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.
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