I bought a Fuji Professional with 175 cranks in 1977 because the mechanic in the shop I worked said I should buy the boxed one in the basement. I did. I knew nothing about fit; that bike was a radical departure from anything I had ever ridden and from day one I was faster than I had ever known. That fall I had a serious crash. The next March I was training to race again in my fix gewar with 168 cranks. Switched to the Fuji to ride 100 miles in prep for the first race that weekend. That was a crank length change combined with poor fitness, lack of stretching and the cold weather of New England. My knees have never been the same. Now switching crank lengths is a big deal so big I don't. I ride 175s on all my bikes, even my three fix gears.

Not saying there is any magic to 175s. They work well for me and there is real risk to my knees making any changes. (I used to be 6'1/2" with long legs. That first dimension is now shrinking.)

Ben