The database of activities defines how much energy is expended for hundreds of activities.
The basic idea behind the database is that activities are assigned a MET value.
A MET values is defined as :
MET (Metabolic Equivalent): The ratio of the work metabolic rate to the resting metabolic rate. One MET is defined as 1 kcal/kg/hour and is roughly equivalent to the energy cost of sitting quietly. A MET also is defined as oxygen uptake in ml/kg/min with one MET equal to the oxygen cost of sitting quietly, equivalent to 3.5 ml/kg/min.
The concept of a MET value and the source for many of the activates in the database comes from The Compendium of Physical Activities.