Metabolic Rates For Typical Tasks

The rate at which we generate heat (the metabolic rate) depends mostly upon our level of muscular activity, partly upon what we eat and drink (and when), and partly on where we are in our normal daily cycle. Our heat production is measured in metabolic (met) units (Table 4.1). One met is defined as 50 kcal/h m2 (equal to 18.4 Btu/h ft2 or 58.2 W/m2). One met is the energy produced per unit of surface area by a seated person at rest. Under these conditions, the total heat produced by a normal adult is about 360 Btu/h (106 W). The more active we are, the more heat we produce.

Thermal comfort

There are a number of interactions between our skin and the rest of our body. These include the sense of touch, the circulation of blood, and the exchange of water vapor. The sensations of touch include pressure and pain as well as heat and cold. The experiences of heat and cold are produced by contact with building or object surfaces and by immersion in air, as well as by radiation. These sensations frequently signal impending shifts in bodily heat regulation, a process that is controlled by the portion of the brain called the hypothalamus.

Metabolic Rate a a ^(a)
met
Activity units b b ^(b) Btu/h f t 2 f t 2 ft^(2) W / m 2 W / m 2 W//m^(2)
Resting
Sleeping 0.7 0.7 0.7 13 40
Reclining 0.8 0.8 0.8 15 45
Seated, quiet 1.0 1.0 1.0 18 60
Standing, relaxed 1.2 1.2 1.2 22 70
Walking (on the level)
2 mph (0.9 m/s) 2.0 2.0 2.0 37 115
3 mph (1.2 m/s) 2.6 2.6 2.6 48 150
4 mph (1.8 m/s) 3.8 3.8 3.8 70 220
Office activities
Reading, seated 1.0 1.0 1.0 18 60
Writing 1.0 1.0 1.0 18 60
Typing 1.1 1.1 1.1 20 65
Filing, seated 1.2 1.2 1.2 22 70
Filing, standing 1.4 1.4 1.4 26 80
Walking about 1.7 1.7 1.7 31 100
Lifting, packing 2.1 2.1 2.1 39 120
Driving/flying
Car 1.0 2.0 1.0 2.0 1.0-2.0 18 37 18 37 18-37 60 115 60 115 60-115
Aircraft, routine 1.2 1.2 1.2 22 70
Aircraft, instrument 1.8 1.8 1.8 33 105
landing
Aircraft, combat 2.4 2.4 2.4 44 140
Heavy vehicle 3.2 3.2 3.2 59 185
Miscellaneous occupational
activities
Cooking 1.6 2.0 1.6 2.0 1.6-2.0 29 37 29 37 29-37 95 115 95 115 95-115
Housecleaning 2.0 3.4 2.0 3.4 2.0-3.4 37 63 37 63 37-63 115 200 115 200 115-200
Seated, heavy limb 2.2 2.2 2.2 41 130
movement
Handling 110-lb (50-kg) 4.0 4.0 4.0 74 235
bags
Pick and shovel work 4.0 4.8 4.0 4.8 4.0-4.8 74 88 74 88 74-88 235 280 235 280 235-280
Machine work
Sawing (table saw) 1.8 1.8 1.8 33 105
Light (electrical industry) 2.0 2.4 2.0 2.4 2.0-2.4 37 44 37 44 37-44 115 140 115 140 115-140
Heavy 4.0 4.0 4.0 74 235
Miscellaneous leisure
activities
Dancing, social 44 81 44 81 44-81 140 255 140 255 140-255
Calisthenics/exercise 3.0 4.0 3.0 4.0 3.0-4.0 55 74 55 74 55-74 175 235 175 235 175-235
Tennis, singles 3.6 4.0 3.6 4.0 3.6-4.0 66 74 66 74 66-74 210 270 210 270 210-270
Basketball 90 7.6 90 7.6 90-7.6 90 140 90 140 90-140 290 440 290 440 290-440
Wrestling, competitive 7.0 8.7 7.0 8.7 7.0-8.7 130 160 130 160 130-160 410 505 410 505 410-505
  • aFor average adult with a body surface area of 19.6 ft2 (1.8 m2). For whole‐body average heat production.
  • bOne met = 18.4 Btu/h ft2 = 58.2 W/m2

The hypothalamus triggers changes in our blood circulation patterns in response to signals from our skin and changes in our core body temperature. If the body temperature is dropping (we are cold), the rate of heat loss from the body needs to be reduced. This is accomplished through a decrease in the flow of blood from the core toward the surface of the skin. This decrease in blood flow toward the surface is called vasoconstriction, and is triggered in part by temperature (cold) signals from our skin. Blood carries heat around the body, and reduced flow to the extremities under cold conditions reduces heat loss. Under this condition, our sweat glands also force less water to the skin surface, which reduces evaporation and thus heat loss.

Note the implications of this zoning arrangement. We strive to maintain, at all costs, a nearly constant core temperature for our vital organs. This protected zone takes thermal precedence over the less vital extremities zone, including the arms and legs and then the fingers and toes. The farther from our central body mass (fingers and toes) and the greater the surface area (ears), the more and faster the temperature will drop in cold conditions. The most variable thermal zone is our skin surface.

When cold conditions worsen, we get goosebumps, symptoms of our skin’s unsuccessful attempt to create insulation by fluffing up our body hair. Because we cannot add insulation this way, we soon increase our metabolic rate, or burn more fuel, by shivering, muscular tension, or increased muscular activity. At the point where shivering incapacitates us, we may reach 6 met. Before this point, we seek help from our second and then our third skins of clothing and buildings.

The opposite occurs when we are too hot: first, blood flow toward the skin surface increases (vasodilation), triggered primarily by warm signals from our core. The sweat glands greatly increase their secretion of water and salt to the skin surfaces. This increases heat loss by evaporation (although salt accumulations impede evaporation by lowering the vapor pressure of water).

FREQUENTLY ASKED QUESTIONS

What is the definition of one metabolic unit (met) and how is it measured?
The metabolic unit (met) is a measure of the energy produced per unit of surface area by a person. One met is defined as 50 kcal/h/m², which is equivalent to 18.4 Btu/h/ft² or 58.2 W/m². This unit is used to quantify the rate of heat production by the human body, which depends on factors such as muscular activity, diet, and time of day.
How does the level of muscular activity affect metabolic rate?

The level of muscular activity has a significant impact on metabolic rate. As a person becomes more active, their metabolic rate increases, resulting in higher heat production. For example, a person engaged in light physical activity such as walking may have a metabolic rate of 2-3 met, while someone engaged in strenuous activity such as running may have a metabolic rate of 8-10 met.

What is the typical metabolic rate for a seated person at rest?

The typical metabolic rate for a seated person at rest is approximately 1 met, which corresponds to an energy production of 360 Btu/h (106 W) for a normal adult. This is the baseline metabolic rate used as a reference point for comparing the metabolic rates of people engaged in different activities.

How does diet and time of day affect metabolic rate?

Diet and time of day can also influence metabolic rate, although to a lesser extent than muscular activity. Consuming a meal can increase metabolic rate temporarily, as the body expends energy to digest and process the nutrients. Additionally, metabolic rate can vary slightly over the course of the day, with some studies suggesting a natural circadian rhythm in metabolic rate.

How is metabolic rate related to thermal comfort?

Metabolic rate is an important factor in determining thermal comfort, as it affects the amount of heat produced by the body. When the body produces more heat, it can lead to discomfort and even heat stress if the surrounding environment is not able to dissipate the heat effectively. Therefore, understanding metabolic rate is crucial in designing and operating HVAC systems that can maintain optimal thermal comfort conditions.

Can metabolic rate be used to estimate energy consumption in buildings?

Yes, metabolic rate can be used to estimate energy consumption in buildings. By knowing the number of occupants, their activity levels, and the duration of their stay, building designers and operators can estimate the total heat gain and energy consumption of the building. This information can be used to size HVAC systems and optimize building energy efficiency.