Garmin and Apple Watch Validation Studies
What independent studies show for Garmin and Apple Watch heart rate, SpO2 and AFib detection, how validation studies work, and where each falls short.
- SpO2 & Oximetry
Short answer: In independent studies against ECG, the Apple Watch agreed with the reference more closely than the Garmin models tested, with a concordance of 0.92 against 0.81 for a Garmin Forerunner 235 during mixed aerobic exercise (Gillinov et al., 2017), and 0.96 against 0.89 for a Garmin Vivosmart HR during treadmill running (Pasadyn et al., 2019). Both brands are reasonably accurate at rest and less accurate during changing activity (Bent et al., 2020). In the Apple Heart Study, 0.52% of 419,297 participants received an irregular pulse notification, and 84% of notifications that coincided with ECG patch wear were atrial fibrillation (Perez et al., 2019).
Garmin and Apple both advertise accurate heart rate tracking, but peer-reviewed validation is narrower than the marketing: each study tests specific models, firmware and activities. This article summarizes what published, independent studies found, and what they did not test.
How Validation Studies Work
Most independent validation studies use a concurrent measurement design:
- Participants wear the watch alongside a reference: an ECG (or ECG chest strap) for heart rate, and arterial blood gas or a medical pulse oximeter for SpO2.
- They perform standardized activities, such as rest, treadmill walking and running, cycling, or elliptical training.
- Agreement is reported with error metrics (mean absolute error, mean absolute percentage error) or concordance coefficients, often with Bland-Altman plots for bias and limits of agreement.
Results apply to the device generation and firmware tested. The studies below used watches from 2016 to 2019, so newer models and algorithm updates may perform differently. Check the device and publication year before applying a result.
Heart Rate During Exercise
Cleveland Clinic aerobic exercise study (Gillinov et al., 2017). In this study, 50 healthy adults exercised on a treadmill, a stationary bike and an elliptical trainer, with ECG as the reference. Across all conditions, Lin's concordance with ECG was:
- Polar H7 chest strap: 0.996
- Apple Watch: 0.92
- TomTom Spark Cardio: 0.83
- Garmin Forerunner 235: 0.81
All devices except the Fitbit Blaze did well on the treadmill (0.88 to 0.93). On the bike, only the Garmin, the Apple Watch and the Scosche Rhythm+ forearm band reached acceptable agreement (above 0.80). On the elliptical without arm levers, only the Apple Watch was accurate. With arm levers, no device was.
Treadmill running in athletes (Pasadyn et al., 2019). In this study, 50 athletic adults ran at 4 to 9 mph wearing a three-lead ECG, a Polar H7 strap and two randomly assigned watches. Concordance with ECG was 0.98 for the Polar H7 and 0.96 for the Apple Watch Series 3. The Fitbit Ionic, Garmin Vivosmart HR and TomTom Spark 3 each reached 0.89.
Sources of error (Bent et al., 2020). This Duke study tested the Apple Watch 4 and the Garmin Vivosmart 3, among other consumer and research wearables, against ECG. Accuracy differed significantly between devices and between activity types, and absolute error during activity was on average 30% higher than at rest. There was no statistically significant difference in accuracy across skin tones. The authors concluded the devices were reasonably accurate at rest and at prolonged elevated heart rate, but differed in how they responded to changes in activity.
Diverse cohort (Shcherbina et al., 2017). This Stanford study compared seven wrist devices, including the Apple Watch, during sitting, walking, running and cycling. Garmin was not tested. The Apple Watch had the lowest overall heart rate error, and six of the seven devices had a median heart rate error below 5% during cycling. No device estimated energy expenditure with less than 20% error.
Two consistent findings: an ECG chest strap remains the most accurate option during exercise, and wrist accuracy depends on the activity, especially activities with arm movement or rapid changes in intensity.
HRV
Both brands report heart rate variability from wrist PPG, Garmin as HRV Status and Apple in the Health app. We did not find an independent peer-reviewed validation of these specific features against ECG to cite here. Wrist PPG is more sensitive to motion than ECG, so treat watch HRV as a personal trend rather than a clinical measurement. See PPG HRV motion artifacts and accuracy.
SpO2
Both companies describe their watch SpO2 readings as wellness features, not medical oximetry.
The clearest independent test of a Garmin watch was done at high altitude. Schiefer et al. (2021) compared the Garmin Fēnix 5X Plus with arterial blood gas in 13 healthy lowlanders at 4,559 m:
- The watch read 7.0 percentage points higher than arterial blood gas on average.
- Its mean absolute percentage error was 9.77%, with limits of agreement from −6.5 to 20.5 points.
- Its intraclass correlation was 0.549.
- A medical Covidien Nellcor oximeter tested alongside it differed by only 0.1 points.
The authors concluded the watch could not be recommended for monitoring SpO2 at altitude. Low saturations like these are where readings matter most. See the wearable pulse oximeter guide.
Atrial Fibrillation Detection
Apple Watch. The Apple Heart Study (Perez et al., 2019) enrolled 419,297 participants without known atrial fibrillation:
- Over a median of 117 days, 2,161 participants (0.52%) received an irregular pulse notification.
- Of the 450 who returned analyzable ECG patches, applied on average 13 days after notification, 34% had atrial fibrillation on the patch.
- When a notification arrived while the patch was being worn, its positive predictive value for atrial fibrillation was 0.84. The positive predictive value of a single irregular tachogram (pulse-interval reading) was 0.71.
The 34% figure is lower than the 0.84 partly because atrial fibrillation can come and go, and patches were applied about two weeks after the first notification.
The FDA granted Apple De Novo authorization for both its irregular rhythm notification feature and its ECG app in September 2018.
Garmin. The Garmin ECG App received FDA 510(k) clearance in January 2023 (K221774). It records a single-lead ECG similar to Lead I on compatible watches and classifies it as atrial fibrillation or sinus rhythm. It is not intended for people under 22 or for users with other known arrhythmias. Availability depends on model and region.
Neither app diagnoses atrial fibrillation. The FDA indications describe the output as informational and say users should not take clinical action without a qualified healthcare professional.
Summary of the Evidence
Scroll horizontally if needed
| Measure | Apple Watch | Garmin | Source |
|---|---|---|---|
| HR, mixed aerobic exercise (concordance with ECG) | 0.92 | 0.81 (Forerunner 235) | Gillinov et al., 2017 |
| HR, treadmill running 4–9 mph (concordance with ECG) | 0.96 (Series 3) | 0.89 (Vivosmart HR) | Pasadyn et al., 2019 |
| HR error, activity vs rest | Error during activity ~30% higher than at rest across tested devices (Apple Watch 4, Garmin Vivosmart 3 and others) | Same study | Bent et al., 2020 |
| SpO2 at 4,559 m vs arterial blood gas | Not tested in these studies | Mean bias +7.0 points, MAPE 9.77% (Fēnix 5X Plus) | Schiefer et al., 2021 |
| AF notification / ECG | 0.52% notified; PPV 0.84 for notifications | ECG App 510(k)-cleared 2023 | Perez et al., 2019; FDA |
Which Should You Choose?
- Training by heart rate: both brands are usable for steady-state aerobic exercise. For intervals, cycling with heavy grip, or elliptical and rowing work, an ECG chest strap is more reliable than either watch.
- Rhythm screening: both offer FDA-authorized single-lead ECG apps on supported models. Apple's irregular rhythm notification has the largest published study behind it.
- SpO2: treat either watch's SpO2 as a wellness reading, not a substitute for a medical pulse oximeter.
- Research use: report the model, firmware and sampling settings, and validate against a reference in your own protocol.
Key Limitations
Both brands share the physical limits of wrist PPG: motion artifacts, cadence locking during running, and reduced perfusion in the cold. Published studies are also small (13 to 60 participants for heart rate and SpO2) and tested older models, so newer watches need fresh independent validation. See the Garmin vs Apple Watch comparison, wrist PPG accuracy limitations, Apple Watch vs chest strap accuracy and smartwatch heart rate accuracy.
FAQ
Is Garmin or Apple Watch more accurate for heart rate? In the two head-to-head studies with ECG as the reference, the Apple Watch agreed with ECG more closely than the Garmin models tested: 0.92 against 0.81 during mixed aerobic exercise (Gillinov et al., 2017), and 0.96 against 0.89 during treadmill running (Pasadyn et al., 2019). These studies used 2016–2018 models, and newer watches have not been compared head to head in a published study.
Are smartwatch heart rate readings accurate enough for training? For steady aerobic exercise such as treadmill running, both brands showed good agreement with ECG. Accuracy dropped for some activities, such as the elliptical with arm levers, and error is higher during activity than at rest. When precise real-time heart rate matters, an ECG chest strap is more accurate.
Can I use Apple Watch or Garmin to diagnose atrial fibrillation? No. Both offer FDA-authorized single-lead ECG apps that classify a recording as atrial fibrillation or sinus rhythm, but the output is informational. A diagnosis requires a qualified healthcare professional.
How accurate is Garmin SpO2? In the one independent test against arterial blood gas, at 4,559 m altitude, the Garmin Fēnix 5X Plus read 7.0 percentage points high on average with a 9.77% mean absolute percentage error. The authors did not recommend it for monitoring SpO2 at altitude (Schiefer et al., 2021).
References
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Gillinov S, Etiwy M, Wang R, Blackburn G, Phelan D, Gillinov AM, Houghtaling P, Javadikasgari H, Desai MY. Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise. Med Sci Sports Exerc. 2017;49(8):1697-1703. doi:10.1249/MSS.0000000000001284
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Pasadyn SR, Soudan M, Gillinov M, Houghtaling P, Phelan D, Gillinov N, Bittel B, Desai MY. Accuracy of commercially available heart rate monitors in athletes: a prospective study. Cardiovasc Diagn Ther. 2019;9(4):379-385. doi:10.21037/cdt.2019.06.05
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Bent B, Goldstein BA, Kibbe WA, Dunn JP. Investigating sources of inaccuracy in wearable optical heart rate sensors. npj Digit Med. 2020;3:18. doi:10.1038/s41746-020-0226-6
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Shcherbina A, Mattsson CM, Waggott D, et al. Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. J Pers Med. 2017;7(2):3. doi:10.3390/jpm7020003
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Schiefer LM, Treff G, Treff F, et al. Validity of Peripheral Oxygen Saturation Measurements with the Garmin Fēnix® 5X Plus Wearable Device at 4559 m. Sensors. 2021;21(19):6363. doi:10.3390/s21196363
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Perez MV, Mahaffey KW, Hedlin H, et al. Large-Scale Assessment of a Smartwatch to Identify Atrial Fibrillation. N Engl J Med. 2019;381(20):1909-1917. doi:10.1056/NEJMoa1901183
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U.S. Food and Drug Administration. De Novo DEN180042, Irregular Rhythm Notification Feature (Apple Inc.), and DEN180044, ECG App (Apple Inc.), decisions of September 11, 2018. DEN180042 · DEN180044
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U.S. Food and Drug Administration. 510(k) K221774, Garmin ECG App (Garmin International, Inc.), decision of January 12, 2023. K221774
Frequently Asked Questions
- Is Garmin or Apple Watch more accurate for heart rate?
- In the two head-to-head studies with ECG as the reference, the Apple Watch agreed with ECG more closely than the Garmin models tested: 0.92 against 0.81 during mixed aerobic exercise (Gillinov et al., 2017), and 0.96 against 0.89 during treadmill running (Pasadyn et al., 2019). These studies used 2016–2018 models, and newer watches have not been compared head to head in a published study.
- Are smartwatch heart rate readings accurate enough for training?
- For steady aerobic exercise such as treadmill running, both brands showed good agreement with ECG. Accuracy dropped for some activities, such as the elliptical with arm levers, and error is higher during activity than at rest. When precise real-time heart rate matters, an ECG chest strap is more accurate.
- Can I use Apple Watch or Garmin to diagnose atrial fibrillation?
- No. Both offer FDA-authorized single-lead ECG apps that classify a recording as atrial fibrillation or sinus rhythm, but the output is informational. A diagnosis requires a qualified healthcare professional.
- How accurate is Garmin SpO2?
- In the one independent test against arterial blood gas, at 4,559 m altitude, the Garmin Fēnix 5X Plus read 7.0 percentage points high on average with a 9.77% mean absolute percentage error. The authors did not recommend it for monitoring SpO2 at altitude (Schiefer et al., 2021).
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Research records with the datasets, papers and published validation results for each device.