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BOX 2-1 
Categories of Intensity of Physical Activity
Categories of Absolute Rate of Energy Expenditure
Aerobic activities are common and important. Such activities include 
walking, dancing, and playing soccer or basketball, all of which involve 
large muscle groups in rhythmic repetitive movement. Aerobic activities 
are performed at a pace that can be continued for more than a few 
minutes, and when done at higher-than-usual levels, improve the 
efficiency and capacity of the cardiorespiratory system (Powell et al., 
2011).
Aerobic activities have commonly been categorized based on the rate 
of energy expenditure, or intensity, at least in part because high rates of 
energy expenditure were known to be important for fitness training and 
were presumed necessary to reduce the risk of heart disease. Although 
energy expenditure can be measured in various units (e.g., kilocalories, 
milliliters of oxygen consumption), metabolic equivalents (METs) have 
become the method of choice in public health circles. One MET is the 
rate of energy expenditure while a person is at rest and is equivalent to 
an oxygen uptake of about 3.5 ml/kg body weight × min. Because METs 
take a person’s body weight into account, the MET rate for any given 
activity is similar for all individuals. For adults, for example, the MET rate 
while sitting at rest is 1.0, for cooking or food preparation is 2.0, for 
walking at 3 miles per hour is 3.5, and for running at 5 miles per hour 
is 8.3. Because of their smaller size, immature motor patterns, and other 
physiologic differences, children’s energy expenditure for most activities 
is slightly higher than that for adults (Torun, 1983). Despite these 
differences, adult MET values provide the best approximation of energy 
expenditure for children for most activities. The exceptions are walking 
and running, for which equations based on age and speed should be 
used (Ridley et al., 2008). 
Although rate of energy expenditure is a continuous variable, splitting 
the range into four categories has become standard (Pate et al., 2008) 
(see the table on the next page). This categorization scheme has 
facilitated conversation, epidemiologic analysis, surveillance, and public 
health recommendations. Although useful, the scheme and its arbitrary 
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Educating the Student Body: Taking Physical Activity and Physical Education to School
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