Australian researchers analyzed 50 studies on running fitness conducted in 28 countries between 1964 and 2010. More than 25 million healthy kids, ages 9 to 17, were part of the studies, all of which gauged fitness in terms of how far kids could run in a set time-frame or how long it took to run a set distance. In the U.S. alone, researchers found that children’s cardiovascular endurance — one of the cornerstones of physical fitness — fell an average of 6 percent per decade between 1970 and 2000.

The reason is simple: they’re carrying too much body fat, making it “more difficult [for them] to move through space,” explains lead researcher Grant Tomkinson, Ph.D., a senior lecturer in the University of South Australia’s School of Health Sciences, whose research was presented Tuesday at the American Heart Association’s annual meeting in Dallas.

Beyond battling obesity, kids also have to contend with an environment that is toxic to activity, he says.