# New scientific study on crank arm length



## Ptor (Jan 29, 2004)

I'm posting here as most SS are interested in crank arm length and more open than many to new data and ideas. I'm not proud of my contribution to a massive flame war a few months back on the 29er forum regarding crank arm length...and hope to avoid something similar occurring here.

Anyway, here's a link to a PezCyclingNews article about crank arm length and summaries of scientific studies. I knew about one of the articles summarized that showed no appreciable difference between significant crank arm length differences when it came to peak power that could be applied to an ergometer. But the new study summarized here is to me quite amazing, enough so that I found the article (here's a link to the abstract) and perused it -- and if this sort of thing interests you, you should read it too. Oh heck, I'll append the whole abstract below, but the bottom line is that shorter armed cranks (170 vs. 175) have a performance advantage in mtb applications. So rather than long legged/tall riders being disadvantaged by not having good access to proportional (longer) length crankarms, they may in fact have an advantage over shorter riders because most crank arms are proportionally shorter for taller riders and provide the observed performance advantage (see below).

The abstract:
The aim of this study was to determine the differential effects of three commonly used crank lengths (170, 172.5 and 175 mm) on performance measures relevant to female cross-country mountain bike athletes (n = 7) of similar stature. All trials were performed in a single blind and balanced order with a 5- to 7-day period between trials. Both saddle height and fore-aft position to pedal axle distance at a crank angle of 90 degrees was controlled across all trials. The laboratory tests comprised a supra-maximal (peak power-cadence); an isokinetic (50 rpm) test; and a maximal test of aerobic capacity. The time to reach supra-maximal peak power was significantly (P < 0.05) shorter in the 170 mm (2.57 +/- 0.79 s) condition compared to 175 mm (3.29 +/- 0.76 s). This effect represented a mean performance advantage of 27.8% for 170 mm compared to 175 mm. There was no further inter-condition differences between performance outcome measurements derived for the isokinetic (50 rpm) maximum power output, isokinetic (50 rpm) mean power output or indices of endurance performance. The decreased time to peak power with the greater rate of power development in the 170 mm condition suggests a race advantage may be achieved using a shorter crank length than commonly observed. Additionally, there was no impediment to either power output produced at low cadences or indices of endurance performance using the shorter crank length and the advantage of being able to respond quickly to a change in terrain could be of strategic importance to elite athletes.


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## sparkie (Oct 7, 2006)

While I am no scientist, I have been playing around with crank lengths on the road. I used to believe a 175 was great for my cross bike ( more leverage for short bursty climbs). I recently built a new cross bike with 172.5 to keep in line with my road bike as opposed to my mountain bike. I found the shorter length more comfortable. I could get up to speed faster and stay there longer. I also took my wife's road bike for a spin which has 170's on it. I felt I could spin faster but couldn't put the maximum amount of watts I could with a 172.5. The bottom line, it depends on your size and riding style. I am 5'9" and love the 172.5 all around for road and off. Just my 2 cents.


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## Cygnus (Jan 7, 2004)

grammar in the abstract is a warning sign about quality of the study. maybe the kiwis don't follow english protocol. 

n = 7? 

why 50 rpm, that's not a typical cadence?

no correlation between crank length and rider height or leg length?

but maybe i'm bring too harsh, and, i've never considered the advantage of long cranks, even on an SS.


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## Stevob (Feb 27, 2009)

Having just recently changed my cranks from 175 to 170mm, I can say that I noticed the difference in leverage straight away, but over the subsequent 6 or so rides, I've now adapted to them, and notice no difference in climbing steep stuff.

Spinning whilst seated is much easier now though. I'm 5'6" with about 30.5 inseam btw. I feel a little stronger and faster, but that could just be all in my mind.


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## jackspade (Jul 23, 2010)

If you ride SS you'll notice right away the change from 175 to 170.

I just change mine from 175 to 170 recently and with few ride I already adapt to it. Few things change are the seatpost height is changing so I have adjust my stem height too.

I am 5'4" which is 165 is better but I think I'll just go with 170. 175 is kinda pushed too far for cadence but now I am glad since I change to 170 my cadence is better although not as good 165.


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## umarth (Dec 5, 2007)

I'm waaaaaay to drunk to make sense of ****ing abstracts. Why can't someone just tell me what crank arm length to ride? I'm 5'2, 540lbs with a 40 inch inseam. I look gawd awful in spandex.

I ride 175 because someone told me to. I have 170's for the fixed gear and they are ****ing rad for spinning like a methhead on.. meth. Yeah. But those cranks are a bloody 130 bcd, which hits the heart warming 5 bolt pattern, but misses the low tooth count I need to be able to bike my sled up even a modest incline.

Also, what does Little Sumpin' Wild + Hop Stoopid equal?


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## johnnyc9 (Jul 27, 2007)

Cygnus - I know I am a couple years late here, but the variable "n" is a universal variable used in the discipline of statistics to indicate sample size. "n" = 7 and so what they are indicating is that the number of female XC athletes used in the study was 7. 

No grammar errors there. Hope that helps.

Thanks!


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## mikhalit (Aug 17, 2011)

PeT, thanks for sharing, good to see the numbers instead of a pure theory (or flaming, that's often the case..). Pity the article is not available for the general public, i'd really like to read more on the experiment setup. For example it's not clear to me whether in both studies researchers have adjusted the gearing or not (seems like not).

I wonder why those studies do not consider really *tall* or short athletes? Hard to find or? By tall I mean people with inseam larger than 35 inches and small is smaller than 30 inches. Or at least they could use cranks from the kids bike for the person of an average height... And why Lennard Zinn cites no other reference but a thought experiment published in a blog.. 

I am riding 205mm long cranks and probably 190mm are worth trying. But would 170mm cranks be any good for me? I can imagine that there might a saturation point after which smaller cranks will not bring an additional improvement, but may actually cause a drop in performance. At least this is how it feels to me after i switched to longer cranks, but i've got no powermeter and I can't measure and be 100% sure what's the placebo effect here.

PS. Jee, i see now how old this thread is! Johnny, thanks for brining it up, was an interesting read.


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## zaskaranddriver (Oct 14, 2009)

Cygnus is a PhD, so I'm pretty sure he knows what n is. His question more likely referred to the adequacy of the sample size.


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## Cormac (Aug 6, 2011)

interesting. I would have thought that longer would = more power. Though I never bothered to look into it. I just know when I go to buy new cranks I plan to purchase ones that are the same length as my stock arms. I like em just fine the way they are and have no idea how long or short they are.


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## ascarlarkinyar (Apr 24, 2012)

study was flawed. all riders rode sitting not standing. duh....


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## Ptor (Jan 29, 2004)

mikhalit said:


> Pity the article is not available for the general public, i'd really like to read more on the experiment setup.


Not sure how Springer-Verlag will feel about it, but the pdf of the article is attached to this post. If you have a college or university nearby, you can usually get your hands on articles like these pretty easily.

I'm often the first to make fun of exercise physiology studies, but there are some unique difficulties associated with the discipline. "N"'s tend to be small, but a well crafted study can get statistically significant measurements. Setting up adequate controls are often difficult (e.g. - quality of rest, sub-acute viral infections or injuries, consistent warm-ups, etc.). I think this is also the reason why there's a tendency to claim individual physiology or genetics as the reason for differences in preferred setup, training, or nutrition. In the end, physiologically and genetically we're not that much different -- it's the externalities and the history we've built in response to those environmental parameters that color our choices (long versus short crank arms, masher versus spinner, PBJ versus Hammer Nutrition). That's my hypothesis, just gotta figure out a way to test it...


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