Heart-rate variability is the most-marketed recovery metric in consumer wearables. The validation literature behind it measures something narrower than the dashboards imply, and the gap matters for anyone making training decisions on a morning number.
A single-morning HRV reading has not been shown to predict next-day performance in individuals. Most validation is against chest-strap ECG, at rest, in small samples of trained adults.
What the validation studies actually measured
Agreement with ECG at rest is strong across consumer devices. Agreement during movement, and agreement on night-to-night change within one person, are weaker and less studied. Those are different questions, and product marketing tends to answer the first while implying the third.
| Question | Grade | Basis |
|---|---|---|
| Accurate (analytic validity) | Good at rest | Agreement with ECG in controlled conditions |
| Meaningful (clinical validity) | Partial | Group-level associations, thin individual-level data |
| Actionable (clinical utility) | Unproven | No trials showing better outcomes from acting on daily readings |
What to watch next
Trials that randomize training decisions to HRV-guided versus fixed protocols, with performance as the endpoint, are the ones that would move this from partial to established.
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