Do you remember a childhood filled with running? Running around the playground, running while playing, running to your friend's house? Youthful scampering appears to be a lost art; childhood now seems more defined by sedentary activities, and in fact, a new study from Australia suggests the same.

Grant Tomkinson from the University of South Australia's School of Health Sciences and his team analyzed 50 studies on running fitness between 1964 and 2010. Combined, they had data for more than 25 million kids between the ages of 9 and 17 in 28 countries.

What they discovered is that children today are around 90 seconds slower per mile than kids were 30 years ago. Running endurance, Tomkinson says, can be a good indicator of future health.

"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," Tomkinson said. "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."

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

In the United States, the decline is slightly worse than other countries. American childrens' cardiovascular endurance fell an average 6 percent per decade from 1970 to 2000; elsewhere, the decline was about 5 percent per decade.

The study found that each country’s fitness was relative to that county’s measurements of weight and obesity in children, suggesting a relationship between the two.

“About 30 percent to 60 percent of the declines in endurance running performance can be explained by increases in fat mass," Tomkinson said.

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

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