According to the definitions shown in the page, power/hr is:
Also known as output / input ratio, a measure of how much output (watts) you produce for a given input (heart rate measured in beats/minute).
and efficiency is:
Efficiency Factor is normalized watts (output) divided by average heart rate (input)
They look pretty similar to me, but I am sure they measure different things (otherwise they wouldn’t be shown together in the activity metrics). What is the difference? Which one should I care more to tell that I am getting aerobically fitter?
I have wondered about this myself and don’t know the answer. There is a linear relationship between power and HR (until threshold anyway) so I am not sure why “Efficiency” uses normalised power which will break this linear relationship.
Necro’ing this thread instead of creating a new one to add: As the Power/HR tooltip makes clear there are many confounding factors and this is most useful for endurance activities.
Those activities are generally steady state, under which normalized power will be close to actual power. It seems to me that treating efficiency as a separate measure may be confounding. Does efficiency have utility in highly variable output cases where NP ≠ power?