Validity of an open-source mobile app to measure fractal correlation properties of heart rate variability during exercise

Purpose: The nonlinear index of heart rate (HR) variability (HRV), based on detrended fluctuation analysis (DFA a1), is recognized for its value in assessing endurance exercise intensity thresholds and autonomic durability. However, a constraint to widespread implementation is lack of a low cost, real-time solution. This study examined whether a free, open-source Android application (Fatmaxxer) could provide reasonable agreement and correlation with the established standard of Kubios HRV software. Methods: Twenty-three participants (10 males) underwent incremental step-ramp-step cycling testing measuring gas exchange variables with recording of raw RR intervals and Fatmaxxer computed DFA a1. The raw RR series was processed by Kubios HRV software for comparisons of HRV HR thresholds (HRVT_K vs. HRVT_FM), DFA a1 responses during the entire incremental ramp and average DFA a1 computed from a post ramp constant intensity interval performed in the heavy domain. Results: Based on t testing, Wilcoxon sign rank testing and Bland Altman analysis, there were small differences present in the direct comparison of DFA a1 responses in the incremental ramp and 10-minute constant interval (bias 0.03–0.05 respectively). Despite this bias, the mean HRVT1/2 determinations were within 1 bpm and the first HRVT comparisons equivalent. All regressions showed excellent alignment with Pearson’s r/Spearman’s correlation/ICC above 0.96. Conclusions: The DFA a1 responses and HRVTs derived from Fatmaxxer closely aligned with those obtained from the established standard Kubios HRV software. This highlights the utility of Fatmaxxer as a free, open-source Android platform capable of real-time monitoring, supporting broader accessibility of DFA a1 tracking.