Agreement between heart rate variability-derived and lactate/ventilatory thresholds during a 4-min stepwise...

Accurate exercise intensity thresholds are key for endurance training prescription. The non-linear heart rate (HR) variability (HRV) index DFAa1 has been proposed as a threshold determination marker, with DFAa1 values of 0.75 (HRVT1) and 0.5 (HRVT2) corresponding to aerobic and anaerobic thresholds, respectively. This study investigated the agreement between these novel HRVT's and the gold standard blood-lactate (LT) and ventilatory thresholds (VT). Twenty-one trained participants completed a 4-min stepwise cycling test (starting 40 W, +30 W/stage) with continuous HR and gas exchange measurement. Capillary blood lactate was sampled after each stage. Agreement between corresponding thresholds was assessed for HR, power output (PO), and oxygen uptake (V̇O₂) using (intraclass) correlation coefficients (ICC) and ANOVA. HRVT1 demonstrated poor agreement with LT1/VT1, while a more personalized determination method (HRVT1_pers) rendered trivial to moderate correlations and poor ICC. Both exhibited substantial mean bias relative to LT1/VT1. Conversely, HRVT2 demonstrated stronger agreement with LT2/VT2, particularly for PO and V̇O₂, with smaller mean biases but wide limits of agreement. ANOVA revealed significant HR differences between VT2 and LT2 and between VT2 and HRVT2. These findings support HRVT2 validity as a surrogate for LT2/VT2 in 4-min stepwise protocols, whereas HRVT1 demonstrates insufficient reliability for determining LT1/VT1.