The American Heart Association said the decline in fitness may indicate worse health in adulthood.
“If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life,” said Grant Tomkinson, Ph.D., lead author of the study and senior lecturer in the University of South Australia’s School of Health Sciences, which was presented at the American Heart Association’s Scientific Sessions 2013 in Dallas, Texas.
The researcher said that while there are different kinds of fitness such as strength and flexibility, “not all 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,” Tomkinson said.
Researchers analyzed 50 studies on running fitness between 1964 and 2010 that involved more than 25 million kids, ages 9 to 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 five to 15 minutes or covered a half-mile to two miles.
Cardiovascular endurance declined significantly within the 46 years, the researchers found. Average changes were similar between boys and girls, younger and older kids, and across different regions, although they varied country to country.
Some key findings showed that kids in the U.S. saw their cardiovascular endurance fall an average 6 percent per decade between 1970 and 2000. Endurance around the world fell by 5 percent, and kids are roughly 15 percent less fit from a cardiovascular standpoint than their parents were as youngsters. In the mile run, for example, kids today are about a minute and a half slower than their peers 30 years ago.
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 American Heart Association says kids should engage in at least 60 minutes of daily activities that use the body’s big muscles, such as running, swimming or cycling.