"The number of sit-ups 10-year-olds can do declined by 27.1% between 1998 and 2008"

Couldn't find the study from any free source so this Guardian article will have to do.
The study is "Handgrip strength in English schoolchildren - Cohen DD, Voss C, Taylor MJ, Stasinopoulos DM, Delextrat A, Sandercock GR."

http://www.guardian.co.uk/society/2011/may/21/children-weaker-computers-replace-activity