Around the world, many children do not run as far or as fast as their parents did when they were kids, according to a large study presented at a scientific meeting in the US recently.
The study concludes that today's kids are about 15% less aerobically fit than their parents were at their age.
And in the US, kids' cardiovascular endurance has fallen by around 6% per decade between 1970 and 2000.
The researchers warn that such a decline in fitness may mean worse health in adulthood.
Lead author Dr. Grant Tomkinson of the University of South Australia's School of Health Sciences, who presented the findings at the American Heart Association's Scientific Sessions 2013 in Dallas, TX, says:
If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life."
Cardiovascular fitness not same as strength, flexibility or skill
Dr. Tomkinson says while there are many ways that young people can be fit, like developing strength by lifting weights, being flexible like a gymnast or being skilled at tennis, this is not the same as having cardiovascular fitness, which is what most relates to health, as he explains:
"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 analzyed kids' running fitness over 46 years
After analyzing kids' fitness for 46 years, researchers found their cardiovascular endurance has decreased by 5% every decade.
Image credit: American Heart Association
He and his colleagues analyzed 50 studies on running fitness conducted between 1964 and 2010 in 28 countries that altogether covered over 25 million kids aged between 9 and 17.
They used how far kids ran in a set time or how long it took to run a set distance as the measure of cardiovascular endurance.
Across the studies, typical running tests lasted either 5 to 15 minutes, or covered between 0.5 and 2 miles (0.8 to 3.2 km).
The analysis found that kids' cardiovascular endurance has fallen significantly over the 46 years it covered - across nations it has declined consistently by about 5% every decade.
There were few differences between boys and girls, older and younger children, or among regions, although there were variations from country to country.
Fall in fitness parallels rises in obesity, body fat
Dr. Tomkinson suggests social, behavioral, physical, psychosocial and physiological factors lie behind the fall in kids' cardiovascular fitness.
The team also found when they looked at the figures country by country, the fall in fitness paralleled rising levels of obesity and body fat, suggesting one may be causing the other.
Dr. Tomkinson says:
"In fact, about 30% to 60% of the declines in endurance running performance can be explained by increases in fat mass."
To develop cardiovascular fitness, he says kids should be exercising at least 60 minutes a day, doing things like cycling, swimming or running, because these use the big muscles in the body.
A study published earlier this year in BMC Medicine suggests children should be physically active for at least 80 minutes a day, of which 20 minutes should be vigorous exercise, to protect them from cardiovascular problems later in life.