Animal And Human Muscle Work

There are no reliable estimates of aggregate animal or human muscle work as such, although the horsepower unit (of work per hour) was originally defined by James Watt to measure the output of steam engines, based on a comparison with the work done by a horse pumping water via a treadmill. It is possible, however, to estimate human and animal outputs of mechanical work crudely on the basis of food or feed intake, multiplied by a biological conversion efficiency. Human muscle work was already negligible by comparison at the beginning of the 20th century. The US population in 1900 was 76 million, of which perhaps 50 million were of 'working age'. Of these, only 25 million were men. Women worked too, perhaps even longer hours than men, but, except for some shopkeepers, teachers and nurses, their labor was not monetized and hence did not contribute to GDP at the time. Despite the impression created by 'working class' songs of the time, such as 'John Henry' and 'Sixteen Tons', at least half of the employed workers were probably doing less physical things like operating telegraphs, entering figures in accounts, driving carriages or trams, caring for animals, cooking or making furniture. In short, they were doing jobs

90 80

J 70

8 50

5 40

1900 1910 1920 1930 1940

1970 1980 1990 2000

Figure 4.1a Percent of coal exergy consumed by type of end-use (USA, 1900-2004)

1900 1910 1920 1930 1940

1970 1980 1990 2000

Figure 4.1a Percent of coal exergy consumed by type of end-use (USA, 1900-2004)

1900 1910 1920 1930 1940 1950

1970 1980 1990 2000

1900 1910 1920 1930 1940 1950

1970 1980 1990 2000

Figure 4.1b Percent of coal exergy consumed by type of end-use (Japan, 1900-2004)

Figure 4.2a Percent of petroleum and NGL exergy consumed by type of end-use (USA, 1900-2004)

Figure 4.2b Percent of petroleum and NGL exergy consumed by type of end-use (Japan, 1900-2004)

1900 1910

1930 1940

1970 1980

1950

1960

1990

2000

1900 1910

1930 1940

1970 1980

Figure 4.3a Percent of natural gas exergy consumed by type of end-use (USA, 1900-2004)

Heat Light

Electricity Prime movers Non-fuel uses

Figure 4.3b Percent of natural gas exergy consumed by type of end-use (Japan, 1900-2004) (no usage prior to 1945)

Heat Light

Electricity Prime movers Non-fuel uses

Figure 4.3b Percent of natural gas exergy consumed by type of end-use (Japan, 1900-2004) (no usage prior to 1945)

1950

1960

1990

2000

Figure 4.4a Percent of total fossil fuel exergy consumed by type of end-use (USA, 1900-2004)

Figure 4.4b Percent of total fossil fuel exergy consumed by type of end-use (Japan, 1900-2004)

that depended more on eye-hand coordination or intelligence than on sheer muscular effort.4

The minimum metabolic requirement for an adult man is of the order of 1500 calories per day, whereas the average food consumption for a working man was (and is) about 3000 calories per day. Thus, no more than 1500 calories per day were available for doing physical (muscle) work above and beyond metabolic needs, for example, to chew and digest food, breath air and circulate the blood. This comes to 18 billion calories per day or about 0.16 EJ per year of food exergy inputs for non-metabolic human muscular effort, as compared to aggregate fossil fuel consumption of 8.9 EJ in 1900. Assuming muscles convert energy into work at about 15 percent efficiency, the overall food-to-useful-work conversion efficiency for the US population as a whole would have been roughly 2.4 percent at that time. In recent years, of course, more and more women have joined the labor force. Given the changing (less physical) nature of modern work, and the much greater life expectancy and longer retirement time, the average conversion efficiency has probably declined significantly. We note, however, that in some developing countries, such as India, the human contribution to physical (mechanical) work, especially in agriculture, may not yet be negligible as compared to the contribution from machines.

Since human labor is treated independently in economic analysis - and since human muscle power is no longer an important component of human labor in the industrial world, as compared to eye-hand coordination and brainwork - we can safely neglect it hereafter. However, work done by animals, especially on farms, was still important in the US at the beginning of the 20th century and remained significant until trucks and tractors finally displaced most of the horses and mules by mid-century.5

According to Dewhurst, 18.5 units of animal feed are needed to generate one unit of useful (physical) work by a horse or mule (Dewhurst 1955, pp. 1113-16, cited in Schurr and Netschert 1960, footnote 19, p. 55). This implies an effective energy conversion efficiency of 5.4 percent for work animals. However, more recent estimates by several authors converge on 4 percent efficiency or 25 units of feed per unit of work done (for example, Gruebler 1998, box 7.1, p. 321 and references cited therein; also Kander 2002). We choose the latter figure, right or wrong. Evidently the work done by animals can be estimated from the feed consumption, which can be estimated with reasonable accuracy.

Luckily, higher precision is probably unnecessary for the quantitative estimates in the US case because even at the beginning of the 20th century the magnitude of animal work was relatively small compared to inanimate power sources. Inanimate sources of mechanical work (hydraulic turbines, steam engines and windmills) exceeded animal work in the US

by 1870. However, again, in some developing countries animal work is still quantitatively important.

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