Thresholds Derived From Muscle NIRS Signal, Heart Rate Variability, and Ventilatory Parameters During an Incremental Cycling Test: An Agreement Analysis

This study aimed to compare different thresholds obtained from the muscle near-infrared spectroscopy (NIRS), heart rate variability (HRV), HR and ventilatory parameters during a maximal incremental cycling test. Twenty trained male cyclists performed an incremental step test with breath-by-breath gas analysis, muscle NIRS-derived signal, HRV and HR continuously recorded. The first and second ventilatory thresholds (VT₁ and VT₂, respectively) were identified and compared to the other methods. The D_MAX method was used to determine the NIRS-derived thresholds by oxygenated hemoglobin (O₂HbT), deoxygenated hemoglobin (HHbT), tissue saturation index (TSIT); HRV-derived thresholds by standard deviation of the instantaneous (SD1T) and continuous long-term RR intervals (SD2T) and HR deflection point. No significant differences were found between VT₁ and O₂HbT (171 ± 25 vs 179 ± 46 W; SEE = 23.0 W), VT₁ and SD1T (171 ± 39 W; SEE = 26.2 W) and VT₁ and SD2T (185 ± 37 W; SEE = 21.7 W); as well as between VT₂ and TSIT (242 ± 28 vs 230 ± 61 W; SEE = 25.1 W), when expressed as power-output, respectively. The standard error of estimate for these comparisons is approximately equivalent to one incremental stage (25 W). However, agreement analysis showed substantial inter-individual error (LoA up to ±107 W). This wide variability limits their precision for individual prescription, suggesting these methods should complement rather than substitute ventilatory thresholds.