Many children cannot run as fast as their parents could when they were young, a study of global fitness says.
Experts say the work - being presented at the American Heart Association's annual meeting - suggests children's fitness levels may be declining.
Researchers analysed data spanning 46 years and involving more than 25 million children in 28 countries.
On average, children today run a mile 90 seconds slower than did their counterparts 30 years ago, they said.
Across nations, cardiovascular endurance - gauged by how far children can run in a set time - has dwindled consistently by about 5% every decade, according to the findings.
The decline is seen in boys and girls and across all ages from nine to 17 years, and is linked to obesity, with some countries faring worse than others.
Lead researcher Dr Grant Tomkinson of the University of South Australia's School of Health Sciences said: "In fact, about 30% to 60% of the declines in endurance running performance can be explained by increases in fat mass."
The problem is largely one of Western countries, but some parts of Asia like South Korea, mainland China and Hong Kong are also seeing this phenomenon.
Dr Tomkinson said children needed to be inspired and encouraged to do more vigorous exercise.
If not, the public health consequences could be dire.
Huff and puff
"If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life," said Dr Tomkinson.
To stay healthy, children and young people need to do at least an hour of physical activity - such as walking or cycling to school and running in the playground - every day. It can be done in small chunks rather than one session.
Prof Michael Gwitz of the American Heart Association said: "The type of exercise is really important."
He says exercise must be something that "makes you sweat" and is "sustained and dynamic" to promote cardiovascular fitness.
Simply going to the gym or belonging to a school sports team might not be enough, unless you are moving around a lot.
Christopher Allen of the British Heart Foundation, said: "It's well established that being physically inactive in childhood can have serious health implications later in life.
"Keeping active can reduce the risk of cardiovascular disease and the sooner kids start, the better.
"By encouraging children to get active, we can help protect their hearts as they grow up. Parents, schools and community groups can all help kids on their way to 60 minutes exercise a day."