Key result
Apple Watch Series 8 matches ECG heart rate during high-intensity exercise, unlike Polar Grit X.
Why the study?
Wrist-worn photoplethysmography devices are widely used for heart rate monitoring, yet their accuracy varies across brands and exercise modalities.
Do wrist-based PPG devices (Apple Watch Series 8 and Polar Grit X) accurately measure heart rate compared to an ECG chest strap during high-intensity exercise in physically active adults?
Observational (n=25)
Do wrist-based PPG devices (Apple Watch Series 8 and Polar Grit X) accurately measure heart rate compared to an ECG chest strap during high-intensity exercise in physically active adults?
The Apple Watch Series 8 provides reasonably accurate heart rate monitoring during high-intensity exercise, whereas the Polar Grit X significantly underestimates heart rate compared to a chest-strap ECG.
Answers are grounded in this paper and cite their sources.
Clinicians should verify wrist monitor accuracy before exercise prescriptions; leaves open optimal device selection pending larger validation studies.
Introduction: Wrist-worn photoplethysmography (PPG) devices are widely used to monitor heart rate (HR); yet, their accuracy varies across brands and exercise modalities. This study compared HR measurements of two wrist-based PPG devices (Apple Watch Series 8 (AWS8) and Polar Grit X (PGX)) to an ECG chest strap (Polar H10) during treadmill high-intensity intervals (HII) and a high-intensity functional training (HIFT) circuit workout. Methods: A total of 25 physically active adults (age = 21.4 ± 9.8 yrs) completed two exercise protocols: treadmill-based HII and a full-body HIFT on two separate days. Participants wore both wrist-based PPG devices and the ECG chest strap for both sessions. HR was recorded at one-second intervals; AWS8 data were interpolated to match sampling. Agreement with ECG was assessed using individual differences analysis and equivalence testing within ±2.5%. Results: During treadmill HII, mean bias for the AWS8 ranged from 0.1 to 3.2 bpm across intensities, whereas the PGX ranged from −12.6 to −5.3 bpm. The AWS8 demonstrated statistical equivalence to the ECG at all intensities, except at 110% of velocity at maximal oxygen consumption ( ). The PGX did not achieve equivalence at any intensity. During the HIFT, mean biases for the AWS8 ranged from −0.4 to 3.1 bpm, while the PGX ranged from −22.3 to −15.7 bpm. The AWS8 showed equivalence for kettlebell swings and box jumps but no other movement. PGX showed no statistical equivalence during any exercise. Conclusion: The AWS8 demonstrated strong accuracy in HR measurement during steady-state and some high-intensity exercise. Measures that were not equivalent for the AWS8 within the strict ±2.5% margin were equivalent at ±5%. The PGX did not achieve statistical equivalence under any of the tested conditions. Consumers and professionals should consider exercise modality, device capabilities, and acceptable margin of error when selecting wearables. Pairing wrist devices with a chest-strap ECG can improve accuracy.
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Ryan et al. (2026) conducted an observational in Physically active adults (n=25). Wrist-based PPG devices (Apple Watch Series 8 and Polar Grit X) vs. ECG chest strap (Polar H10) was evaluated on Heart rate measurement agreement with ECG using individual differences analysis and equivalence testing within ±2.5%. The Apple Watch Series 8 demonstrated statistical equivalence to an ECG chest strap for heart rate monitoring during most high-intensity exercises, whereas the Polar Grit X did not.
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