What I would like to find is something to relate beats per watt per duration. This would give a true measure of aerobic ability. You could do it easily enough for an hour as average watts/average HR. But I can’t work out how I would model the effects of long duration fade.

That is a very interesting idea, I think I'll analyse my recent Seiler zone 1 rides (similar to MAF) and see what I can come up with. I don't think it could be called a "true" measure though but a useful personal indication.

First analysis based on my Seiler Zone 1 rides (similar to MAF in that they are targeted to be below 80%HRmax) during January (before it got cold) show no correlation between average W/bpm and duration.

Data here:

Three rides were completely in Z1, of durations 90, 184 and 206 minutes, from these it would seem that the efficiency is increasing with time, i.e. more power/HR, but another 3 rides were only 11% or less above Z1 and if you consider these as well then the P/bpm is not showing a trend.

I have including the Aerobic coupling value as calculated by the Golden Cheetah algorithm for Trevor if he finds it useful.

Everyone is different so if someone else were to do this intervention (n=1) then the results could of course be different.

PS it was a rough and ready exercise so I missed off units from the table, I'd have marked my students down for that obviously!

PPS If you spot anything I didn't please let me know, as well as any other things worth looking at and/or evaluating.