Muscle power: How energy resides
How food becomes force, and how much is lost
What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.Chemical energy in food and fodder, converted by muscles into mechanical work. A working adult sustains about 75 W; a horse about 750 W for a few hours. At best about a quarter of digested food energy becomes work.
Form Chemical energy in food (carbohydrates, fats, proteins) converted to mechanical work in muscle Origin Indirect solarHow it got there
Fusion in the Sun Sunlight Photosynthesis in crops and pasture Food and fodder Digestion and metabolism Muscle contraction (mechanical work)How concentrated it is
| Measure | Value | Note |
|---|---|---|
| Sustained power of an adult human | 75 W1 | |
| Typical rating of manual (hand-powered) equipment | about 30 W2 | |
| Sustained power of a horse | 746 (1 horsepower) for about 3 hours W1 | About three-quarters of a horsepower over an 8-hour day; peaks up to 15 hp briefly. |
| Power of draught animals | ox 350-500; donkey 75-200; camel 400-700 W2 | FAO table prints the unit as kW, evidently a typo for W (its own table gives about 120 W for one ox driven constantly). Lower limits should be cut by about 40% for animals on a typical African diet. |
| Conversion of digested food into work | up to about 25%2 | Overall efficiency can fall to about 2% when animals are used for only part of the year, as for ploughing. |
Muscle power is solar energy at two removes: plants store sunlight as food, and animals (including people) oxidise that food to make ATP, the molecule that powers muscle contraction. Most of the food energy keeps the body alive and warm; only a fraction can be turned into useful work, at best about a quarter of what is digested.
The power available is small. A person working steadily delivers about 75 W, enough to light a bright bulb; an ox delivers perhaps five times as much and a horse ten times as much, but only for a few hours a day. For most of history this was nevertheless the main source of mechanical energy for farming, building and transport, and it still is for hundreds of millions of smallholders.