Researchers believe that just like weight, our bodies have their own "set point" when it comes to exercise, meaning that it compensates for extra movement by making us lazier later on.

The New York Times reports on a study of kids ages 8 to 10 who were each given different PE requirements. The kids at the school with the strictest requirements spent 65% more time exercising during the schoolday than kids at other institutions. However, when researchers looked at activity levels across the whole day, they actually evened out. Turns out that these kids who moved around a lot at school were lazier when they got home. Study author Terence J. Wilkin coins it as "activity at one time is met with less activity at another."

The findings might help explain why people have a hard time losing weight through exercise, showing that their bodies may be working against their efforts. Some have questioned Wilkin's conclusions, pointing out that other studies show environment has a profound effect on activity levels. He defends the study by saying:
Exercise is extremely good for the health of young people, as it is for all of us. It improves metabolic profiles and cardiorespiratory fitness. Our results should not be interpreted to mean that exercise is not worthwhile.