Metabolic Equivalents (METs) offer a simple, standardized way to measure the energy cost of physical activities relative to the energy spent at rest. This concept allows health professionals and individuals to quantify how much harder the body is working during exercise compared to sitting still. By using a single numerical value, METs standardize the comparison between activities as varied as light gardening and competitive running. The higher an activity’s MET value, the greater the demand for oxygen and the more energy the body expends per unit of time.
Defining the 1 MET Baseline
The foundation of the Metabolic Equivalent system rests on the definition of 1 MET, which represents the energy expenditure of the body while sitting quietly at rest. Physiologically, this baseline is defined by the rate of oxygen consumed. Specifically, 1 MET is conventionally set as the consumption of 3.5 milliliters of oxygen per kilogram of body weight per minute (3.5 mL/kg/min).
This value was originally derived from the resting oxygen uptake of a single reference subject. The 3.5 mL/kg/min figure serves as a universal, standardized resting metabolic rate for comparison in large-scale studies. While the true resting metabolic rate (RMR) varies among individuals based on age, sex, and body composition, this conventional 1 MET value remains the fixed standard against which all activity intensity is measured.
For instance, an activity rated at 4 METs means a person is consuming oxygen and expending energy at four times the rate they would while sitting still. The convenience of the fixed 1 MET standard allows for simple, consistent calculation across different activities. This standardized baseline allows experts to create widely applicable tables of activity values, such as those found in the Compendium of Physical Activities.
Classifying Physical Activity Intensity
MET values provide a practical framework for classifying physical activities into three distinct intensity levels. These classifications help individuals understand the effort required for various tasks and meet weekly activity goals.
Light Intensity (Less than 3.0 METs)
Activities requiring less than 3.0 METs are categorized as light intensity, demanding minimal effort above the resting rate. Examples include folding laundry, slow walking, and light household chores. While important for breaking up sedentary time, these activities generally do not significantly improve cardiorespiratory fitness.
Moderate Intensity (3.0 to 5.9 METs)
Moderate-intensity activities significantly elevate the heart rate and breathing, allowing a person to talk but not sing during the effort. Common examples include brisk walking (2.5 to 4 mph), playing doubles tennis, or raking the yard. Health authorities recommend accumulating 150 minutes of moderate-intensity activity per week.
Vigorous Intensity (6.0 METs or More)
The highest category is vigorous intensity, which encompasses any activity requiring 6.0 METs or more. These activities cause a person to breathe hard and fast, making it difficult to say more than a few words without pausing. Because vigorous activities require a higher energy output per minute, the recommended weekly goal can be met in half the time—typically 75 minutes. Examples include:
- Running
- Swimming laps
- Singles tennis
- Vigorous step aerobics
Translating METs into Energy Burn
The utility of the MET system lies in its ability to estimate the total energy expenditure, or calories burned, during physical activity. Since 1 MET is directly linked to a rate of oxygen consumption, and oxygen consumption is proportional to energy expenditure, a mathematical relationship allows for calorie calculation. The formula used to calculate the estimated calories burned per minute is: (METs x 3.5 x Body Weight in Kilograms) / 200.
The constant 3.5 represents the oxygen consumption rate in mL/kg/min, and the division by 200 is a conversion factor that translates the oxygen consumption value into kilocalories. For example, a person performing an activity with a MET value of 5.0 would use this formula to estimate their caloric burn rate per minute. Multiplying this per-minute rate by the total duration provides a total estimated caloric expenditure for the workout.
This quantitative estimate is valuable for individuals focused on weight management, as it helps determine the energy deficit needed for weight loss goals. This calculation provides an estimate based on the standardized 1 MET value, meaning it does not perfectly account for individual factors like fitness level, age, or body fat percentage. Nevertheless, the MET calculation provides a reliable metric for comparing the energy cost of different exercises and tracking overall activity levels.