Lead author Grant Tomkinson, a senior lecturer in the University of South Australia's School of Health Sciences, and colleagues analyzed 50 studies on running fitness between 1964 and 2010 that involved more than 25 million kids ages 9-17 in 28 countries.
They gauged cardiovascular endurance by how far kids could run in a set time or how long it took to run a set distance. Tests typically lasted 5-15 minutes or covered a half-mile to 2 miles.
The meta-analysis found cardiovascular endurance declined significantly within the 46 years. Average changes were similar between boys and girls, younger and older kids, and across different regions, although they varied country to country, Tomkinson said.
"If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life," Tomkinson said in a statement
"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."
In the United States the study found children's' cardiovascular endurance fell an average 6 percent per decade from 1970 to 2000. However, across nations, endurance declined consistently by about 5 percent every decade.
In a mile run, kids today are about a minute-and-a-half slower than their peers 30 years ago, Tomkinson said. Declines in cardiovascular endurance performance are probably caused by social, behavioral, physical, psychosocial and physiological factors, Tomkinson said.
Country-by-country fitness findings are mirrored in measurements of overweight/obesity and body fat, suggesting one factor may cause the other.
"In fact, about 30 percent to 60 percent of the declines in endurance running performance can be explained by increases in fat mass," Tomkinson said.
The findings were presented at the American Heart Association's Scientific Sessions in Dallas.
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