When operating close to the 'impedence match' condition, the pedal force provided by the rider has the form shown below. Between locations 1 and 16 there is always a positive force on the pedal. Under these conditions the foot is not used to produce an

upward force on the pedal. Only under slow rotation conditions, such as during a hill climb, will the rider tend to pull up on the pedal as well as push down during the 'power' part of the stroke. As the rotation speed is increased, the magnitudes of the force vectors decrease, until at the maximum rotation speed of which the rider is capable the forces drop to zero. The forces are also time dependent at a constant rotation rate. At the beginning of a pedaling period anaerobic work can be done by the muscle using stored oxygen. As the pedaling time increases this oxygen is depleted and the activity requires an oxygen supply - an aerobic muscle activity.

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