Originally Posted by
bijan
As to efficiency that's hard to say, and I'm not convinced that is the most important criteria. It depends what you mean by efficient. If you mean wasting less energy when spinning high rpms, then probably clipless pedals are "more efficient". On the other hand considering efficiency from a purely mechanical point of view, spinning at higher rpm should be less efficient, because for a given power output, you are wasting extra energy accelerating your legs. However it is clear that mashing hard is going to tire out one's leg muscles, so maybe it's that spinning results in better performance, though with greater energy use.
This was just mentioned over in another thread -- I think it was Clipless Pedal Question over in the Road forum.
Briefly, mashing at a slow cadence becomes an anaerobic thing, which will tire out your muscles quicker. Spinning an easier gear at a higher cadence is an aerobic exercise, and it takes a lot longer to tire out your lungs and heart.
One analogy that was posted was, "Think of doing curls with a 50-lb free weight versus doing them with a 5-lb dumbbell. Which one requires more power, but also, which one do you think you can do for four hours?"
To me, mashing a tall gear is like climbing stairs, while spinning is like going for a jog. Climbing stairs for an hour sucks, but an hour easy jog is manageable for me.