Studies of children around the world show kids today are less fit than their parents were.
An analysis of those studies by the American Heart Association shows kids don't run as fast or as far as their parents did when they were young.
Heart-related fitness has declined 5 percent per decade since 1975.
"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," study researcher Grant Tomkinson, senior lecturer at the University of South Australia's School of Health Sciences, explained.
"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," he said.
Health experts recommend children 6 and older get 60 minutes of moderately vigorous activity throughout each day. Only one-third of American kids do now.