I think the problem is that cyclists get more efficient over time on their bike and on the kind of course they are familiar with. In this study, they didn't even have clip-in pedals: "Because not all of the runners were familiar with cycling shoes, all participants cycled in their own running shoes on flat pedals without toe clips or straps."

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.

Although I use clipless pedals on both my road and mountain bikes, most of the research I've seen recently suggests that clipless do not provide any efficiency gain. This video provides links to numerous peer reviewed articles on the topic https://www.youtube.com/watch?v=KUEaN9FKGLE. Of course I'm sure there is research that would suggest otherwise, and I would be interested in seeing counter arguments.

There's seems to be questioning on powering the upstroke, but that has never been "the reason" for foot retention systems. Except for one specific type of cycling-- track cycling, where upstroke IS USED in high acceleration starts like in Match Sprint or short time trials. There are track versions of clipless systems which require a lot more force and twist to get out of.

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.

Thanks, that's a great video and I'm going to look through the references. Lots of great points made in the video.

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.

As a flat pedal rider (and also clips) and someone who has trained fairly extensively in pedaling form here is my 2c. The form of the stroke is to increase the length of the downstroke. So making power transfer happen as early as possible and to maintain it as much as possible at the base. This translates into pushing forward and then down at the beginning and down and back at the end.

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.

It's a fair point; I'm not sure where you'd draw the line between physiological effects and skill effects and equipment; things like ligament spring-loading sound like they would be subconscious techniques that are also accompanied by physiological adjustments.

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.