Hello,
The Forerunner 165 automatically overwrites Threshold Power (FTP) after hard sessions, and there is no option to disable auto-detect. This can result in unrealistic FTP values.
Important: Firstbeat algorithms for VO₂, EPOC, Recovery Time, and Training Load do NOT use FTP, only HR, HRV, and physiological trends.
Documents:
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VO₂ estimation: https://assets.firstbeat.com/firstbeat/uploads/2015/10/white_paper_vo2_estimation.pdf
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EPOC estimation: https://assets.firstbeat.com/firstbeat/uploads/2015/10/white_paper_epoc.pdf
Workaround for power zones:
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Set a very high Threshold Power (e.g., 999 W)
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Adjust the zone percentages manually to match your real physiology
This keeps your power zones accurate without affecting other metrics.
Important note about power zones:
Even if power zones are displayed and edited in watts, Garmin stores them internally as percentages of Threshold Power (FTP).
This means that any automatic FTP update will still modify the zones, even when they appear fixed in watts in the settings.
That’s why controlling or neutralizing FTP auto‑detect is necessary if you want stable power zones.