Here is the simple answer:
You'll be able to ride longer with less fatigue. You should have your feet attached to the pedal (either with toe cages or clipped in) to ride at 90 RPM comfortably if you are not used to this pedal speed.
Here's the Science:
90 RPM recruits more Type 1 (“slow twitch”) muscle fiber which generally use aerobic forms of energy (stored fat for fuel) and allows you to cycle for longer periods of time without getting fatigued.
Pedaling slower, say 60 to 80 RPM, tends to recruit the Type 2 “fast twitch” muscles (ironic, right?) which are larger and more powerful. Because of their high demand, Type 2 muscle fibers tend to use anaerobic energy production (glycogen stores) for powerful bursts of speed.
Glycogen stores are limited, fat is not. Let's use more fat and pedal at 90 RPM.
I'll add that most people whom I've worked with who have never had any cycling training start out pedaling slower, say at 60-75 rpm. I think this is because…
1) No one has taught them good pedal stroke (how to pull back, up, and over the pedal in addition to pushing down).
2) Their feet are not attached to the pedal – it is hard to impossible to have a good pedal stroke when your feet are not somehow attached to the pedal (cleats, toe cages).
3) No one has explained the whole idea of pedaling faster to conserve energy. And they haven't practiced it to make it smooth and comfortable.
4) No one has explained that pedaling slowly into a hill will only S L O W your pedal speed even more – and maybe you'll end up walking up the hill. If you head into the hill at a faster cadence, then you have more room to slow your pedals as the hill gets longer or steeper. (Don't forget to shift or reduce resistance!)
How to Count your Cadence – Watch the video below
Does your Indoor cycle or bike monitor show RPM or Cadence? (by the way, RPM – Revolutions per Minute – and cadence mean the same thing)
If not, here is how to count your RPM:
* Take a 15 second count
* As you pedal, tap your right knee each time at the top of the pedal stroke, count how many times you tap.
15 taps = 60 RPM
17-18 taps = 70 RPM
20 taps = 80 RPM
22-23 taps = 90 RPM
25 taps = 100 RPM
27-28 taps = 110 RPM
Here is a video to show you exactly how to do this: How to Count your RPM
CEM (Chief Executive Mole)
www.powertopedal.com <= We teach you about pedal stroke and speed in these videos!