Can anyone help me with what S1,S2 & S3 mean in my HR zones?
Also can I change the HR zones from a 7 zone model to a more traditional 5 zone model? Thanks
That’s Intervals’ implementation (approximation) of the Seiler zones, which is not 100 % accurate but comes close. S1= Sum of Zones 1 & 2, S2 = Sum of Zones 3 & 4, S3 = Sum of all higher Zones. The correct way would be S1 = anything under LT1, S3 = anything above LT2 and S2 anything in between LT1 and LT2.
If you want 5 HR zones, change it on the settings page:
But on the calendar, the zones displayed are the combined zones and these stats use the Power zones if power is available. And if not, HR or Pace. It’s like on te Totals page.
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Many thanks 
These Seiler zones are for polarized training: Polarized Training - A Detailed Look with Stephen Seiler - FLO Cycling and used to calculate that in that week you’re pyramidal. There’s evidence to suggest if you get this value above 2, you might get better improvement in some metrics.
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Except in the Norwegian method, where you explicitly try to avoid S3.
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