An Observational Study of the Reliability and Concurrent Validity of HRV Monitors in Athletes

Hedvig Johansson¹Emily Adderley¹Sean Clarke¹Patrick Mcintyre¹Garreth Reilly¹Brian Caulfield¹Sinead Holden^1,2*

• ¹School of Public Health, Physiotherapy and Sports Science, University College Dublin, Dublin, Ireland
• ²institute for sport and health university college dublin, dublin, Ireland

Background: Heart rate variability (HRV) is a non-invasive indicator of autonomic nervous system function and is increasingly used in athlete monitoring. While electrocardiography (ECG) is the gold standard for HRV measurement, its use is limited in field settings. Objective: To evaluate the intra-session reliability and concurrent validity of a smartphone-based PPG app for HRV measurement in athletes, compared to a Polar H10 chest strap and ECG. Methods: This observational study included 37 trained participants (17 female; mean age 21.95 ± 3.69 years). HRV was recorded concurrently via ECG, Polar H10 chest strap, and the CameraHRV smartphone app in two repeated trials on the same day. Data were processed using device-specific software. Intra-class correlation coeRicients (ICC), coeRicient of variation (CV%), mean absolute percentage error (MAPE), and Bland–Altman plots were used to assess reliability and agreement. Results: All three devices showed good-to-excellent intra-session reliability for RMSSD (ICC range 0.83–0.90) and pNN50 (ICC range 0.87–0.92). The Polar chest strap had the highest consistency and lowest error compared to ECG (RMSSD MAPE: 2.16%). The PPG app also demonstrated strong validity (RMSSD MAPE: 17.49%) but wider limits of agreement. Conclusion: Both the Polar chest strap and smartphone PPG app demonstrated acceptable reliability and validity for short-duration HRV assessment in athletes. While the chest strap outperformed the PPG app in precision, the PPG app may oRers a practical, low-cost alternative for athlete monitoring.