diabetes and high blood pressure.