As the time of pedaling increases, oxidation products build up in the muscles and fatigue starts to decrease the forces and hence the sustainable power. The diagram shows the results of several different measurements of human power output as a function of time. After about 20 min the 300 W plateau gives rise to a slow decrease in sustainable power output, with the one hour level being between 75 and 200 W.

From: Abbott & Wilson, "Human-Powered Vehicles,"
Human Kinetics (1995)