Determination of maximal lactate steady state in healthy adults: Can NIRS help?

PURPOSE: We tested the hypothesis that the maximal lactate steady state (MLSS) can be accurately determined in healthy subjects based on measures of deoxygenated hemoglobin (deoxyHb), index of oxygen extraction measured non-invasively by near-infrared spectroscopy (NIRS). METHODS: 32 healthy men (average age 48±17 yrs, range 23-74 yrs) performed an incremental cycling test to exhaustion and square wave tests for MLSS determination. Cardiorespiratory variables were measured bbb and deoxyHb was monitored non-invasively on the right vastus lateralis with a quantitative NIRS device. The individual values of V O2 and heart rate (HR) corresponding to the MLSS were calculated and compared to the NIRS-derived MLSS (NIRSMLSS) that was, in turn, determined by double linear function fitting of deoxyHb during the incremental exercise. RESULTS: V O2 and HR at MLSS were 2.25±0.54 L•min-1 (76±9 % V O2max) and 133±14 b•min-1 (81±7 %HRmax) respectively. Muscle O2 extraction increased as a function of exercise intensity up to a deflection point, NIRSMLSS, at which V O2 and HR were 2.23±0.59 L•min-1 (76±9 % V O2max) and 136±17 b•min-1 (82±8 %HRmax) respectively. For both V O2 and HR, the difference of NIRSMLSS from MLSS values was not significant and the measures were highly correlated (r2=0.81 and r2=0.76). The Bland Altman analysis confirmed a not significant bias for V O2 and HR (-0.015 L•min-1 and 3 b•min-1, respectively) and a small imprecision of 0.26 L•min-1 and 8 b•min-1. CONCLUSIONS: A plateau in muscle O2 extraction was demonstrated in coincidence with MLSS during an incremental cycling exercise, confirming the hypothesis that this functional parameter can be accurately estimated with a quantitative NIRS device. The main advantages of NIRSMLSS over lactate-based techniques are the non-invasiveness and the time/cost efficiency.