Kids today are about 15 per cent less fit from a cardiovascular standpoint than their parents were as youngsters, according to a new study which included data from 28 countries.

Many kids don't run as far or fast as their parents did, researchers said, noting that the decline in running fitness may indicate worse health in adulthood.

"The most important type of fitness for good health is cardiovascular fitness, which is the ability to exercise vigorously for a long time, like running multiple laps around an oval track," said Grant Tomkinson, lead author of the study and senior lecturer in the University of South Australia's School of Health Sciences.

"If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life," he said.

Researchers analysed 50 studies on running fitness between 1964 and 2010 that involved more than 25 million kids, ages 9 to 17, in 28 countries.

They gauged cardiovascular endurance by how far kids could run in a set time or how long it took to run a set distance. Tests typically lasted five to 15 minutes or covered 800 m to 3.2 km.

Cardiovascular endurance declined significantly within the 46 years, the researchers found.

Average changes were similar between boys and girls, younger and older kids, and across different regions, although they varied country to country.

The study is the first to show that kids' cardiovascular fitness has declined around the globe since about 1975.

In the US, kids' cardiovascular endurance fell an average 6 per cent per decade between 1970 and 2000.

Across nations, endurance has declined consistently by about 5 per cent every decade.

The study found that in a 1.6 km run, kids today are about a minute and a half slower than their peers 30 years ago.

Declines in cardiovascular endurance performance are probably caused by social, behavioural, physical, psychosocial and physiological factors, Tomkinson said.

Country-by-country fitness findings are mirrored in measurements of overweight/obesity and body fat, suggesting one factor may cause the other, researchers said.

"In fact, about 30 per cent to 60 per cent of the declines in endurance running performance can be explained by increases in fat mass," Tomkinson said.

Kids should engage in at least 60 minutes of daily activities that use the body's big muscles, such as running, swimming or cycling, he said.

The research was presented at the American Heart Association's Scientific Sessions 2013 in Dallas.