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Example of RR intervals recorded by ECG and a PPG-based device (Polar H7) during recovery after exercise in one subject. (a) RRE = RR interval series recorded by ECG; (b) RRP = RR intervals series recorded by the PolarH7. Compared to the ECG, the PPG method overestimates the HRV. Reprinted from [55]. No changes were made to the image, which has been published under CC BY 4.0 license (https://creativecommons.org/licenses/by/4.0/legalcode).
