What constrained sustained, high-intensity mechanical work before the steam engine?

Answer

Flow of rivers, wind, or brute strength of humans/animals

Before the widespread adoption of steam power, work requiring substantial, continuous energy was strictly limited by the availability of moving water, prevailing wind conditions, or the physical labor capacity of humans and animals.

What constrained sustained, high-intensity mechanical work before the steam engine?
inventionmachinepoweradvantagesteam engine