By Nick Triggle
Health reporter, BBC News
Most children are not doing enough physical activity
Sedentary lifestyles are making children less fit - even among those who are not obese, a study suggests.
Essex University staged fitness tests on 600 10-year-olds a decade apart in an area with low levels of obesity.
They found significant falls in fitness levels, concluding the average 10-year-old in 1998 could beat 95% of youngsters in 2008 in running tests.
The researchers said the focus on obesity was obscuring the health risks of wider declines in fitness levels.
Children are routinely weighed and measured in schools in England as part of the government's drive to tackle rising obesity rates, but there is no equivalent for fitness.
The Essex team of sports experts chose to focus on Chelmsford, an affluent town with traditionally low levels of obesity, to illustrate how being a normal weight did not necessarily equate to having good fitness.
In 1998, they carried out 20m shuttle run tests - commonly known as the bleep test - on 303 children from six schools.
In 2008, the tests were repeated on a similar number of 10-year-olds, the Archives of Disease in Childhood reported.
While obesity levels had hardly changed, there was a significant shift in fitness which was "large and worrying".
Researchers said similar if not worse findings would be expected in areas with high levels of obesity.
Lead researcher Dr Gavin Sandercock said: "The measurement of obesity alone may not be sufficient to keep an eye on children's future health. We need some form of monitoring of fitness.
"We have a generation of children who are spending more and more time in front of a screen, whether it is a TV or a computer.
"Schools are now trying to do more, but it is the lack of unstructured activity outside that is the problem."
Professor Alan Maryon-Davis, president of the Faculty of Public Health, said: "We have been concerned about the sedentary lifestyles of children for some time.
"But the focus on obesity is right at the moment because it is more directly linked to chronic conditions such as diabetes and heart disease."
A Department of Health spokesman said promoting physical activity remained a "top priority" and a key part of the obesity drive.
She added Change4Life, the government's campaign to promote healthy lifestyles, had "kick-started a lifestyle" revolution since it was launched in January.