The evidence to the contrary came in the form of power-meter data from pro cyclists. It shows that professional cyclists instead use their power only in the down stroke of the peddle and have a far less “even” power through the cycle. Their technique is much more similar to a piston engine than a rotary.
Doesn't "spinning out" mean your power output is bottlenecked because you can't move your legs fast enough? It only happens when your gear is too low. I don't see how that's possible while climbing, regardless of what pedals you use. I ride with flats, and there's a 10% incline hill not far from where I live. I've climbed it several times (at about walking pace) and never had any problem.
Alternating left-right pushes feels very different than trying to maintain a continuous circular motion with quasi-equal force throughout. But unscientifically I can't notice any difference.
Clipping in is mostly a cargo-cult among non-pro riders
Otherwise there is no benefit and a lot of downsides in terms of how likely you are to get injured by poor cleat placement or saddle position.
On a constant-speed training bike without clip-in pedals, a road cyclist will be pretty far out of their element. A lot of the efficiency gain in cycling comes from adding rigidity to the power train between the foot and the road, and from the continuous power application that clip-in pedals and shoes enable.
The main reason for clipless, as I understand it, is for safety and maintaining one's foot on the pedal. When you're pedaling, if you are apply any down-force whatsoever on the upstroke, you're wasting power. So, a trained cyclist will always have their foot practically floating on the upstroke and their foot will drift position as they pedal unless something is keeping it in place.
As he acknowledges in the end, a lot of cyclists prefer clip-in pedals just due to stability. In my experience, it's not so much about the upstroke (which to be fair I am surprised was not measured to provide a power benefit), but the knowledge that I will never lose my footing. Without that knowledge, I simply wouldn't apply as much power.
This efficiency simply means getting a longer ‘glide’
Its very, very telling when climbing and also trying to attack as when applying extra good form will eek out benefits.
And I am sure that you would make a physical adaptation to a sufficiently different frame/fit geometry, though if your fit was "good enough" then upgrading might not incur much change on the physiological level.
At the end of the day I've seen massive improvements in my cycling ability when regularly doing a HIIT bike routine (such as sufferfest for example) on a trainer such as the wahoo kickr.
A pretty big caveat. I’m not surprised that cycling on a stationary bike at a fixed cadence has very little room for skill/technique improvements. This invalidates the results vis. real cycling in my book. Gear selection, out-of-saddle technique, etc. Though having said that I’d expect the skill-based component to be much smaller in riding than running as the movement pattern is much more constrained.
Still an interesting result, since it does show that there is a skill-based efficiency gain in running.