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Does Crank Length Matter?

Renowned cycling researcher Dr. Jim Martin from the University of Utah joins the podcast this week to discuss the implications of crank length on cycling. Dr. Martin’s research was originally motivated by examining kid’s standard crank length and attempting to find the optimal solution. Although his research on this topic was published in 2001, many people have strong feelings about specific crank lengths and how it affects cycling. Dr. Martin provides a colorful and detailed picture of the science behind crank length as well as multiple examples from his long career of working with national and world level athletes.

THE DIVE: Ep. 8 Topics

  • Custom-built equipment in the University of Utah Lab: Inertia load cycling system for maximum power testing, a biomechanical system with force sensing pedals, and an isokinetic bike.
  • “If you only have store-bought equipment, you can only do store-bought research.”
  • The importance of leg flexion in cycling
  • The negative impacts of poor leg flexion: blood flow, aerodynamic impact, and hip impingement.
  • Who should ride on shorter cranks?
  • Maximum power impact on crank length
  • Pedal Speed vs. Cadence
  • Muscles need to relax during the cycle
  • Cadence’s profound effect on muscle relaxation
  • Cycling is concentric but may provide an eccentric effect
  • Pedaling is not a sophisticated technique–regardless of crank length, the muscles still respond to the same motor control program
  • Looking at muscles individually vs. the whole in cycling
  • How much of a crank length change is required to experience a “difference.”
  • Short cranks make you feel taller
  • Dr. Martin’s thoughts on Bike Fitting
  • A crank length suggestion for national and world champion track cyclist Robert Forstemann
  • A reduction in crank length requires an increase in gear ratio.
  • Cycling effect on diabetes study
  • One leg cycling
  • Cycling inefficiency
  • Future research in ACL reconstruction and one leg cycling to restore symmetry
  • PRs with Birkenstocks
  • Dr. Martin’s favorite track cyclist

The interesting thing about crank length is how amazingly sensitive people are to crank length.  Most will know immediately that their cranks are different.  Where I think it goes wrong is, just because you can tell there’s a difference, does it make a difference?  All our work shows it [crank length] does not compromise power or efficiency.

Dr. Jim Martin

Dr. Jim Martin Biography:

Associate Professor in the Department of Nutrition and Integrative Physiology, within the College of Health at The University of Utah and a Fellow of the American College of Sports Medicine. He teaches graduate and undergraduate courses in biomechanics and muscle physiology. His research areas include neuromuscular function, biomechanics, physical activity, and performance modeling. Applications of these areas range from optimizing elite sport performance to facilitating physical activity in desk-bound office workers. He is an author on 50 papers in peer-reviewed journals and three book chapters which have been cited over 2000 times. He has been an investigator on research funding totaling approximately two million dollars. He is the sole inventor on one patent and a co-inventor on one other. He has served as a consultant to several sports organizations including the USA Cycling, Australian Institute of Sport, the English Institute of Sport, Canada’s Own the Podium, High Performance Sport New Zealand, and Oracle Team USA where he has worked with World and Olympic Champions. He holds a Bachelors’s degree in Mechanical Engineering and Masters and Doctoral Degrees in Exercise Science.  Dr. Martin’s publications can be viewed on his Google Scholar Profile.  When not working, he enjoys spending time with his family, hiking, cycling, and woodworking.

Find out more about his research or contact Dr. Martin via the University of Utah website. 

Read More about the Research Referenced on the Podcast:

Determinants of maximal cycling power: crank length, pedaling rate and pedal speed.

Effects of Pedal Speed and Crank length on Joint Powers during Submaximal Cycling

Biomechanics of Counterweighted One-Legged Cycling

Effect of Crank Length on Joint-Specific Power during Maximal Cycling

6 Comments

  1. Rick Schultz

    Dr. Martins podcast is RIGHT ON the money! I sent him a copy of my latest research and he liked it. You should interview me to take this topic to the next level!

    Reply
  2. Dan Kennison

    Great Interview – 100% – this is exactly why we scale the crank lengths through the size rage of our bikes.

    Reply
  3. Frank Radaker

    What formula do you use to determine foot velocity given crank arm length and cadence?

    Reply
    • Damon Wyatt

      Answer from Jim:

      “Pedal speed in meters/second = RPM * 2 * pi / 60 * CL in meters. eg 90 * 2 * pi / 60 * .1725 = 1.63 m/s”

      Reply
  4. Steve Stuart

    What are Jim’s and your thoughts of crank length across different cycling disciplines such as XC mountain biking?

    Reply
    • Damon Wyatt

      Here are the comments directly from Jim:
      “My thoughts on CL(Crank Length) across disciplines have to do with torso position and hip angle. Triathlon and TT, and even road racing, with a low torso mean crank length is very important in terms of keeping hip angle open. Upright cycling such MTB mean the hip angle will be more open and CL is less important except in extreme cases of short riders with 175s.”

      Reply

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