(HealthDay News) – Cardiovascular fitness has declined substantially among children and youth around the world since the 1960s, according to research presented at the American Heart Association’s 2013 Scientific Sessions, held from Nov. 16–19 in Dallas.

Grant Tomkinson, PhD, from the University of South Australia in Adelaide, and colleagues identified and reviewed 50 studies from 1964–2010 of more than 25 million healthy children and youth 9–17 years old from 28 countries. The studies analyzed time changes in maximal long-distance running performance.

The researchers observed significant declines in cardiovascular performance with time. Although there was variation from country to country, globally there was a 5% decline in endurance per decade, with an average 6% decline per decade since 1970 in the United States. Children and youth today are about 15% less fit than a generation ago, taking about 1.5 minutes longer to complete a mile run. Increases in fat mass can explain 30–60% of the decline in performance, according to the authors.

“There is overwhelming evidence for substantial global declines in cardiovascular endurance performance of children and youth in recent decades,” Tomkinson and colleagues conclude. “Time-related declines in cardiovascular endurance performance are probably caused by a network of social, behavioral, physical, psychosocial, and physiological factors.”

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