[sfricks]

Since fatigue during cycling comes from how much pressure you put on the pedals, not how fast you spin the pedals, you would think that the faster you spin the pedals, the longer you can ride and the greater your endurance. However, all cyclists learn sooner or later that they will reach a point when they spin too fast and the slow down and tire earlier.
A study from Purdue University in Indiana shows why you slow down when you pedal faster than your brain can coordinate your muscles (Medicine & Science in Sports & Exercise, April 2007). You get your maximal force during pedaling when the force of your foot is directed downward. When you pedal faster than is comfortable for you, the downward maximal force is applied later in the pedaling cycle so you never reach your maximum downward force of your feet on the pedals. Since you don't go down on your pedals with as much force, you lose force that starts you in your upward motion and have a weaker up stroke.
The key to pedaling efficiency is to pedal as fast as you can and still retain your maximum sustainable force on your pedals. Most people can tell when they are pedaling too fast because they start to feel uncoordinated when they raise their feet in the up stroke. Optimal cadence for competitive racers is between 90 and 105. Optimal cadence for most experienced riders is 80-85. Anyone pedaling at fewer than 70 revolutions per minute needs instruction and more practice.