Smartphone Accelerometer Assessment of HRV: Concurrent Agreement of the KubiosHRV App

Heart rate variability (HRV) is widely used to monitor athlete recovery and adaptation. Recent technological advances have enabled HRV assessment through smartphone applications without the need for external devices. This study aimed to assess the concurrent agreement between the accelerometer-based HRV measurement feature of the KubiosHRV app and the Polar H10 in supine, seated, and standing positions. A total of 52 participants (30 males and 22 females) across performance tiers 0–4 completed a 2 min resting recording, with the first minute discarded and the second minute analysed in each position. HRV was measured simultaneously using a Polar H10 chest strap and the accelerometer-based feature of the KubiosHRV app on an iPhone 14. The analysed variables included Mean RR, RMSSD, SDNN, LF power, HF power, SD1, and SD2. Data were automatically processed by the KubiosHRV app and manually extracted into a database. Results showed excellent intraclass correlation coefficients (≥0.985) and small magnitudes of error for time-domains and non-linear variables in the three analysed positions with the reference criterion. Frequency-domain variables showed larger error estimates. Bland–Altman analysis showed no evidence of proportional bias across the measurement range (p > 0.05). These findings suggest that, during 1 min recordings, the accelerometer-based feature of the KubiosHRV app showed strong concurrent agreement with the Polar H10 for time-domain variables and acceptable agreement for selected non-linear variables across positions. Therefore, it may represent a practical alternative for routine monitoring of these HRV metrics when external hardware is unavailable.