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Relative Rate of Energy Expenditure
The rate of energy expenditure expressed in METs, kilocalories, or kilo-
joules is an absolute measure of intensity. For all people, higher rates 
of energy expenditure require a greater level of effort, and the above 
categorical terms for the commonly used categories—sedentary, light, 
moderate, vigorous—reflect that fact. For most healthy and reasonably 
fit youth and adults, the categorical terms and the individual’s perceived 
level of effort closely align. Older and unfit persons, however, require a 
greater relative, or perceived, level of effort to accomplish a given physi-
cal activity (see the table below). For example, a healthy person with the 
capacity to expend energy at a rate of 12 METs feels as though he or 
she is performing vigorous activity when jogging (MET level above 7), 
whereas an older or unfit person with a capacity of only 5 METs feels 
as though he or she is performing vigorous activity when walking (MET 
level of 3-4). Recognizing that level of fitness determines the rate of 
energy at which physical activity can be performed is important when 
planning programs to improve the fitness of any individual or group.