Your left leg rises on every pedal stroke. Something has to make it rise. That something can be the left leg muscles or the right leg muscles. From an energy expenditure point of view it doesn't matter - it's the same amount of work. And in fact as you yourself explain below, at high cadence it has to be the right leg pushing down that lifts your left leg, since the left hamstrings aren't fast enough to keep up with the pedal revolutions.


That is probably true, and even clipless pedal bashers acknowledge they may help here, but there are also lots of people who ride up steep slopes with platforms - take bmx riders for instance, or half-pipe tricksters.

When I used to ride clipless and I would swap them for platforms in winter boot weather, I would notice that too, for the first day or two, and then it goes away. In fact, I noticed it happened as the pedal goes over the top, not halfway up as you would expect if you are pulling up. So I concluded it was more an effect of leg momentum, and clipless making you lazy - you're used to the pedal dragging your foot over the top so you forget to actually keep your foot in contact with the pedal.