Disclaimer: I use the term FTP here but Humon may be measuring LT, CP or some other measure that correlates somehow with aerobic endurance.
The slope 0.1661 was based on 5-6 separate tests over a year or so. I plotted the value of FTP that Humon reported for each test as a function of time (in seconds) of the test when the color chart of the workout Humon shows you in the Ap turned red. The best fit was a straight line with that slope. I wasn’t sure how they handled incremental data (30W intervals) yet gave continuous results FTP. They just use that slope.
As an aside, when the Humon first came out, there was no dedicated FTP protocol in the iphone Ap. You would select a workout (endurance, intervals) and it would just show you SmO2 in a color to indicate recovery/SS/nearing limit/limit. Based on some emails with Humon on FTP testing, I made a TR workout with a smoothly increasing slope (no intervals). It didn’t work. SmO2 turned red way below predicted FTP, could flop back to blue and green at higher efforts. This would all be OK if the pattern was reproducible from test to test but it was all over the place. It’s really too bad because what could better than a FTP test where you just increase power until you hit FTP and then stop?
Here is that workout if anyone wants to play with it. Log In to TrainerRoad