February 24, 2011 cburnham

Efficiency and Cadence

I love reading about new gadgets and gizmos that are supposed to improve a cyclists efficiency, help them pedal perfect circles, and boost power output by 373%. OK, maybe that last part was an exaggeration but reading the marketing speak on some of these products isn’t too far off. So are these things for real and does pedaling perfect circles improve performance? Let’s break down some pedaling dynamics to find out. (If your not interested in the geekness, skip to the bottom for the basic points).

First off we need to define efficiency. Efficiency is simply a measure of work performed (output) versus work generated (input). For most cyclists including pros, we are 20 – 25% efficient with 75 – 80% of the energy generated being lost as heat through the skin. This has been determined by using oxygen as a proxy for energy expenditure: 1 liter of O2 results in 20.9 kJ, or 70 – 87 watts generated per liter of O2 at 20 -25% efficiency.

Stronger riders simply stomp harder on the pedal down stroke (0 – 180deg).

Does cadence effect efficiency? It has been shown through several studies that efficiency depends almost entirely on the percentage of slow twitch muscle fibers represented in the working muscles and not on any specific pedaling cadence or improving power distribution through out the pedal stroke ( i.e. pedaling circles).  At a constant power, most people will naturally self select the lowest cadence that activates the smallest amount of fast twitch muscle fibers for a given power output.  As we increase our power output we also need to recruit more fast twitch muscle fiber resulting in our optimal cadence increasing as power increases. Activating fast twitch muscle fibers, which work primarily anaerobically, results in more glycogen usage and a higher rate of lactate production which are both not good things for maximum performance.  It has been common advice to pedal fast and work on pedaling perfect circles with claims that this will result in less stress on the musculature of the legs, leaving more reserves for riding longer and faster. If this was true why don’t we see riders pedaling at consistent 125+ cadences? Because we are good at self selecting efficient means of movement and there is a higher metabolic cost to pedaling at extreme ranges of cadence.

It is interesting to note that thermodynamically, lower cadence/higher force pedaling has been shown to be slightly more efficient, but metabolically accelerates the demand for ATP production and increases the rate of lactate production resulting in an overall more costly pedaling style. Again, at a constant power, most people will naturally self select the lowest and most efficient cadence that activates a minimal amount of fast twitch muscle fibers.

So all of the above is referring to steady state efforts (time trials, typical endurance riding, long climbs, etc.) and not to the dynamic elements of crits, mountain bike racing, sprinting, or the typical stochastic elements of road races. In those critical moments acceleration and pure power are more important than efficiency and often requires us to pedal outside the most efficiency cadence. In those situations, pedaling at a more inefficient higher cadence allows us to produce a higher power.

So what can we determine from all of this? You are better off riding at your naturally self selected cadence for steady state events. It may be valuable to include some higher cadence work in training to prepare for key moments in cross country mountain bike, road races, or criteriums. The best way to improve power output is by improving the respiratory capacity of the muscles through sustained moderate to relatively high intensity riding.  Efficiency is primarily set by genetics with a very small range existing between most cyclists.  A Toyota Prius won’t win too many drag races.

 

  • Carnevale, T.J., and G.A. Gaesser.  Effects of pedaling speed on the power-duration relationship for highintensity exercise.  Medicine and Science in Sports and Exercise 23(2):242-6, February 1991
  • Coast, J.R., R.H. Cox, and H.G. Welch.  Optimal pedalling rate in prolonged bouts of cycle ergometry.  Medicine and Science in Sports and Exercise 18(2):225-30, April 1986.
  • Coyle, E.F., A.R. Coggan, M.K. Hopper, and T.J. Walters.  Determinants of endurance in well-trained cyclists.  Journal of Applied Physiology 64(56):2622-30, June 1988.
  • Coyle, E.F., et al.  Physiological and biomechanical factors associated with elite endurance cycling performance.  Medicine and Science in Sports and Exercise 23(1):93-107, January 1991.
  • Coyle, E.F., L.S. Sidossis, J.F. Horowitz, and J.D. Beltz.  Cycling efficiency is related to percentage of Type I muscle fibers.  Medicine and Science in Sports and Exercise 24(7):782-88, July 1992.
  • Foss, O., and J. Hallen.  The most economical cadence increases with increasing workload.  European Journal of Physiology 92(4-5):443-51, August 2004.
 

Comments (2)

  1. jfeder

    At first I was happy to see a nicely technical article, but very quickly it degenerated into non-scientific confusion.
    “This has been determined by using oxygen as a proxy for energy expenditure: 1 liter of O2 results in 20.9 kJ, or 70 – 87 watts generated per liter of O2 at 20 -25% efficiency.” The problem is that it gives the amount of energy per volume of “O2”, but then relates that back to watts. watts is a unit of power, not energy, so how do we relate them with an efficiency calculation? We can’t. All of the units must be either power or energy (since power is energy per unit time).
    Also, when people give a volume of O2, do they really mean a volume of O2 or the volume of air? Air is only about 20% O2 by weight.
    Also, there is a nice graph of torque vs. pedal position in angle. The surrounding text and title of the article are referencing cadence and NOT pedal position. There is a potential for confusion since the maximum torque occurs near 90 degrees, which is close to many people’s optimum cadence in RPM. This is not technically an error, but could be a cause for confusion.

    • Thanks for your comment. You are correct that watts is measure of power and that sentence should of stated that 20.9kJs with a 25% of efficiency would result in 87 watts for 1 minute. I did leave the time component out. 1 joule for 1 second equals 1 watt so it is possible to translate energy over to work load with a time constraint. I like to give athletes a metric that they can relate to and most cyclists know watts, but Kjs is still a bit abstract. This o2 calculation is also from work done by Coyle, et. al. which does relate to o2 content, not air.

      The torque graph was just to show where we produce power in the pedal stroke. I did come back and write another article on the myth of “pedaling circles” that dives into this more.

      Thanks again and happy cycling!

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