Comparison of NIRS-derived [HHb] breakpoints and concordance with lactate thresholds during speed- and slope-based incremental running tests.

Although physiological responses to flat versus incline running have been previously studied, the influence of slope on muscle deoxygenation and its relationship with physiological thresholds remains unclear. This study aimed to: (I) compare the deoxyhemoglobin breaking point (HHb-BP) between a slope-based and speed-based incremental running test, and (II) examine the agreement between HHb-BP and the second lactate threshold (LT2) in both tests. Fifteen recreational trail runners completed two incremental running tests: (I) a speed-based test with a fixed 1% gradient and 1 km h-1 increments every 3 min, and (II) a slope-based test at a fixed speed (50% of peak speed from the speed-based test) with 2% gradient increase every 3 min. Cardiorespiratory variables, blood lactate concentration, and gastrocnemius muscle oxygenation were assessed. The %V̇O2max at HHb-BP, first lactate threshold (LT1), and LT2 was higher in the slope-based test, along with absolute V̇O2 values at LT1 and LT2. In the speed-based test, HHb-BP occurred at a lower intensity than LT2, with poor agreement between them, whereas no significant differences were found in the slope-based test. In conclusion, HHb-BP occurred at a higher %V̇O2max in the slope-based test. In the speed-based test, HHb-BP was observed at a lower intensity than LT2, with weak agreement between them. However, the differences in LT1 and LT2 between tests were dependent on the method used to determine them. These findings highlight the importance of the method applied to determine lactate thresholds and the influence of the slope in peripheral physiological threshold. ReBEC trial registration: ensaiosclinicos.gov.br/(RBR-4hx89v9).