Many children are unable to run as fast or as far as their parents could when they were the same age, major new research has revealed.

The findings indicate that children's levels of cardiovascular fitness are declining worldwide.

Australian researchers analysed 50 studies on running fitness that were carried out between 1964 and 2010. These studies involved over 25 million children aged between nine and 17 and living in 28 countries worldwide.

Cardiovascular endurance was determined by measuring how far the children could run in a set time, or how long it took them to run a particular distance. Most of these trials lasted between five and 15 minutes or covered distances of between a half a mile and two miles.

The findings revealed that cardiovascular fitness declined significantly over the 46-year period, but particularly since 1975.

The researchers found that from a cardiovascular standpoint, children today are about 15% less fit than their parents were when they were children.

In a mile run, today's children were found to be about 90 seconds slower than children 30 years ago and throughout the world, endurance has consistently declined by around 5% every decade.

According to the lead author of the study, Dr Grant Tomkinson of the University of South Australia, if a person is generally unfit as a child, ‘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," he explained.

Dr Tomkinson suggested that this decline in cardiovascular fitness is probably due to a number of factors, including social, behavioural and physical factors.

He also noted that in each country, fitness levels were mirrored by measurements of overweight/obesity and body fat. In other words, the more overweight or obese a population was, the less fit they were.

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

He reminded parents that children should be getting at least 60 minutes of physical exercise every day and these exercises should use the body's big muscles. The best exercises for this include swimming, running and cycling.

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

Details of these findings were presented at the American Heart Association's Scientific Sessions 2013.