The recession affected Americans in many ways, and it appears that one of those ways was in their exercise habits. Due to the recession, Americans took part in more voluntary exercise, but also suffered drops in physical exertion. The increase in available time due to unemployment can help explain the increase in voluntary exercise, and the loss of work-related activities (walking to work, etc.) can explain the decrease in overall physical exertion despite the fact that Americans voluntarily exercised more.

Source: Freakonomics Blog