Muse 2 forehead photoplethysmography agrees with Polar H10 for heart rate but overestimates ultra-short RMSSD
While the Muse 2 forehead PPG sensor demonstrates near-perfect agreement with the Polar H10 ECG for mean heart rate and good agreement for SDNN, it significantly overestimates ultra-short RMSSD due to independent timing errors, rendering the two metrics non-interchangeable for this specific variability measure.