The diagram below shows the time dependence of human power output. Note that the time scale is represented logarithmically. After a brief plateau in power output at about

the 300 W level, the output decreases continuously as the time increases with the last data point being based on a value estimated from cross-country bicycle races.
This limited power dictates that the bicycle structure be as light as possible and that all power losses be minimized.

Many of the motions involved in the power train are circular and correspond to a shaft rotating in a bearing. In order to reduce energy absorbing friction forces in the bearings, lubricated ball bearings are used. These were introduced into bicycle design in about 1870.

From: Drela & Langford, "Human-powered Flight,"
Scientific American, November 1985, p 144