Combining Near-Infrared Spectroscopy and Heart Rate Variability Derived Thresholds to Estimate the Critical Intensity of Exercise

Fleitas-Paniagua, PR, de Almeida Azevedo, R, Trpcic, M, Murias, JM, and Rogers, B. Combining near-infrared spectroscopy and heart rate variability derived thresholds to estimate the critical intensity of exercise. J Strength Cond Res 38(1): e16-e24, 2024-Critical intensity determination often requires costly tools and several testing sessions. Alternative approaches display relatively large individual variation. Therefore, simpler estimations with improved precision are needed. This study evaluated whether averaging the heart rate (HR) and oxygen uptake (VO2) responses associated with the muscle deoxyhemoglobin concentration breakpoint ([HHb](BP)) and the heart rate variability (HRV) given by the detrended fluctuation analysis second threshold (HRVT2) during ramp incremental (RI) test improved the accuracy of identifying the HR and VO2 at the respiratory compensation point (RCP). Ten female and 11 male recreationally trained subjects performed a 15 W<middle dot>minute(-1) RI test. Gas exchange, near-infrared spectroscopy (NIRS), and RR interval were recorded to assess the RCP, [HHb](BP), and HRVT2. Heart rate (mean +/- SD: 158 +/- 14, 156 +/- 13, 160 +/- 14 and, 158 +/- 12 bpm) and VO2 (3.08 +/- 0.69, 2.98 +/- 0.58, 3.06 +/- 0.65, and 3.02 +/- 0.60 L<middle dot>minute(-1)) at the RCP, [HHb](BP), HRVT2, and HRVT2&[HHb](BP) average (H&H-Av), respectively, were not significantly different (p > 0.05). The linear relationship between H&H-Av and RCP was higher compared with the relationship between [HHb](BP) vs RCP and HRVT2 vs RCP for both HR (r = 0.85; r = 0.73; r = 0.79, p > 0.05) and VO2 (r = 0.94; r = 0.93; r = 0.91, p > 0.05). Intraclass correlation between RCP, [HHb](BP), HRVT2, and H&H-AV was 0.93 for VO2 and 0.79 for HR. The [HHb](BP) and the HRVT2 independently provided VO2 and HR responses that strongly agreed with those at the RCP. Combining [HHb](BP) and the HRVT2 resulted in estimations of the VO2 and HR at the RCP that displayed smaller variability compared with each modality alone.