Ah, the cadence question. Whether 'tis nobler in the legs to pedal hard and slow or to take strokes lightly, and by pedaling fast, save energy? All right, the strained Shakespeare reference aside, the question of whether it's better to pedal slow or fast depends on your training goals.
Exercise leads to fatigue, and the cadence you use during cycling can affect how fatigue impacts your riding. When you pedal slowly, you're pushing against more resistance with each pedal stroke, which means you have to recruit a lot of muscle fibers in your legs to generate enough power to keep going. The trouble is, many of those fibers fatigue quickly, no matter how fit you are. Pedaling faster reduces the resistance you're pushing against with each stroke, which shifts a good portion of the stress of pedaling from your leg muscles to your heart and lungs. Since your heart and lungs don't fatigue the same way skeletal muscles do, this shift allows you to keep riding longer before your legs get tired.
Sample Workout: Muscle Tension
Find a gradual climb (5 to 8 percent), shift into a big gear that you can only push at a cadence of 50 to 55 rpm. Stay seated and relax your upper body, and focus on pulling your feet back through the bottom of the pedal stroke and pushing forward over the top of the stroke. Continue grinding your way uphill for five to eight minutes, rest ten minutes, and repeat for a total of two or three intervals.
High-cadence cycling received a lot of attention during Lance Armstrong's first Tour de France victory in 1999 because his pedal speed in the mountains and time trials was notably faster than his rivals'. During his comeback from cancer we discovered that he could produce more power, go faster, and maintain that speed longer by pedaling faster instead of harder. Cancer peeled 17 pounds of muscle from his frame, and mashing big gears with that remaning muscle led to fatigue very quickly. As a result, it made sense for him to purposely shift as much work as possible from his leg muscles to his aerobic engine.
Pedaling faster puts more stress on your aerobic system, but with training, your aerobic system will adapt and you'll be able to sustain a high pace on flat ground and hills for longer periods of time.
Sample Workout: Fast Pedal
On a relatively flat road, shift into an easy gear and bring your cadence up to 15 to 16 pedal revolutions per ten-second count. This equates to a cadence of 90 to 96 rpm. Stay seated with your upper body relaxed, and try to pedal even faster while keeping your hips from bouncing. If your hips start to bounce on the saddle, you're pedaling faster than you can control, and you should back off until you can pedal smoothly again. Intervals should be five minutes of continuous pedaling, separated by five to ten minutes of normal cruising cadence riding.