Physical activity is something I do more than something I study.  But sedentary behavior and its health outcomes intrigue me.  Many of the articles I read on obesity causes and prevention strategies, will mention the amount of physical activity that Americans engage in and that we have more sedentary time.
The article I mentioned Sunday, by researcher Roland Sturm, went over some of the changes in our energy expenditure - supporting still that the increase in the calories we consume is far greater than the decrease in the ones we burn.  
He was discussing that it is a myth that Americans spend less time in leisure activities.  In fact, we have increased the amount of time we spend doing things besides work and chores.  But Sturm, as Swinburn has, notes that the type of leisure activity we engage in has changed.  Our leisure is sedentary - or sitting, passive.  
Not all of the change was purposeful.  One instance he noted was related to a technical advance and an unintended consequence.  Golf and the golf cart.  Sturm said that golf carts came into the scene so that people who were unable to walk more than a few feet could still participate.  Then some people who did not need carts started to use them too - THEN, walking golfers started causing backups on the course and began to get dirty looks and such. Now most people use golf carts and it is not as active a sport as it used to be.