Here's something about spinning that has bothered me: The upper leg at least, has to decelerate to zero angular momentum at the bottom and top of its stroke and then reverse direction. This is like the pistons in an engine: The amount of reciprocating mass has to be as small as possible, and this limits an engines top RPM, right? Isn't the same true for legs - that you will reach an RPM where the power input equals the power required to pump the mass up and down with zero throughput to the pedals?