Videnskab posted an article about sports researcher Glen Nielsen's PhD project called "Children 's Daily Physical Activity", at the University of Copenhagen.
It is in danish but is worth reading (google translate), as he followed more than 500 Danish and 500 New Zealand children and put accelerators (motion sensors) on them to monitor their levels of activity.
Here's a part roughly translated:
vast majority of children's physical activity going on as so-called '
self-organized physical activity ' in the schoolyard or at the youth
center where the intensity is as high as in recreational and school
was very surprising how much of children's physical activity that comes
from what might be called free play, while organized sports had only a
very small contribution to the total volume of activity ," says Glen