Pulse vs. R-R Interval: Validity and Reliability of the Garmin Elevate v4 Sensor for Ultra-Short HR and HRV Monitoring in Healthy Adults

Accurate resting heart rate (HR) and heart rate variability (HRV) monitoring is crucial for evaluating autonomic nervous system balance and guiding training. This study investigated the validity and reliability of the Garmin Forerunner 955 smartwatch (Elevate v4 sensor) against an electrocardiogram (ECG) reference for measuring resting HR and photoplethysmography (PPG)-derived pulse rate variability (PRV), evaluated through the root mean square of successive differences (rMSSDPRV), compared to reference ECG-derived HRV (rMSSDHRV). Twenty-three healthy adults underwent simultaneous assessments using the smartwatch and a Biopac ECG during short recordings in supine and seated positions under controlled breathing. For resting HR, the optical device demonstrated a near-perfect linear association (r ≥ 0.94) and substantial concordance with the ECG. While rMSSDPRV demonstrated good-to-excellent intrasession test–retest reliability, its agreement with ECG-derived rMSSDHRV was limited, showing significant proportional bias and wide limits of agreement. A significant proportional bias was detected, producing back-transformed limits of agreement that demonstrated relative measurement errors ranging from –43% to +52% in the supine position and from –41% to +118% in the seated position in isolated individual cases. Ultimately, the Garmin Elevate v4 is highly reliable for basic HR tracking and within-subject PRV trends, but it lacks the absolute accuracy required to replace reference ECG-derived HRV for individual resting autonomic evaluation.