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 (LT 2 ) 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 %V̇O 2 max at HHb-BP, first lactate threshold (LT 1 ), and LT 2 was higher in the slope-based test, along with absolute V̇O 2 values at LT 1 and LT 2 . In the speed-based test, HHb-BP occurred at a lower intensity than LT 2 , with poor agreement between them, whereas no significant differences were found in the slope-based test. In conclusion, HHb-BP occurred at a higher %V̇O 2 max in the slope-based test. In the speed-based test, HHb-BP was observed at a lower intensity than LT 2 , with weak agreement between them. However, the differences in LT 1 and LT 2 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).