Originally Posted by
gregf83
That's true for the weighted average power Strava reports but my recollection was the Fitness number matched my CTL number from Golden Cheetah within a few points. Whatever formula they used also worked pretty well with just HR. I didn't notice much difference when selecting power vs HR on Strava's numbers.
I don't use Golden Cheetah, but I seem to recall somebody mentioning that it uses the same formula for NP as Strava, which if true would explain why you didn't see a difference. In fact I wouldn't be surprised if Strava got their calculations from Golden Cheetah since it's open-source.
I use a browser extension for chrome called Stravistix, one of the extra data fields it adds to a strava workout is "A. Coggan Normalized Power", which I've found is always higher than Strava's weighted power.
It's not a huge difference for most workouts, and as long as you know what your fitness/freshness numbers mean for you, it's probably not a big deal at all. I only pointed it out because even the Coggan/TrainingPeaks formula is more heavily weighted towards duration than intensity than it should be IMHO. A 120 TSS VO2Max interval workout definitely takes more of a toll on me than 120 TSS from riding zone 2, and the former takes longer to recover from as well.