As a machine designed for work, the efficiency of the body is quite low. This means that a significant amount of energy is wasted compared to the amount of work done. This energy isn’t actually wasted, it is turned into heat which the body must get rid of. Even when resting or relaxing, the body is producing heat.
The human body maintains a basic minimum rate of heat production at about 60 Watts during sleep and about 70-120 Watts when awake but sedentary. As bodily activity increases, the rate of oxidation of food will increase, along with this release of energy. The level of heat production for light work is about 150-200 Watts, with an extreme value for very heavy work being about 700 watts.
The following table details standard metabolic activity rates for a range of activities and relates them to heat output from the human body. On average, the surface area of the human body is around 1m².
| Activity Level | W/m² | Met |
|---|---|---|
| Reclining | 46 | 0.8 |
| Seated relaxed | 58 | 1.0 |
| Clock and watch repairer | 65 | 1.1 |
| Standing relaxed | 70 | 1.2 |
| Sedentary activity (office, dwelling school, laboratory) | 70 | 1.2 |
| Sports -Running, 15 km/h | 550 | 9.5 |
| Car driving | 80 | 1.4 |
| Graphic profession - Book Binder | 85 | 1.5 |
| Standing, light activity (shopping, laboratory, light industry) | 93 | 1.6 |
| Teacher | 95 | 1.6 |
| Domestic work -shaving, washing and dressing | 100 | 1.7 |
| Walking on the level, 2 km/h | 110 | 1.9 |
| Standing, medium activity (shop assistant, domestic work) | 116 | 2.0 |
| Building industry -Brick laying (Block of 15.3 kg) | 125 | 2.2 |
| Washing dishes standing | 145 | 2.5 |
| Domestic work -raking leaves on the lawn | 170 | 2.9 |
| Domestic work -washing by hand and ironing (120-220 W) | 170 | 2.9 |
| Iron and steel -ramming the mould with a pneumatic hammer | 175 | 3.0 |
| Building industry -forming the mould | 180 | 3.1 |
| Walking on the level, 5 km/h | 200 | 3.4 |
| Forestry -cutting across the grain with a one-man power saw | 205 | 3.5 |
| Agriculture -Ploughing with a team of horses | 235 | 4.0 |
| Building industry -loading a wheelbarrow with stones and mortar | 275 | 4.7 |
| Sports -Ice skating, 18 km/h | 360 | 6.2 |
| Agriculture -digging with a spade (24 lifts/min.) | 380 | 6.5 |
| Sports -Skiing on level, good snow, 9 km/h | 405 | 7.0 |
| Forestry -working with an axe (weight 2 kg. 33 blows/min.) | 500 | 8.6 |