Kids today cannot run as fast or as far as their parents did, which may indicate their health will worsen as adults, according to new research from the University of South Australia's School of Health Sciences.

To determine children's cardiovascular fitness over the years, researchers analyzed 50 studies about running fitness conducted between 1964 and 2010. The studies involved more than 25 million children between the ages of 9 and 17 in 28 countries. Most of the studies gaged cardiovascular endurance by measuring how far participants could run in a set time or how long it took them to run a set distance.

Analysis of the studies' data showed that kids' cardiovascular fitness levels have been declining since about 1975, and children today are roughly 15 percent less fit than their parents were as kids. Data also showed that the endurance of children around the world decreased by 5 percent each decade between 1970 and 2000. In the United States, children's endurance has decreased by 6 percent each decade.

Declines in cardiovascular endurance performance could be caused by a variety of factors, but 30 percent to 60 percent of the declines in running performance can be explained by increases in fat mass, Grant Tomkinson, lead author of the study, said in a statement.

To improve their cardiovascular fitness and prevent health issues, children should participate in physical activities that use the body's big muscles, such as running or swimming, he added.

"If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life," Tomkinson said. "We need to help to inspire children and youth to develop fitness habits that will keep them healthy now and into the future. They need to choose a range of physical activities they like or think they might like to try, and they need to get moving."

Tomkinson presented his research earlier this week at the American Heart Association's Scientific Sessions in Dallas, but it has not yet been published in a peer-reviewed journal.