Originally Posted by
Moisture
I dont see how this would apply if you stay within the 21.6% range.
The 21.6% range is a BS formula perpetuated by a guy that just happens to sell longer cranks.
As
rubiksoval states, the longer the crank, the closer your knees get to your chest (closed hip angle) this is why it's not uncommon for TT riders to use 145mm cranks.
The longer your cranks, the more your cadence will have slowed and become more 'choppy'.
Think of the tendons in your hips, knees and ankles like coiled springs being constantly flexed and un-flexed.
If you keep their motion in a range where the spring isn't nearing it's limit, it is easier for your muscles to control and you get a nice smooth action. If you stretch the spring further, to the limit of its flex, it requires slightly more effort but results in a fast and snappy response.
This is what happens with your leg joints when you use longer cranks (Unless you truly are a giant; which you are not); hence the fallacy of them being more powerful. Because although you DO attain slightly more torque in certain positions, this is entirely offset by the reduction in cadence and un-even deflection from your joints (resulting in more fatigue).
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It's pretty well demonstrated here:-