Measurement of the power output of people as a function of the knee extension rate measured in degrees per second has the form shown below. Initially the power curve increases linearly with knee velocity, but then saturates at a knee velocity of about 240 degrees per second.

In designing a gear system for a bike, this maximum knee velocity must be taken into account. As the knee velocity is increased further, the power output falls since the force the cyclist can apply to the pedals decreases. Under these higher velocity conditions the inefficiencies of the muscle system increase and useful output from the legs decreases. This situation can be summarized in the curve shown on the next page which shows a range of foot velocities over which there is a good 'impedance match' between the cyclist and the bicycle system.

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