Determinants of 24-hour Energy Expenditure in Man, Journal of Clinical Investigation, 1986
Lots of processes in our bodies require energy, burn calories if you will. Eating uses energy. Thinking uses energy. Repairing tissue uses energy. Breathing uses energy. Sex uses energy. Moving molecules within cells and between cells uses energy. And, of course, activity or physical exertion uses energy:
"A highly significant correlation was observed between activity and energy expenditure (P < 0.0001 in the 118 subjects)."But not all activity is the kind you see in aerobics classes. There are less overt forms, such as opening the refrigerator door, or "movements of the limbs with little displacement of the body's center of gravity," what these authors called "fidgeting." These are what was measured as "activity" in the above quote.
In this study, 118 subjects spent time in a respiratory chamber and "were not allowed to carry out physical exercise such as isometric exercises or calisthenics." Nonetheless, the spontaneous physical activity they could perform, "strolling around the chamber" and fidgeting cost them between 95 and 903 kcal/day.
The authors concluded:
"A large portion of the variability of [24-hour energy expenditure] among individuals, independent of differences in body size, was due to variability in the degree of spontaneous physical activity, i.e., "fidgeting."
A Role for Brain-Specific Homeobox Factor Bsx in the Control of Hyperphagia and Locomotory Behavior, Cell Metabolism, 2007.