Heart Rate Variability-Established Thresholds to Determine the Ventilatory and Lactate Thresholds of Endurance Athletes

This study aimed to determine the use of heart rate variability (HRV)-established thresholds to accurately estimate endurance athletes' ventilatory threshold 1 (VT₁), respiratory compensation point (RCP) and lactate threshold 2 (LT₂). Eleven cyclists (aged: 23.7 ± 3.1 years) from the African Continental Development Cycling team and ten middle- and long-distance athletes (age: 21.2 ± 1.8 years) from a South African university participated in this study. Before the start of an incremental cycling or running maximal oxygen consumption (V̇O2max) test each participant was fitted with a Fixed Polar HR Transmitter Belt and Monitor to determine the R-R intervals of the last 60 seconds of each stage. The Kubios HRV Premium software package was used to analyze the R-R-intervals. Blood samples were taken 30 seconds before the end of each stage and analyzed for blood lactate. Ventilatory threshold points were identified using the criteria of an increase in V̇E/V̇O₂ with no increase in V̇E/V̇CO₂ (VT₁) and an increase in both V̇E/V̇O₂ and V̇E/V̇CO₂ (RCP). The LT₂ was identified as the second increase in blood lactate concentration from one increment to the next. Concerning HRV thresholds, VT₁ was determined at a DFAα1 value of 0.75, the first breakpoint in the standard deviation of the instantaneous (SD₁) and continuous long-term RR interval (SD₂) curves, the visual deflection in the squared root of the mean squared differences between successive R-R intervals (RMSSD) curve, and the first abrupt increase in high-frequency (HF) power x HF frequency. The RCP was detected at a DFAα1 value of 0.5 and the final abrupt increase in HF. The Bland-Altman plots revealed that the absolute power outputs at VT₁SD₁ and VT₁RMSSD showed significant agreements with the absolute power outputs at VT₁. Large paired-sample correlations were also found between the absolute power outputs at VT₁SD₁, VT₁RMSSD and VT₁. In conclusion, coaches, sport scientists and other professionals are encouraged to use SD₁ and RMSSD to determine the VT₁-related training program workloads of endurance-trained athletes, especially in areas where laboratories are not available.