I don’t know where this came from, but I spotted this interesting graphic on Bikeforums:
I’d once naively thought that cycling involved mainly the quadriceps. So I was very surprised when a minor calf strain kept me off my bike — why was that damn muscle twinging when my thighs were supposed to be doing all the work? Turns out that the calf does do a bit of work at 5-6 o’clock. The other surprising thing (surprising to the naive beginner cyclist that is) is the major role of the gluteus maximus, instead of the quads, at 12-3 o’clock. The natural instinct when one is waiting at a stop light is to bring the pedal up to somewhere at 1 or 2 o’clock for the accelerating power stroke. But going by the diagram the most power you can apply is from 3 o’clock onwards, and I can’t say I’m surprised by that — after commuting for long enough I began noticing that I couldn’t accelerate much at 1 or 2 o’clock.
Update Oct 6 2009: The graphic was originally from here.