IF you have a limited range of motion in your knee, you need shorter cranks!
You make up for the loss of leverage by changing the cassette to lower gears and make up the difference with a faster cadence.
Picture this-
The longer the crank, the more you lower the seat to get proper leg extension when the crank is in the 6 o' clock position.
Now, when the crank is in the 12 o'clock position, the difference is 2X since the pedal is higher above the ground AND you have lowered the seat.
You might look at this old thread I created. Go to post 110 for my results.
BTW, I've shrunk to 5-11" from a scant 6-1", in my old age- (kind of short legged)
http://www.bikeforums.net/showthread...Too-Long/page4