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Short answer: An early independent comparison reported that Apple Watch Series 12 heart-rate readings closely tracked a Polar H10 chest strap during indoor cycling, running, and weightlifting. That does not establish accuracy for every wearer, and the report provides no Series 12 result for HIIT. The figures are correlations—not errors measured in beats per minute—so they cannot show how far readings differed at each moment.
What the Series 12 chest-strap comparison found
In a report published September 18, 2026, 9to5Mac described The Quantified Scientist’s comparison of Apple Watch Series 12 with a Polar H10 ECG chest strap. The report covered indoor cycling, running, and weightlifting, and gave a correlation for each activity:
| Activity | Reported correlation with Polar H10 | What the report says |
|---|---|---|
| Indoor cycling | 1.0 | The traces were described as nearly identical for the comparison. |
| Running | 0.98 | Minor deviations appeared at higher heart rates. |
| Weightlifting | 0.97 | This was the lowest of the three reported correlations. |
These are results reported by 9to5Mac from the tester’s comparison, not universal accuracy rates. The available report passage does not give participant count, test duration, mean absolute error, or a range of bpm differences. A correlation describes how closely readings move together; it does not establish that the watch and strap agree within a particular number of beats per minute at every point. Read 9to5Mac’s report.
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What those results mean for each workout
Indoor cycling
The reported 1.0 correlation is strong evidence that the watch trace tracked the Polar H10 trace closely in the tester’s indoor-cycling comparison. It is not a claim that the watch was “100% accurate,” nor does the available report provide the bpm error needed to judge the size of any differences.
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Running
The report gives a 0.98 correlation and notes minor deviations at higher heart rates. It also says the Series 11 performed slightly better in the tester’s longer-term running observations. The evidence therefore does not support a blanket claim that Series 12 is more accurate than its predecessor while running.
Strength training
The 0.97 figure applies to the report’s weightlifting test, not every kind of strength workout. Different lifts can involve different wrist positions, grip, and movement patterns, so this result should not be generalized to all strength training. It was the lowest correlation among the three activities reported, but the passage does not quantify the actual bpm differences.
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HIIT
The independent report does not give a Series 12 HIIT result. Apple’s 2024 technical paper describes testing on earlier Apple Watch generations that included HIIT and chest-strap or ECG references, but that is not evidence for Series 12. Apple’s 2024 activity paper is useful background, not a substitute for a model-specific HIIT comparison.
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Apple says Series 12 has the most accurate heart-rate sensing in a wearable, citing a study with more than 1,000 participants comparing it with leading wearables in a diverse population. That is Apple’s company claim. The announcement does not provide detailed error tables or enough protocol information to independently assess the claim or compare its results directly with the Polar H10 correlations above. Apple’s Series 12 announcement.
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The two kinds of evidence answer different questions: Apple describes a larger study but, in the announcement, does not provide the detail needed to calculate or inspect an error distribution; the independent report names the chest strap and activities but is an early tester comparison rather than a large population study. Neither supports a precise statement about the Series 12’s bpm error across all people and workouts.
Why wrist readings can differ from a chest strap
A Polar H10 measures electrical activity at the chest; Apple Watch uses an optical sensor at the wrist. Wrist readings can be affected by how the watch sits against the skin and by workout conditions. Apple says cold can reduce skin perfusion, some tattoos can interfere with light reaching the sensor, and irregular movement may make readings less reliable than rhythmic activities such as running or cycling.
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- Understand how much you slow down when your foot hits the ground with step speed loss, and improve your running form with additional running dynamics, including stride length, vertical oscillation and ground contact time balance (requires compatible smartwatch)
- During activities where you can’t wear a watch, such as team sports, HRM 600 will record the workout and sync data, including heart rate, calories, speed, distance and more, directly to the Garmin Connect smartphone app
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- Wear the watch snugly on top of your wrist, with the sensor close to your skin.
- Consider whether cold conditions, a tattoo beneath the sensor, or irregular wrist movement could be affecting the reading.
- For a workout where consistent readings matter and the watch is not giving them, Apple says a compatible Bluetooth chest strap can be connected.
Apple’s guidance covers fit and possible sources of inconsistency; it does not promise that adjusting fit will eliminate every discrepancy. See Apple’s measurement guidance and Apple’s heart-rate monitoring information for details.
Verdict: promising for three tested activities, unproven here for HIIT
The reported comparison is encouraging for indoor cycling, running, and weightlifting, with the clearest reported match during indoor cycling. But the correlations do not tell you the bpm error, the report represents a tester comparison rather than a large independent study, and it contains no Series 12 HIIT result. If your workouts involve intervals or you need a chest reference, do not treat these results as proof that the watch will match a strap in your specific conditions.
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