On average, it takes children 90 seconds longer to run a mile than their counterparts did 30 years ago, according to the American Heart Association. Mike Egerton/PA Wire/AP

Today's youth around the world run slower than their parents did when they were young, according to an analysis studying millions of children.

On average, it takes children 90 seconds longer to run a mile than their counterparts did 30 years ago. Heart-related fitness has declined an average of 5 percent per decade since 1975 for children ages 9 to 17.

The American Heart Association, whose conference featured the research on Tuesday, says the study is the first to show that children's fitness has declined worldwide over the past three decades.

"It makes sense. We have kids that are less active than before," said Dr. Stephen Daniels, a University of Colorado pediatrician and spokesman for the association.

Health experts recommend that children 6 and older get 60 minutes of moderately vigorous activity accumulated over a day. Only one-third of American kids do now.

"Kids aren't getting enough opportunities to build up that activity over the course of the day," Daniels said. "Many schools, for economic reasons, don't have any physical education at all. Some rely on recess" to provide exercise.

Greg Welk, chair of FITNESSGRAM Scientific Advisory board, told Al Jazeera that though the federal government and non-profit organizations are doing "tremendous things" to combat obesity, physical activity should be encouraged at home as well.

"More responsibility should fall on parents. There's too much pressure on school," Welker said. "Physical activity has to be facilitated by parents" otherwise children will resort to indoor, sedentary activities.

"Physical inactivity is both a cause and a consequence of being overweight," Welk added. FITNESSGRAM, a tool used to track exercise and activity across the U.S., is also the assessment test used for the Presidential Youth Fitness Program.

In a speech to the conference on Monday, Sam Kass, a White House chef and head of first lady Michelle Obama's Let's Move program, stressed the need to get kids to be more active.

"We are currently facing the most sedentary generation of children in our history," Kass said.

The new study was led by Grant Tomkinson, an exercise physiologist at the University of South Australia. Researchers analyzed 50 studies on running fitness — a key measure of cardiovascular health and endurance — involving 25 million children ages 9 to 17 in 28 countries from 1964 to 2010.

The studies measured how far children could run in 5 to 15 minutes and how quickly they ran a certain distance, ranging from half a mile to 2 miles. Today's kids are about 15 percent less fit than their parents were, researchers concluded.

A separate report by the Bogalusa Heart Study found that the proportion of American children 5 to 17 years of age who were obese was five times higher in 2008-09 than in 1973-74. The study said that more than 23 million children in the U.S. are obese.

The decline in fitness seems to be leveling off in Europe, Australia and New Zealand, and perhaps in the last few years in North America. However, it continues to fall in China. Japan never had much falloff — fitness has remained fairly consistent there. About 20 million of the 25 million children in the studies were from Asia.

In China, annual fitness test data show that the country's students have become slower and fatter over the past several decades.

Other research discussed global declines in activity.

Fitness is "pretty poor in adults and even worse in young people," especially in the United States and Eastern Europe, said Dr. Ulf Ekelund of the Norwegian School of Sport Sciences in Oslo, Norway.

World Health Organization numbers suggest that 80 percent of young people globally may not be getting enough exercise.

Ehab Zahriyeh contributed to this report. Al Jazeera and The Associated Press