Optimal Crank Length

posted Oct 4, 2009, 7:09 AM by Donald Vescio   [ updated Sep 2, 2013, 5:03 AM ]

In a recent article, I looked at how moving to a shorter crank length could have a positive impact on power output by opening up hip angle while in an aero position.  The idea is that by shortening crank length by x-amount  and by raising aerobars and seat heights by the same amount, a rider can maintain his/her current aero position with less hip impingement.  For those who are struggling to get into a lower aero position, reducing crank length enables a lower frontal position for the same hip angle.  The concept is relatively simple:

Shortening crank length means that seat height needs to be raised in order to maintain the same overall extension of the leg.  Once this is done, the rider can decide whether to raise his/her aerobars the same amount, thus opening hip angle and possibly enabling greater power generation, or to leave the aerobars at the same height, which will result in a lower, more aerodynamic position.

Selection of crank length is somewhat of a black art and science, combined.  There are many who support the proportional crank length argument, which is that crank length should be match to a specific morphological characteristic, such as inseam length.  For instance, Dr. Michael Fararri (who has some degree of notoriety for reasons other than his biomechanical analyses of cycling)  recommends:

  • from 75 to 80 cm "inseam" = 170mm crank length
  • from 81 to 86 cm = 172.5mm
  • from 87 to 92 cm = 175mm
  • 93 cm and higher = 177.5mm

 He then further refines the recommendations above by factoring in a femur/tibia ratio—that is, if “the ratio femur/tibia is superior to 1.13, it might be suggested a slightly longer crank (usually by 2.5mm) than the one indicated by the inseam measure.” 

 Finally, pedaling style needs to be accounted, too.  Dr. Ferrari  suggests that “rouleurs,” those who prefer to pedal in a seated position, might opt for slightly longer cranks, as opposed to those who climb out of the saddle (typical of lightly built climbers).

 All of this can get overwhelming.  There are, though, others who advocate a simple formula approach.  Such formulas are based on the notion of an average femur length:

 

Inseam length in mm x .219 = Crank Length .

 

So far we have accounted for inseam length and physical morphology; we still need to account for predominate muscle fiber type.  In very simple terms, there are two types of muscle fibers: Type 1, slow twitch (recruited during endurance events); Type 2, fast twitch (recruited during explosive, anaerobic events, such as sprints).  Two basic principles need to be kept in mind.  All exercise initially will recruit Type 1, slow twitch muscle fibers; if the event is long in duration and short in intensity, then Type 1 fibers might be all that is used for the entire session.  Second , each athlete has a different proportion of Type 1 and Type 2 muscle fibers; research is inconclusive as to whether the proportion of Type 1 and Type 2  fibers is a genetic or acquired trait.

 While a needle biopsy is necessary to determine with any specificity the fiber type of a specific muscle, athletes can gain a general sense of their fiber type ratios by performing the following test as outlined by Jason R. Karp:

An indirect method that can be used in the weight room to determine the fiber composition of a muscle group is to initially establish the 1RM (the greatest weight that they can lift just once) of your athletes. Then have them perform as many repetitions at 80% of 1RM as they can. If they do fewer than seven repetitions, then the muscle group is likely composed of more than 50% FT fibers. If they can perform 12 or more repetitions, then the muscle group has more than 50% ST fibers. If the athlete can do between 7 and 12 repetitions, then the muscle group probably has an equal proportion of fibers

 

What does all of this mean within the context of crank length selection?  According to Dr. Ferarri,

  • A longer crank could be adopted by those riders who have predominance in red fibers (or Type I) and prefer high pedaling cadences, while suffering high force peaks.
  • A longer crank in facts requires, at a given power output, lower force peaks for each pedal stroke, due to the longer arm.
  • Those riders who have a higher percentage in white fibers (or Type II) instead, more used to bear high muscle tensions, could opt for shorter cranks, allowing inferior articular excursions and reducing internal friction.

 

In simpler terms, I recommend the following as a starting point:

  • If you are a lightly built endurance athlete who tests primarily for Type 1, slow twitch fibers, you might want to opt for a crank length equal to (or slightly longer than) Dr Ferarri’s inseam proportion recommendations (listed above).

  • If you are a muscular athlete who excels in explosive efforts (i.e., has a high percentage of Type 2 (fast twitch) fibers, then you might want to opt for a crank length equal to ( or less than)  Dr. Ferarri’s recommendations.


In a future article, I will explore the final variable in crank length selection, which is cadence and foot speed.



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