Macdermid 2010 Study
There is, of course, a severe caveat that I must emphasize again. This study only looked at maximal power output and NOT anything to do with aerobic performance that might be relevant to the vast majority of cycling. Therefore, while logistically really difficult because of the need for people to be trained and adapted to different crank lengths, the more appropriate study would be replicating this design with more time trial type of tests.
This leads to an interesting Kiwi-Aussie 2010 study that looked at more "realistic" crank lengths and endurance exercise, with the specific context and population of female cross-country mountain bikers (1). In brief, cranks of 170, 172.5, and 175 mm were tested with maximal sprint, a VO2max test, and also an isokinetic (constant velocity) test at 50 rpm. The latter is certainly representative of grinding up a big climb off-road. No differences in the isokinetic or VO2max tests were observed across the three crank lengths. What was really interesting, however, was that, while peak power was not different, the time required to achieve peak power was much lower with the 170 mm (2.57 s) than the 175 mm (3.29 s) cranks.
This has significant practical applications. The shorter cranks permitted you to generate high power outputs faster. This ability is critical in closing a gap, accelerating in a sprint, and also laying out that massive and quick burst of power to get you over an obstacle on the trail or out of a turn in cyclocross.
More: Variable Gearing With Dave Scott
Test Yourself
Therefore, the best thing to do appears to be a matter of testing and understanding your inherent style and the demands of your sport. One great way to do it is to give yourself time to try out different bike positions and crank lengths during the offseason or the "easy" base period of training. There are several systems available for altering crank lengths on the fly, with likely the simplest and most adjustable system that from PowerCranks.
The "basic" PowerCranks model can be adjustable from 145 to 182.5 mm, and the "long adjustable" version can be adjusted all the way from 85 to 220 mm crank lengths. PowerCranks can either be purchased for your own bike or for variety of indoor training ergometers, making it possible for you to test your own preferred or optimal crank length under different conditions. The important thing is to give yourself time (at least a few easy rides) to adapt to each different crank length before doing any specific testing like a five-minute effort.
Have fun and ride safe!
More: What Is the Future of Electronic Shifting?
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