Environmental Engineering Reference
In-Depth Information
variety of actual needs determined by the type and dura-
tion of activities.
Human energy costs can be measured directly and
with great accuracy. The first human respiration chamber
was built in the 1860s (Pettenkofer and Voit 1866), and
the first ingenious calorimeter was used to determine
both heat production and respiratory exchange just
before the end of the twentieth century (Atwater and
Benedict 1899). Since that time we have accumulated
thousands of measurements for scores of activities, with
energy cost now usually expressed in terms of physical
activity ratio (Durnin and Passmore 1967; FAO 2003)
or as MET (Ainsworth 2002). Thinking is at the bottom
of that scale. The brain's constantly high BMR goes up
only marginally during challenging mental tasks, which
means that science is, energetically at least, very light
work. Standing requires the deployment of large leg
muscles, resulting in MET of 1.2-1.3. Light exertions
are typical of numerous service jobs that now dominate
modern economies. Typing, truck driving, food retail, or
car repair have usual physical activity ratios of 1.8-2.0.
Most modern manufacturing and construction work as
well as mechanized farming belong to the same category.
Activities requiring moderate or heavy exertions are
most common in traditional farming: plowing, manual
planting, weeding, and harvesting have MET 4-6; dig-
ging and cleaning of irrigation canals, tree cutting, and
some fishery tasks rank above 6. But even for the most
demanding occupations average long-term expenditures
may be only in the medium category, as the spells of
taxing exertions are interspersed with periods of less
demanding activity or rest. In traditional farming these
disparities also had a seasonal flux. Averages for a group
of North China peasants showed 11.5 h of moderate to
heavy work during the summer harvest, with 6.5 h of
5.5
Energy costs of cycling. Based on D. G. Wilson (2004).
sleep, whereas during the mid-winter there were only
7 h of light work and 10 h of sleep (Chen 1981).
Energy costs of leisure and sport activities depend on
their intensity. Cycling is a perfect example of this differ-
ence: its energy demands can range from very moderate
in sedate pedaling to extremely heavy in record-breaking
races (fig. 5.5). Because it deploys very efficiently the
body's largest leg muscles, cycling is also the fastest
mode of human locomotion. The fastest human-
powered machines on land, water, and in the air have all
been propelled by accomplished cyclists (D. G. Wilson
2004). Competitive cross-country skiing and medium-
