Horsepower from a Horse
I am sure most people have wondered, why is the output of engines and motor vehicles for that matter termed "horsepower". Does it really measure the power of an engine equivalent to how much power a given number of horses are able to produce?
Apparently it does! The invention of steam engines to replace horses could be the root reason for this term. In order to provide a yardstick for the amount of power steam engines could produce, the term horsepower was conceived.
"In 1702, Thomas Savery wrote in The Miner's Friend: So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…" (http://www.history.rochester.edu/steam/savery/)
In comparison, normal human can produce about 1.2 hp briefly, and sustain about 0.1 hp indefinitely; athletes can manage up to about 2.5 hp briefly and 0.3 hp for a period of several hours.
In 1993, R. D. Stevenson and R. J. Wassersug published an article calculating the upper limit to an animal's power output. The peak power over a few seconds has been measured to be as high as 14.9 hp. However, Stevenson and Wassersug observe that for sustained activity, a work rate of about 1 hp per horse is consistent with agricultural advice from both 19th and 20th century sources.
Source: Wikipedia.org
