Strava's Fitness Score versus CTL

Thanks for the episode on CTL, it was very informative.

I don’t use TrainingPeaks, just Strava.

I think Strava’s fitness score (like CTL) and relative effort (like TSS) are calculated based on heart rate instead of power.

Does anyone know how exactly Strava’s fitness score is calculated and how it would compare to CTL?

Great question, Rob. I looked that up before to try and get a better sense for just how they calculate it, but it’s hard to find a straightforward answer to compare to CTL. They do cite Banister’s impulse-response model, but then cite Coggan’s modeling as well, so tough not sure what functions they selected in their calculations.

TrainingPeaks will calculate your “hrTSS” with HR data only, but calculate “TSS” if power data is provided. It seems like Strava does something similar since you can select power and/or Relative Effort to display that fitness chart. There is a nice article explaining more about how they came to the Relative Effort calculation on a Strava engineering page.

In terms of comparison, I’m not sure how exactly it would compare, but have noticed that they are pretty close in terms of the overall picture provided, even though absolute numbers are off. Here’s my TP from the last year:

And Strava from the same time period:

I have Power and Relative Effort selected for Strava, and TP is a mixture of hrTSS and TSS depending on which bike or sport type I do, so they should all be showing the same sessions since my Wahoo device pushes to each platform.

Maybe someone will have a better idea of the exact calculations used, but hope this helps somewhat with some of the info for comparison.

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That all makes sense. I’m happy to the similarities in the pattern between the two. It sounds like I can apply the same principles to the strava scoring system. Thanks for the response.

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