Muscles, wood, water and wind: energy before fossil fuels: The human body as an energy converter

1.1 Most of the food we eat simply keeps us alive

A human is a heterotroph: unable to capture sunlight, we must eat what plants have made, directly or through animals. The first claim on that food is basal metabolism, the energy needed at rest to keep the heart, lungs, liver, kidneys and brain working and the body at 37 C. For a 70 kg man it is about 7 MJ (about 1,700 kcal) a day, which is a continuous power of about 80 W. For a sedentary adult, basal metabolism takes 60 to 70% of all the energy used in a day.

Total daily need is basal metabolism multiplied by a physical activity level. Office work gives a level of about 1.4 to 1.5; moderate manual labour 1.7 to 2.0; heavy farm or construction work above 2.0. A 60 kg woman with a basal rate of 5.6 MJ doing moderately active work at a level of 1.6 needs about 9 MJ, or about 2,150 kcal, a day. A man hauling loads all day at a level of 2.2 might need 15 MJ.

The brain is unusually expensive. It is about 2% of adult body mass but uses about a fifth of basal metabolism, and in infants more than half. Smil links the growth of the human brain over evolutionary time to better-quality diets, with more meat and, later, cooked food, which deliver more energy and protein per bite and per hour of digestion.

Energy is not the only need. Adults require about 0.8 g of good-quality protein per kilogram of body weight per day, more for growing children. Requirements are uncertain and have been revised several times by FAO and WHO. Also, national food supply figures are not what people eat: in rich countries a third or more of supplied food is wasted between farm and plate.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M3 'The human body as an energy converter'
• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M3 'The human body as an energy converter'
• Smil, Should We Eat Meat? Evolution and Consequences of Modern Carnivory (2013) · via Smil Mastery Course, M3 'The human body as an energy converter'

1.2 A person can sustain only 50 to 100 W of useful work, at about a quarter efficiency

The useful mechanical power a healthy adult can keep up for hours is surprisingly small: about 50 to 100 W. Trained cyclists can hold over 400 W for an hour, and elite sprinters touch 1,500 to 2,000 W for a few seconds, but no one works a full day at those rates. A farm labourer digging, hoeing or carrying averages well under 100 W of useful output.

Muscles convert roughly 20 to 25% of the food energy they burn into work. The rest is heat. So delivering 75 W of useful power for an hour means burning 300 to 400 W of food energy over that hour, on top of basal needs. This is why heavy labour can double a person's food requirement, and why, in traditional societies, the cost of human work was largely the cost of the grain to fuel it.

Simple machines did not add power, but they let people apply it better. Levers, pulleys, wedges, inclined planes and windlasses traded force for distance, so a person could lift a heavy stone slowly rather than a light one quickly. Treadwheels used the body's strongest muscles, the legs, and let a person deliver a steady 50 to 100 W to lift building materials or grind grain. Medieval cranes on cathedral sites were often treadwheels with one or two people walking inside.

The Energy table lists this as Muscle power among the natural sources, and the Rockets and human power tile shows that human power still matters in bicycles, rickshaws and hand tools. The basic limit, though, has never changed: a few tens of watts per person, which is why moving from human muscles to any other prime mover multiplied what a society could do.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M3 'Limits of human performance'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M3 'Limits of human performance'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M4 'Prime movers: muscles and animals'

1.3 Endurance, not speed, is the human strength, and organisation multiplied it

The real limit on sustained human work is heat. Three quarters of the energy a working muscle burns becomes heat, and it must be shed or the body overheats. Humans are exceptional at this. We are nearly hairless and have millions of sweat glands, and we can lose more than a litre of sweat an hour. Each litre that evaporates carries away about 2.4 MJ, the same latent heat that drives the water cycle.

This gave early humans an unusual hunting method. We are slow sprinters, but over long distances in the midday heat we can outlast most mammals, including antelopes and, in hot conditions, even horses. Smil describes persistence hunting: following an animal at a steady trot until it collapses from overheating, because it cannot sweat as we do and must stop to pant. Body shape also adapts to climate. Tall, slender bodies shed heat better in the tropics; shorter, stockier ones conserve it in the cold.

The second multiplier was organisation. Monumental projects were not built by superhuman effort but by planning, logistics and steady work by modest numbers of people over long periods. Smil's calculation for the Great Pyramid of Khufu, using the energy needed to quarry, move and lift about 6 million tonnes of stone, is that it needed fewer than 10,000 workers at any one time, and probably under 7,000, over roughly two decades. That is far fewer than the 100,000 Herodotus claimed. The hard part was feeding, housing and coordinating them, not the raw muscle power.

The pattern holds across early civilisations. Human energy was cheap per person but tiny per person, so anything large demanded either many people or many years.

• Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M3 'Limits of human performance'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M3 'Limits of human performance'
• Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M3 'Foraging: gathering, hunting, fishing'

See it in the table

Check your understanding

A man has a basal metabolism of 7 MJ a day and a physical activity level of 2.0. What is his daily food energy need?

A labourer delivers 80 W of useful work at 20% muscle efficiency. How much food power does that work burn?

What made persistence hunting possible?