Kids Worldwide Getting Less Heart Fit: Study
Large study of running fitness in recent decades found today's children are much slower
Obesity also plays a part. "We are fatter today, so from a weight-bearing perspective it's harder to move our bodies through space," Tomkinson said, noting that about 30 percent to 60 percent of declines in endurance running performance can be explained by increases in body fat mass.
So what's the solution? To Tomkinson, it's simple -- kids need to be exposed to prolonged exercise that leaves them exhausted.
"You want exercise to be fun, but there needs to be some huff and puff there as well," he said. "It needs to make them somewhat tired."
Kids need to engage in at least 60 minutes of physical activity that uses the body's big muscles, like running, swimming or cycling, Tomkinson said.
It doesn't have to be all at once, however. Tomkinson said kids can "snack" on physical activity throughout their day -- engaging in a walk for 10 minutes in the morning, or playing an active game for 10 minutes at recess, for example.
Parents also need to engage with their kids, he added. They should limit a kid's sedentary time to no more than two hours a day, while also exposing them to a range of physical activities they might enjoy.
Kass of Let's Move! called on Americans to work toward making physical activity easier and more fun, both for children and adults.
"We know we need to break this cycle of passing physical inactivity down from one generation to the next," Kass said. "There's no one solution. It's got to happen in a comprehensive way. We have to integrate physical activity in our lives. It's got to be a part of our daily lives."