Zwift rides and Strava Fitness

Hi, everyone.

Does anyone know why Zwift sessions would affect the Strava Fitness graph differently to outdoor rides?

I can’t seem to attach an image but, a single ride outside during the summer often made the line rise by 4-5 points. Whereas, every Zwift ride only ever increases it by 1 and sometimes not at all.

On the road I don’t have a power meter, and I see that as the only difference to an indoor ride where my Kickr reads the power.

Appreciate your help,
Ste.

Purely a guess as I dont use use Strava’s fitness stats, but it will be estimating your watts and whatever else its not getting from Zwift (cadence?). Never heard anyone say Strava estimates these things accurately.

Yeah, in the absence of a power meter, Strava has to rely on HR to estimate your power, producing a less reliable TSS.

You probably already control for this, but your indoor rides are probably shorter too, and duration definitely impacts your TSS.

I assume you’re looking at the phone app, which uses a mix of relative effort and power data to generate the graph. On the desktop version you can select whether to show only relative effort or only power data. If you switch it to relative effort, then it treats outdoor and indoor rides equally, all based off of heart rate data. It’s just one more gripe of mine to add to the list, that the phone app and desktop versions of Strava don’t have the same functionality.

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That’s the answer, thank you.

As you said, app uses relative effort and power whereas the website allows you to sort it.

For comparison, my current fitness is ‘39’, when based on the same as my outdoor rides. Far different to the shown in the app. The app is effectively inflating my outdoor rides.