Cadence is a matter of balancing your muscles vs. your cardiovascular system. At any given speed, there's a required amount of power that you have to generate to overcome aero and rolling-resistance. How you generate that power is up to you, but it's given by the following:
POWER = PedalForce X RPM
At any given power-output, you can push hard at low-RPM or push lightly at high-RPM.
Then in order to balance the muscles vs. lungs, you determine which one is the limiting step in your riding. If your muscles get tired, sore and cramped up first, then you'll want to spin higher-RPMs with lower force (lower gears) to preserve your muscles.
If your lungs & heart are falling out and your legs don't feel a thing, then use higher gears and push the pedals harder.
Originally Posted by Mentor58
I may have been guilty of not being as precise in my language as possible. What I'm looking at is finding a cadence that will maximize my endurance and still allow for effective cycling. Having done some touring and a few (for me) long distance rides, (300 miles in 3 days), I feel that my usual high cadence may be working against me. Based on some of the studies that I've seen on the web, a high cadence tends to be more cardio-vascular stressful than lower (which makes sense), and also utilizes a larger percentage of Fast Twitch Muscles. A lower cadence will tend to utilize Slow Twitch Muscles, and would be better for maximizing performance if your metric is "riding long distances with speed being of secondary concern".
I think you're mixing up two different concepts, cardio vs. muscular balance (determined by RPM) and fast vs. slow-twitched as determined my muscle-force. We've gone over cadence above.
the percentage ratio of force generated from fast vs. slow-twitch muscles is determined by total force. Your muscles can only generate up to a certain amount of max-force, as determined by a 1-rep max-lift. At this load level, you're using 100% of both your fast & slow-twitch muscles with the primary contribution coming from fast-twitch anaerobic muscles.
At the lower load-levels, say a 40-50 rep lift, you'd use primarily aerobic slow-twitch muscles.
On the bike, there's actually a 3d graph of muscle-load X RPM X fast/slow-twitch depending upon the power-required. The higher the power-generated, the more pedal-force requried and more utilization of fast-twitch muscles. HOWEVER, at any given power, you'll
still use more slow-twitch muscles at higher-RPMs due to the lighter loads on your muscles. For example, take the following two power-outputs and the required pedal-force
200-watts output ~19mph
90 rpms = 27 lbs average force on pedals
60 rpms = 39 lbs average force on pedals
400-watts output ~25mph
90 rpms = 53 lbs on pedals
60 rpms = 80 lbs on pedals
You'll see that the lower-cadence at any given power-output will require higher force at the pedals. Depending upon where your LT is, a function of max-lift strenght and aerobic efficiency, you'll use a ratio of fast vs. slow-twitch muscles. But as you get closer and closer to LT with higher pedal forces, you'll get a larger and larger portion of force from the fast-twitch muscles.
There is yet another variable (resulting in 4D graph) of fat vs. carb utilization which affects endurance on long rides. Fast-twitch, high-force muscles use primarily carbs for energy while slow-twitch, low-force muscles use a lot of fat. Pushing hard on teh pedals in high-gear will have you burn up carbs at a faster rate than pushing ligher in low-gears which would burn up a higher percentage of fats. If your endurance-limit is partly determined by energy-supply & bonking, this will play a role as well.
So to find your "optimum" cadence... compare the end-results you have now and see how varying your cadence will affect it. Is your "endurance limit" a muscular-fatigue issue? Like your muscles are sore, cramped and tired? Then a higher-cadence would actually reduce muscle-loads and increase your endurance. If your lungs and heart tire out first and your legs feel fine after a double-century, then you might to push harder on the pedals and use a higher-gear.