Millions of children around the world cannot run as fast or as far as their parents were able to at their age, fresh analysis from the American Heart Foundation has found.

On average, it takes children 90 seconds longer to run a mile than their counterparts did 30 years ago. Heart-related fitness has declined 5% each decade since 1975, for children aged 9 – 17.

The American Heart Association, whose conference featured the research on Tuesday, said it was the first to show that children's fitness has declined worldwide over the past three decades.

"It makes sense. We have kids that are less active than before," said Dr Stephen Daniels, a University of Colorado paediatrician and spokesman for the association.

World Health Organisation numbers suggest 80% of young people globally may not be getting enough exercise.

Health experts recommend children aged six and older get 60 minutes of moderately vigorous activity accumulated over a day. "Many schools, for economic reasons, don't have any physical education at all," Daniels said. The study was led by Grant Tomkinson, an exercise physiologist at the University of South Australia.

Researchers analysed 50 studies on running fitness – a key measure of cardiovascular health and endurance – involving 25 million children aged 9 –17 in 28 countries from 1964 to 2010.

The studies measured how far children could run in five to 15 minutes and how quickly they ran a certain distance, ranging from half a mile to two miles. Today's children are about 15% less fit than their parents were, researchers concluded.

The decline in fitness seems to be levelling off in Europe, Australia and New Zealand, and perhaps in the last few years in North America. However, it continues to fall in China, and Japan never had much falloff – fitness has remained fairly consistent there. About 20 million of the 25 million children in the studies were from Asia.

In China, annual fitness test data show the country's students are getting slower and fatter over the past couple of decades. Experts and educators blame an obsession with academic testing scores for China's competitive college admissions as well as a proliferation of indoor entertainment options such as gaming and web surfing for the decline.

Data from China's education ministry show that in 2010 male college students ran 1,000 metres 14 to 15 seconds slower on average than male students who ran a decade earlier. Female students slowed by about 12 seconds in running 800m.