The most used instruments for the determination of free-living physical activity and energy expenditure are; whole-room indirect calorimeter, Accelerometers and pedometers and also the Doubly-Labeled Water. Indirect calorimetry measures gas exchange from which energy expenditure can be estimated, Double labeled water is the most relevant method for measuring free-living energy expenditure in animals and humans, it is based on exponential disappearance from the body of the stable isotopes (H and O) after a bolus dose of water. Accelerometers and pedometers provide an inexpensive overall measure of physical activity, however they are unable to asses the intensity, frequency and duration of activity or to estimate energy expenditure.
