The Forerunner 955 reads resting heart rate within half a beat of a laboratory ECG, a 0.9% difference across 23 adults, or 99.1% accuracy, if you prefer. Now the bad news.
- Sitting up, a single heart rate reading could be -6.4 to +5.3 bpm.
- HRV readings from the same sensor compared to the ECG over 2 minutes ranged from -43% to +118%
- The HRV error is largest in the best recovered subjects.
The work comes from Universidad Miguel Hernández and was published in the journal Sensors on 15 October. Participants lay down and sat still, breathed to a metronome, and wore a Forerunner 955 on one wrist, wired to a Biopac ECG. The watch measured through Garmin’s Health Snapshot, a two-minute spot check using the old-generation Elevate 4 sensor.
Resting heart rate: as good as the lab kit
| Position | Average difference from ECG | Range for one reading |
|---|---|---|
| Lying down | +0.52 bpm | -2.4 to +3.4 bpm |
| Sitting | -0.55 bpm | -6.4 to +5.3 bpm |
The researchers set their acceptability level at ±5 bpm before testing. Lying down, the watch cleared it but exceeded it slightly when sitting.
For anyone watching a morning resting heart rate trend, half a beat of average error changes very little.
HRV: the error grows with your recovery
| Position | Agreement with ECG | Range for one reading |
|---|---|---|
| Lying down | Poor (0.89) | -43% to +52% |
| Sitting | Poor (0.78) | -41% to +118% |
The error is not random. The higher your true HRV, the more the watch under-reads it, in both positions. i.e., the fittest, best recovered athletes get the least accurate numbers.
The authors spell out the consequence.
Athletes should not measure HRV lying down. Sit up at least, and an orthostatic test is better still.
The second study this year, this time it’s peer-reviewed
An April thesis by Kayla Porter under Andrew Flatt at Georgia Southern University tested the same Health Snapshot feature on a Forerunner 265, which has the same Elevate v4 sensor as the Forerunner 955, and found the same split: accurate heart rate and inaccurate HRV. Flatt’s summary was that Garmin reads low HRV too high and high HRV too low. The Spanish data show the same pattern. That thesis has not been peer-reviewed. This study has.
An all-day clinical ECG comparison published in February found errors above 100 milliseconds in some participants, running in both directions, on an older Garmin sensor.
Why a repeatable number can still be wrong
Take the same reading twice in a row, and the watch varies by about 3%. Lying down, it could pick up a smaller real change than the ECG could. The sensor is more consistent with itself than the gold standard is.
The two disagree for a mechanical reason. An ECG times the electrical signal that makes the heart beat. An optical sensor times the pulse arriving at the wrist, and the delay between the two changes from beat to beat with how stiff or relaxed your arteries are. That delay varies most in well-recovered people, and the wrist algorithm doesn’t correct for it.
What this means for HRV Status and Body Battery
The study tested Health Snapshot rather than the overnight HRV measurement that feeds Training Readiness and Body Battery, so nothing here measures overnight HRV accuracy directly.
It does support how Garmin frames those scores. Each is read against your own rolling baseline rather than an absolute figure, and a sensor that repeats itself to within 3% can track its own baseline. The number on the screen still means nothing compared with an ECG value, a chest strap value, or another athlete’s.
Garmin HRM 600
High-end, watch-free heart rate recording strap.
+other retailersWhere this doesn’t apply
- Nobody moved. Every measurement was taken lying down or sitting still, breathing to a metronome, so the study says nothing about optical heart rate away from rest or during exercise.
- Nobody stood up, where the earlier Forerunner 265 study found its biggest disagreement.
- The cohort was 23 adults averaging 29 years old, with no elite athletes. The authors say the error may be larger still in people with HRV (rMSSD) above 150ms.
- Everything came from one lab session, so the repeatability figures describe a single sitting rather than day-to-day consistency.
For HRV you can act on, the advice here is unchanged. Sit up, take it at rest on waking with a chest strap, and use the Polar H10 as the reference against which other devices are measured. Garmin’s top-end HRM 600 reads electrically rather than optically, which avoids the timing problem above, though testing on this site still put HRM 600’s RMSSD about 6.6ms below the H10.
Quick answers
Is Garmin's optical heart rate accurate?
For resting heart rate, yes. The Forerunner 955 matched a laboratory ECG within half a beat on average, and within about 3 bpm for a single reading taken lying down. The study tested nothing in motion, so it says nothing about accuracy during a run.
Can I use a Garmin instead of a chest strap for HRV?
Not if you want a figure comparable with an ECG or a strap. Individual readings ranged from -43% to +118% against the ECG value. The watch is consistent enough to track its own baseline, which is what HRV Status does, but the absolute number is unreliable.
Why does my Garmin HRV look low when I feel recovered?
The sensor under-reads HRV by more as true HRV rises, so the best recovered athletes see the largest underestimate. A real improvement in recovery can show up on the watch as a smaller gain than it was, or as no gain at all.
Last Updated on 18 October 2026 by tfk the5krunner
Small things I always use
- Garmin 90-degree charging adapter: Charges your Garmin from any USB-C cable. Essential when travelling. I use one.
- Body Glide: The blue anti-chafe stick swimmers and runners carry. I use it.
- Speedo earplugs: Fewer ear infections and no cold-water dizziness. I use them.
- Dryrobe Advance: My winter changing robe for open-water swims. I use one. A cheaper option is the AONYIYI changing robe.
- Maurten: Race fuel that stays easy on the stomach. I use them.
- Beet It Sport nitrate shots: Beetroot shots I take in the days before a race. I use them.
- Garmin charging puck: The quickest way to top up before a session. I use one.
- Injinji: Toe socks that stop rubbing, so you run more. I use them.
- Ultimate toe protectors: Stop black toenails on long runs. They work. I use them.
- Compressport R2 v2 calf sleeves: My calf sleeves of choice. I use them.
- Cellucor C4: My pre-workout before hard sessions.
- TPU tubes: Schwalbe Aerothan, Tubolito and Pirelli SmarTube. Light, tiny spares. I use all three.
- G8 insoles: Adjustable insoles that are good. I use them.
- Cycplus Cube: A 97g electric pump that gets a road tyre rideable in two minutes. I carry one. 5% off with code THE5KRUNNER.
Watches and sensors
- Polar H10: My daily driver for waking HRV.
- Polar Verity Sense: My backup arm-worn HR sensor.
- Nomad bedside charger: Charges my iPhone and Apple Watch overnight. I use it, and many of their bands.
- Train.Red: The best muscle oxygen sensor I’ve used.
- Tymewear: Accurate heart rate and reliable ventilatory zones. I use it to pace endurance rides over an hour.
- Garmin Forerunner 970: A pro-grade triathlon watch. I use this.
- Apple Watch Ultra 4: My daily smartwatch. No phone needed on rides.
- Garmin Fenix 9 Pro: I’ve just bought one.
On the bike
- Continental GP5000 for everyday riding, and Aero 111 on the front for special occasions.
- Helmets: Specialized S-Works Evade 3 for road riding, and Rudy Project Wing for TT-style races. I use both.
- Ravemen FR300: A front light that mounts under your Garmin or Wahoo. I use one.
- Wahoo TRACKR Radar: I use it on one bike and Varia on the other. They perform the same.
- Garmin Varia RTL515: Radar that warns you of cars from behind. I use this model.
- Wahoo ELEMNT Roam 3: The bike computer that’s easy to use. I use mine on most rides.
- Favero Assioma Pro RS2: Accurate power pedals that swap easily between bikes. I use this model.
Home and indoor training
- NPE Runn: Makes my basic treadmill smart, sending more accurate speed and distance to my watch and Zwift. I use it.
- Google Wifi: Keeps Zwift connected in my pain cave. I use it.
- Apple TV 4K: Zwift on the big screen. I use two.
- Google Nest Doorbell (wired, 2nd gen): My video doorbell, running in both Google Home and Apple Home. I use it.
Health, recovery and sleep
- FreeStyle Libre 3 Plus: Shows how my blood sugar responds to diet.
- hDrop: Measures my sweat loss in summer and indoors.
- Fourth Frontier Zone: Checks my heart under load in hard sessions.
- Oura Ring 5: My partner’s recovery tracker of choice.
- Eight Sleep: A cooling mattress cover my partner and I use nightly.
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tfk is the founder and author of the5krunner, an independent endurance sports technology publication. With 20 years of hands-on testing of GPS watches and wearables, and competing in triathlons at an international age-group level, tfk provides in-depth expert analysis of fitness technology for serious athletes and endurance sport competitors. See the gear tfk uses. ID
