The metabolic equivalent of task (MET) is the objective measure of the ratio of the rate at which a person expends energy, relative to the mass of that person, while performing some specific physical activity compared to a reference, currently set by convention at an absolute 3.5 mL of oxygen per kg per minute, which is the energy expended when sitting quietly by a reference individual, chosen to be roughly representative of the general population, and thereby suited to epidemiological surveys. A Compendium of Physical Activities is available online, which provides MET values for hundreds of activities. A primary use of METs is to grade activity levels for common household activities (such as cleaning) and common exercise modalities (such as running). Vigorous household chores can add up to as much energy expenditure as dedicated exercise, so it is necessary to include both, suitably pro rata, in an assessment of general fitness. An earlier convention defined the MET as a multiple of the resting metabolic rate (RMR) for the individual concerned. An individual's resting metabolic rate can be measured by absolute gas exchange, absolute thermal output, or steady-state diet in a sedentary condition (with no reference to body mass); or it can be estimated from age, sex, height, body mass, and estimated fitness level (which in part functions as a proxy for lean body mass). As a relative measure, it might correlate better with rating of perceived exertion. This definition is more common in colloquial use on the Internet concerning personal fitness, and less common in the recent academic literature. As a relative measure suited to judge exertion level for the individual athlete, many coaches now prefer a measure indexed to maximum heart rate, which is easy to monitor continuously with modern consumer electronics. Exercise equipment with an accurate delivered-wattage indicator permits the use of relative METs for the same purpose, assuming a known ratio of biological efficiency in converting metabolic energy to mechanical energy, commonly estimated as around 25%. A benefit of relative METs over heart rate is that it tracks fairly directly to caloric consumption, and can be used to judge the impact of task exertion on fed or fasted states in various dietary regimes, such as intermittent fasting; fast duration in this context is sometimes denominated in MET⋅hours (effectively RMR⋅hours), where sedentary hours count as unitary. An alternative convention for the absolute MET replaces the mass of a reference individual with the body surface area of a chosen reference individual. Health and fitness studies often bracket cohort activity levels in MET⋅hours/week.
Quantitative definitions
Based on oxygen utilization and body mass The original definition of metabolic equivalent of task is the oxygen used by a person in milliliters per minute per kilogram body mass divided by 3.5. Other definitions which roughly produce the same numbers have been devised, such as:
1 MET = 1 kcal kg × h = 4.184 kJ kg × h = 1.162 W kg {\displaystyle {\text{1 MET}}\ =1\,{\frac {\text{kcal}}{{\text{kg}}\times {\text{h}}}}\ =4.184\,{\frac {\text{kJ}}{{\text{kg}}\times {\text{h}}}}=1.162\,{\frac {\text{W}}{\text{kg}}}}
where
kcal = kilocalorie kg = kilogram h = hour kJ = kilojoule W = watt
Based on watts produced and body surface area Still another definition is based on the body surface area, BSA, and energy itself, where the BSA is expressed in m2:
1 MET = 58.2 J s × BSA = 58.2 W m 2 = 18.4 Btu h × ft 2 {\displaystyle {\text{1 MET}}\ =58.2\,{\frac {\text{J}}{{\text{s}}\times {\text{BSA}}}}\ =58.2\,{\frac {\text{W}}{{\text{m}}^{2}}}=18.4\,{\frac {\text{Btu}}{{\text{h}}\times {\text{ft}}^{2}}}}
which is equal to the rate of energy produced per unit surface area of an average person seated at rest. The BSA of an average person is 1.8 m2 (19 ft2). Metabolic rate is usually expressed in terms of the unit area of the total body surface (ANSI/ASHRAE Standard 55).
Based on resting metabolic rate Originally, 1 MET was considered as the resting metabolic rate (RMR) obtained during quiet sitting. Although the RMR of any person may deviate from the reference value, MET can be thought of as an index of the intensity of activities: for example, an activity with a MET value of 2, such as walking at a slow pace (e.g., 3 km/h) would require twice the energy that an average person consumes at rest (e.g., sitting quietly).
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