Choosing the Cranks

The length of the cranks is one of the most controversial arguments among cycling specialists and connoisseurs. 
Even in scientific literature there is no unanimous opinion on how to calculate the ideal measure for each cyclist. 

How important is the length of the legs, the proportion between femur and tibia, the type of muscle fibers or the pedaling style? 

- “Inseam” measure - 

Usually measured with a carpenter’s square, 2 cm wide, positioned in the middle of a cyclist’s legs, standing against a wall, without shoes. 
Try to position the square as high as possible, marking the height on the wall. 
The measure already gives a first indication on the choice for the best cranks: 

- 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 

- Femur/tibia ratio - 

Usually measured seated on a stool, properly adjusted in height so that the femur is parallel to the ground, the tibia is perpendicular, the feet are firmly on the floor and the lower back leans against the wall: measure the “femur” length from the wall to the most forward point in the kneecap, and the “tibia” length from the highest point in the kneecap all the way down to the floor. 
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. 

- Type of muscle fibers - 

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. 

- Pedaling Style- 

Those cyclists who prefer to pedal mostly in a seated position, such as medium-large sized riders and rouleurs, might use longer cranks, while those cyclists who are often pedaling out-of-the-saddle, such as light-weight climbers, should avoid cranks that are too long. 

If a rider wishes to change the length of the cranks on his bike, he should do it carefully, allowing time to adapt muscles, joints and tendons to the change, even though the history of cycling saw many champions using cranks that were 2.5mm or even 5mm longer than usual for a single mountain stage or a time trial. 
Anquetil and Merckx were among them, while Hinault and Moser preferred never to change the same cranks (172.5mm) on all the courses.