Your HRV is wrong. Use it anyway

What was tested

One wrist device, Biostrap Kairos Spot Check, against a clinical ECG. Short 3-minute resting readings in the lab, lying down and sitting. Healthy young adults.

What it found

Heart rate matched ECG very closely. HRV was fine for patterns, not precise numbers. Time-based scores did best: SDNN strongest, RMSSD close. SD1 was OK. LF, HF and SD2 were poor. As HRV gets higher, the wrist reads lower and the error widens, so big improvements at the top end can be under-reported.

Caveat

No external funding. Biostrap supplied one device. One author did paid education work with Biostrap in 2018. Treat results as promising but look for replication.

Other brands, from the paper’s discussion

Resting heart rate is often accurate on Polar Vantage V2, Empatica E4 and recent Apple Watches in controlled tests. Fitbit Charge 4 and Samsung Active2 were less precise in shorter samples. For HRV, studies on Garmin Venu 2S and Apple Watch show the same pattern many wearables share: lower accuracy as HRV rises and a tendency to pull high values down. These are separate studies, not part of this experiment.

How to use this

Track trends over 3–7 days. Be cautious with very low or very high single-day values. For important decisions, confirm with ECG or an ECG-quality chest strap.

Source

Flatt AA et al. Sensors, 2025: https://www.mdpi.com/1424-8220...

On my mindUnderrated truth

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