Muscle power: Fluctuation in time
Fatigue, harvest calendars and the march of machines
What this tab covers How availability changes over seconds, days, seasons, years and longer, how predictable it is, and what that means for the plants that use it.Available on demand but only for limited hours a day; constrained by fatigue, food supply and the farming calendar, and in long-term decline as machines take over.
Variation by time scale
| Time scale | What happens | How large |
|---|---|---|
| Seconds to minutes | Short bursts far exceed sustainable output.1 | A horse can briefly reach about 15 hp vs about 1 hp sustained |
| Daily | Work is limited by fatigue, heat, shade, health and feeding.2 | Oxen 5-6 hours a day; donkeys 3-5 hours; a horse sustains 1 hp for about 3 hours |
| Seasonal | Animals are needed mainly for ploughing and planting, a fraction of the year.2 | Overall food-to-work efficiency as low as about 2% |
| Decades | Farm power shifts from hands to animals to tractors as incomes rise.6 | Sub-Saharan Africa in the late 1990s: 65% of cultivated area prepared by hand, 25% by animals, 10% by tractor |
How predictable it is
High in the sense that work can be scheduled, but output is capped by fatigue and nutrition, and the peak labour demand of planting and harvest is the bottleneck in manual farming systems.
Capacity factors and utilisation
| Plant or measure | Value |
|---|---|
| Working hours per day, ox (about 21-25% of the day, calculated) | 5-6 hours2 |
| Working hours per day, donkey | 3-5 hours2 |
Unlike most flows, muscle power is dispatchable: a worker or a team of oxen can start when needed. But it is time-limited. Animals typically work five or six hours a day, and a horse's famous one horsepower lasts about three hours before falling to around three-quarters of that over a full day.
Over the year the mismatch is larger: draught animals are essential for a few weeks of land preparation but eat throughout the year, which is why their overall efficiency can drop to a few per cent. Over decades, as tractors and engines spread, muscle power declines first in field work and transport and lingers longest in poor, smallholder farming.
Sources
- Why one horsepower is more than the power of one horse Institute of Physics (IOPSpark)
- Research on diversified uses of work animals: needs, experiences and methods (Thomas, Oram and Tembo) FAO
- Contribution of farm power to smallholder livelihoods in sub-Saharan Africa (Agricultural and Food Engineering Technical Report 2) FAO