Originally Posted by PeteHski

I definitely agree that there is no meaningful power generated on the up-stroke. Enough studies of hundreds of elite cyclists have repeatedly shown that to be the case at normal cadence and higher power levels.
The weight of the leg itself does cancel out over a half-stroke. You are simply creating and releasing potential energy each time you raise and lower your leg. BUT you still have to expend energy in raising that leg (even if using the other leg to do it) even though you effectively get that energy back in terms of powering the bike forward. So having heavier legs raises your minimum energy expenditure required to pedal. It's no different to standing on the spot and raising your knee in the air and then letting it drop down again. It requires a minimum amount of energy (mgh) to lift your knee in the first place, even though that energy is released as you let it drop back down.

So in a way both of you have a valid point!