Subject: Feedback on Zone Sense accuracy: Short intervals and Zone 2 threshold shifts

Dieter1960

I’ve been testing Zone Sense recently and wanted to share some detailed feedback regarding its behavior during specific sessions (treadmill and outdoor runs). I compared the Suunto data with Garmin’s LTHR-based zones and performed a deep dive into the raw HRV data.
Testing Conditions:
For these tests, I used a Garmin HRM strap paired simultaneously to both my Suunto Race 2 and my Garmin Epix Pro (Gen 2). This ensures that the heart rate and HRV (R-R intervals) data source is identical and highly accurate for both devices.

Short Intervals Visibility (Treadmill)
During 1-minute intervals at 13 km/h on the treadmill, Zone Sense remains almost “blind” to the intensity shift. Even with the accuracy of the chest strap, the rapid change in effort doesn’t seem to register within the algorithm’s detection window, likely due to the short duration of the burst.
Underestimation of the First Threshold (Aerobic)
During steady Zone 2 sessions (based on Garmin LTHR outdoor baselines), Zone Sense consistently marks me as being above the first threshold (moving into “Yellow/Red” zones in the app).
Data Verification
To verify this, I performed the following analysis:
• Exported the .fit files from both Suunto or Garmin.
• Converted them to .csv via fitfileviewer.com to access raw R-R intervals.
• Analyzed the data through a specialized AI model to identify physiological breakpoints.
The result: The raw R-R intervals confirm that for the Zone 2 session, my heart rate variability markers stayed well within the aerobic zone. I never actually crossed the first threshold, which contradicts the current Zone Sense interpretation. It seems the algorithm is currently too “conservative” for my profile.
Has anyone else noticed this bias regarding the first threshold or a lag in detecting short, high-intensity intervals? I’m happy to share further data if it helps the dev team!