[David]

Maybe I'm wrong, but it seems to me that a crank driven chain drive is still the most efficient means of converting human power to drive power.

When I was an engineering student at UT and a bicycle racer, I thought it would be a good idea to join the UT engineering human powered vehicle team. I went to one of their meetings with another bicyclist/engineering student and to our dismay they had decided to use a little two push pedal drive system like in a kid's car. We tried to explain the error of their ways, but they didn't want to discuss it. Needless to say we left.