Grinding vs. Spinning

In a previous post, I offered a hugely simplified potential energy analysis to determine a lower bound on the amount of power draw on my body on a recent climb.

The model consisted of two parts. The first was the potential energy of moving the mass of bike+rider up a hill. The second was the potential energy involved in moving the mass of the legs around and around on the cranks. Using these, I computed 198 Watts for the mass up a hill part and 46.44 Watts for the legs moving around on the cranks (at a 55 rpm cadence) part.

Here, I am curious about what this model might say about relative power draw for the same climb and same rate of climb except with variations in cadence. What if I climbed at the same rate but geared for a cadence of 60 rpms, 70, 80 or 90? What is the difference in power draw and how large a fraction of the total does the moving legs part become?

Using the same appraoch explained in a previous post, I've computed and tabluated the results below.

Tabulation of power due to two separate contributions; moving mass up a hill
and mass of legs in motion around cranks and as a percentage of the total.
This is based on same climb and speed of climb but just differnt gearings.

Climbers are often classified into two types; grinders and spinners. Grinders, like Jan Ulrich, opt for lower cadences on climbs, often below 60 rpms. Spinners, like Lance Armstrong, go for higher cadences, often above 85. All other things being equal though, there is a price to pay for that. A spinner at 90 rpms will draw more power just to keep the mass of his/her legs in motion at a higher cadence. In my example, the power difference is a tad over 30 additional watts. Another way to think about this is that each additional rpm requires approximately one additional watt of power from my body.

What this means for you, the rider, is that your cardio system is going to be working that much harder to accomodate the extra power draw due to your faster moving legs. But, your leg muscles and feet will have a bit of an easier go of it because although they contract more often they don't have to contract as forcefully and the pedal pressure is lower. So, this is a way in which you have some ability to tradeoff cardio capacity for muscular strength during a ride. By shifting to lower gears and spinning more, we place more load on our cardio system but lessen the load on our leg muscles and feet and vice versa.

For me and for most people I think, higher total power draw means higher heart rates too. I like to do whatever I can to keep my heart rate as low as practical. So, I tend to favor grinding. But, eventually, my leg muscles simply tire of the forceful contractions and I have no choice but to reduce gears and spin more or, more likely, reduce speed.

That suggests to me why spinning has more or less become the adopted standard among most elite racers. Late in a ride, after a lot of grinding, the leg muscles become deeply fatigued, unable to either contract with the same force or contract more frequently. Your only option is to reduce speed. However, spinning doesn't result in the same depth of fatigue in the leg muscles. Because it's placing more load on the cardio system your legs muscles can continue contracting at the same rate for longer.

If you can train yourself to master both grinding and spinning styles of riding, you give yourself an option to distribute the load on your body a bit depending on how you are feeling during a ride.