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Garmin articles
3 articles on Garmin, newest first. Every one is sourced and links to the calculator it applies to.

How Garmin Calculates Your Max Heart Rate (HRmax)
Garmin estimates HRmax from your age (208 − 0.7 × age), then raises it from new peaks. How auto-detection works, what it drives, and how to set it.

How Garmin Calculates Your VO2 Max (and How Accurate It Is)
Garmin's Firstbeat algorithm reads pace against heart rate in short credible segments. Within 3-5 ml/kg/min of a lab with a chest strap, worse without.

Why Your LTHR Zone 2 Is Higher Than Your %HRmax Zone 2
Switched your watch to LTHR zones and Zone 2 jumped 20–30 beats? Nothing is broken. Why %HRmax and LTHR zones differ and why Zone 2 means three things.