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, set by convention at 3.5 mL of oxygen per kilogram per minute, which is roughly equivalent to the energy expended when sitting quietly. 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: 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: 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). Use MET: 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 MET concept was primarily designed to be used in epidemiological surveys, where survey respondents answer the amount of time they spend on specific physical activities. MET is used to provide general medical thresholds and guidelines to a population. A MET is the ratio of the rate of energy expended during an activity to the rate of energy