Most children today cannot run as far or as fast as their parents did, according to research.

The decline, which may indicate worse health in adulthood, was discovered following an international study of running fitness in children over the last 50 years.

It found that children's cardiovascular fitness had dropped markedly worldwide since about 1975.

Over a mile run, youngsters today would be about a minute and a half slower than children 30 years ago.

Dr Grant Tomkinson, the study's lead author from the University of South Australia, said: "If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life.

"Young people can be fit in different ways. They can be strong like a weightlifter, or flexible like a gymnast, or skillful like a tennis player. But not all of these types of fitness relate well to health.

"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."

Researchers analysed 50 studies on running fitness between 1964 and 2010 of more than 25 million children aged nine to 17 in 28 countries.

They gauged cardiovascular endurance by how far they could run in a set time or how long it took to run a set distance, typically lasted five to 15 minutes or covering half a mile to two miles.

Cardiovascular endurance declined significantly within the 46 years, the researchers revealed at the American Heart Association's annual meeting yesterday.

On average, endurance has declined consistently by about 5% every decade, while children are about 15% less fit on a cardiovascular measure than their parents were at their age.

In the US, children's cardiovascular endurance fell by an average of 6% per decade between 1970 and 2000.

The a verage changes were similar between boys and girls, younger and older children, and across different regions, although they varied country to country.

The decline in cardiovascular endurance performance was probably caused by social, behavioural, physical, psychosocial and physiological factors, Dr Tomkinson said.

The fitness findings were mirrored by measurements of being overweight or obesity and body fat, suggesting one factor may cause the other.

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

He added that children should take part in at least 60 minutes of daily activities that use the body's big muscles, such as running, swimming or cycling.

"We need to help to inspire children and youth to develop fitness habits that will keep them healthy now and into the future," Dr Tomkinson said. "They need to choose a range of physical activities they like or think they might like to try, and they need to get moving."