The study conducted at the University of Strathclyde in the UK also found that there is no evidence to indicate that the decline is greater among girls than boys.
The long-term study states that the prevailing view among policy makers and health professionals is that physical activity levels during childhood are adequate, but fall sharply during adolescence - with the decline is significantly greater among girls.
According to the research, there is actually very little firm evidence to back this up and what research has been carried out in this area has mostly been done before the impact of new technologies would have been felt.
"We did not set out to examine the reasons behind the changes, but finding out why around one in five of the boys managed to maintain levels of moderate to vigorous physical activity throughout the study period might help to inform future policy and practice," Reilly said.
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"Future research and public health policy should focus on preventing the decline in physical activity, which begins in childhood, not adolescence, and providing an improved understanding of the determinants of the different physical activity trajectories, including an understanding of the relative importance of biological and environmental influences," Reilly said.
To quantify the timing of any changes, the researchers tracked the physical activity levels of a representative sample of around 400 children.
These children took part in the Gateshead Millennium Cohort Study in North East England between 2006 and 2015.
Overall, the total volume of physical activity fell from the age of seven onwards in both boys and girls during this time, with declines no steeper during adolescence than in earlier childhood.
Published in the British Journal of Sports Medicine, the study is observational and firm conclusions cannot be drawn from it about cause and effect.
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