Crank Length

One for the hardcore.

Introduction

I think it’s fair to say that, by and large, cyclists are quite a conservative bunch. Many moons ago, before the invention of colour (I assume…), crank length was a crucial element in a bike’s performance. At this time, crank lengths in the region of ~170 mm were deemed to be about optimal. However, gone are those black-and-white days, yet that same dogma still persists: small frames come standard with 170mm cranks, and large frames come standard with 175mm cranks. That is what cyclists are used to, so that is what cyclists want, so that is what manufacturers supply—and the cycle continues (pun very much intended).

Recommendations from the Research

Determining the optimum crank length can be approached in one of two ways. Firstly, crank length can be optimised on the basis of oxygen uptake. That is, identifying the crank length that incurs the lowest metabolic cost all else being equal (i.e. power output, seat height etc.). Alternatively, it can be optimised on the basis of power output.

Crank Length for Maximum Power

This is of particular interest to the sprinters, who look to produce the highest possible wattage irrespective of metabolic efficiency.

First off, it is important that we dismiss the commonly held belief that longer cranks, in and of themselves, afford greater leverage. In reality, crank length represents just one lever in a series of levers that translate force from the pedal to the ground (e.g. sprockets, wheel size, chainrings). As such, gearing can offset any leverage that you lose with a shorter crank.

By enlarge, the ability of muscles to produce maximum power is constrained by something termed “cyclic velocity”1.

explanation of cyclic velocity

Specifically, cyclic velocity is maximized at around 4.27 Hz·m/s1,3.

What does this mean in practice? It means that you can produce very similar power outputs with a wide range of crank lengths (145–195 mm)2, provided that the resistance is set to optimise cyclic velocity3. The difference is the cadence at which maximum power is achieved—shorter cranks require a higher cadence to achieve maximum power, and vice versa2.

However, there is still some individual variability in the ability to produce maximum power with various crank lengths, which may be attributable to anatomical differences. This variability is in the order of 3-4% (!) for a range of crank lengths between 145 mm and 220 mm2,3. On this basis, it has been suggested that the optimum crank length for maximum power is 41% of lower leg length (measured from the knee to the lateral malleolus), but this is a weak predictor (R2= 0.215)2, so isn’t really all that useful. Therefore, cyclists looking to maximize power output should probably look at other factors to guide their choice of crank length3 (e.g. comfort/flexibility, aerodynamics, personal preference).

the lateral malleolus and greater trochanter

Crank Length for Maximum Efficiency

Varying crank length (or, more specifically, pedal speed) can elicit a ~4–5% variation in metabolic cost4. While this is a significant effect for athletes, it perhaps undermines the fiercely partisan debates surrounding the topic.

Firstly, bear in mind that it is pedal speed that is significant in this respect, as pedal speed acts as a surrogate measure of muscle contraction velocity1,3,4, which has implications for contractile efficiency7. As pedal speed = cadence x crank length, crank length per se will only affect efficiency when cadence is held constant (and vice versa). If we assume that the metabolically optimum pedal speed is around 1.2–1.3 m/s (based on the data from McDaniel et al.4), then the interactive effects of cadence and crank length are shown in the table below.

how to maximise pedal speed based for optimum efficiency based on crank length and cadence

Therefore, riding at a cadence of 80-100rpm (as most competitive cyclists tend to5), pedal speed might be optimised at a crank length of 140-155mm—which is in line with the conclusions of the other studies6,9.

However, this predicted optimum can only take us so far—specifically, it probably accounts for about half of the variability seen in response to different crank lengths/cadences4. The individual variability that remains may, again, be attributable to anatomical differences. As such, the following formula has been proposed for optimum crank length (OCL):

OCL (mm) = (18.971 x LLL) - (7.438 x TLL) + 90.679

Where:

  • LLL = lower leg length (measured from just below the knee to the bottom of the lateral malleolus).
  • TLL = total leg length (measured from the greater trochanter to the floor, while standing barefoot).

— Sprules, 20006

Be aware that the predictive capability of this formula is limited. In reality, the best method for determining an athlete’s metabolically optimum crank length is by direct measurement of oxygen uptake10.

The take home message is that, from an efficiency perspective, taller riders do generally warrant longer cranks than shorter riders, but the optimum probably falls in a lower range than the industry standard of 170–175 mm.

The Case for Shorter Cranks

There does seem to be a growing movement towards shorter cranks - championed, in part, by Chris Boardman (for example, see this interview). There have also been a few high-profile athletes moving to shorter cranks lengths (e.g. Craig Alexander). Some advantages of moving to shorter crank lengths include:

  • Aerodynamics: Assuming that the angle of your torso (which largely dictates your frontal area) is limited by a certain minimum hip angle, using shorter cranks (while correcting saddle height) will permit a more aerodynamic position.
  • Blood flow: Hip flexion (i.e. bringing you’re knee towards your chest) is associated with a degree of blood flow restriction through the iliac arteries (which supply blood to all of the leg muscles)11. Thus, minimising hip flexion with shorter cranks could enhance performance—assuming you don’t slam the handlebars so much that hip angle remains unchanged.
  • Acceleration: Unlike laboratory tests, real world cycling is dynamic and involves frequent changes in pace/power. For the same gear ratio, acceleration is improved with shorter cranks12,13.
  • Cadence: Put simply, power (W) = torque x cadence (and torque = pedal force x crank length). Therefore, to produce a given power output with a shorter crank, you either apply more force to the pedal or you ride at a higher cadence. Assuming that most people will tend towards the latter, riding at a higher cadence can improve blood flow & oxygen delivery14 and reduce leg muscle stress15.

References

  1. Martin JC, Brown NA, Anderson FC, Spirduso WW. A governing relationship for repetitive muscular contraction. Journal of Biomechanics 33: 969–974, 2000.
  2. Martin JC, Spirduso WW. Determinants of maximal cycling power: crank length, pedaling rate and pedal speed. Eur J. Appl. Physiol. 84: 413–418, 2001.
  3. Barratt PR, Korff T, Elmer SJ, Martin JC. Effect of Crank Length on Joint-Specific Power during Maximal Cycling. Med. SCi Sports Exerc. 43: 1689–1697, 2011.
  4. McDaniel J, Durstine JL, Hand GA, Martin JC. Determinants of metabolic cost during submaximal cycling. J. Appl. Physiol. 93: 823–828, 2002.
  5. Lucia A, Hoyos JS, Chicharro JL. Preferred pedalling cadence in professional cycling. Med. Sci Sports Exerc. 33: 1361–1366, 2001.
  6. Sprules EB. The Biomechanical Effects of Crank Arm Length on Cycling Mechanics [Online]. University of Guelph. 1998. https://circle.ubc.ca/bitstream/id/26716/ubc_2000-0583.pdf.
  7. He Z-H, Bottinelli R, Pellegrino MA, Ferenczi MA, Reggiani C. ATP Consumption and Efficiency of Human Single Muscle Fibers with Different Myosin Isoform Composition. Biophysical Journal 79: 945–961, 2000.
  8. Hansen EA, Jorgensen LV, Jensen K, Fregly BJ, Sjogaard G. Crank inertial load affects freely chosen pedal rate during cycling. Journal of Biomechanics 35: 277–285, 2002.
  9. Gonzalez H, Hull ML. Multivariable optimization of cycling biomechanics. Journal of Biomechanics 22: 1151–1161, 1989.
  10. Morris DM, Londeree BR. The Effects of Bicycle Crank Arm Length on Oxygen Consumption. Can. J. Appl. Physiol. 22: 429–438, 1997.
  11. Schep, Bender, Kaandorp, Hammacher, de Vries. Flow Limitations in the Iliac Arteries in Endurance Athletes. Current Knowledge and Directions for the Future. Int. J. Sports Med. 20: 421–428, 1999.
  12. Macdermid PW, Edwards AM. Influence of crank length on cycle ergometry performance of well-trained female cross-country mountain bike athletes. Eur. J. Appl. Physiol. 108: 177–182, 2009.
  13. Ansley L, Cangley P. Determinants of “optimal” cadence during cycling. European Journal of Sport Science 9: 61–85, 2009.
  14. Gottshall R, Bauer T, Fahrner S. Cycling Cadence Alters Exercise Hemodynamics. Int. J. Sports Med. 17: 17–21, 2007.
  15. Marsh AP, Martin PE, Sanderson DJ. Is a joint moment-based cost function associated with preferred cycling cadence? Journal of Biomechanics 33: 173–180, 2000.