Crank Arm Size? Is bigger better?

The other day I got asked the question on the sizing of crank arms.  The individual who asked me the question was a tall female with a 914 mm inseam.  I honestly did not know the answer to the question.  She went on to say that someone she knew who was tall with a longer inseam switched to a longer crank arm and it increased their power.  I got very curious.  I always become extremely curious when someone states a singular item for the reason their power increases.  My experience, as an athlete and a coach, tells me that a singular item is seldom the result.  

I started to read on the topic.  What I did know about crank arm length included:

• Crank arms influence cadence and the leverage you can exert on the pedals.  I think this is obvious.
• Longer crank arms are generally used for pushing large gears at low cadence.  Long crank arms are used for time trialing and climbing hills.  You would have longer crank arms on a mountain bike for better leverage in climbing.
• Shorter crank arms are generally used for higer cadenece with smaller gears.  Short crank arms are used for track sprints and criteriums.

What I did not know is how complicated the debate on crank arm length is.  There are many schools of thought.  I concluded that whoever comes up with the formula for proper crank arm length will be a very rich person, and will change the bicycle industry forever.

Blindly changing your crank arm length can lead to a whole lot of problems / issues.  It changes positioning on the bike.  Saddle height would likely have to change, and possibly the stem.  If all factors are not considered, injury and a decrease in power may actually result.  What is important to understand is for every increment you lengthen the crank, your knee is double that increment closer to you torso as you start to apply power to the pedals at the top dead center. 

Here are some links on the topic.  It is interesting reading.

http://www.stevehoggbikefitting.com/blog/2011/06/crank-length-which-one/

http://www.slowtwitch.com/Tech/Crank_arm_lengths_for_tri_727.html

http://bikedynamics.co.uk/FitGuidecranks.htm