Near-infrared spectroscopy (NIRS) has emerged as a potential alternative method for determination of breakpoints equivalent to lactate thresholds. However, the optimal NIRS location remains unclear, particularly in Nordic skiing, which requires both upper- and lower-limb contributions. This study aimed to evaluate the feasibility and accuracy of NIRS-derived breakpoints determination (i.e., BP1 and BP2) compared to first (LT1) and second (LT2) lactate thresholds and to compare different muscle sites in male and female world-class Nordic skiers. Fifty-two world-class Nordic skiers (29 males, 23 females) performed an incremental treadmill test on roller skis. NIRS sensors were located simultaneously on four muscles: vastus lateralis (VL), biceps femoris (BF), biceps brachii (BB), and triceps brachii (TB). Oxygen saturation (SmO2) was collected and analysed to detect BP1 and BP2 vs. LT1 and LT2. First, BP1 was too often undetectable or inaccurately detected, suggesting an unsuitable practical use. Second, BP2 was detected in VL (88.5%), BF (96.2%) and BB (86.5%) but not in TB (24.1%). Third, there was a very good accuracy (i.e., bias [95% CI] in heart rate between BP2 and LT2 in VL (-0.6 bpm [-8.9, 7.8]), BF (+1.3 bpm [-2.8, 4.2]) and BB (+1.0 bpm [-7.5, 9.5]). Finally, no significant differences were found between male and female athletes. NIRS appears as an effective non-invasive method for detecting breakpoint equivalent to LT2 in both male and female world-class Nordic skiers, especially if positioned on both BB and BF. Highlights - What is the central question of this study? Can near-infrared spectroscopy (NIRS) muscle oxygen saturation breakpoints (BP1, BP2) validly replace lactate thresholds (LT1, LT2) in world-class Nordic skiers? Which muscles provide the best signal (vastus lateralis, biceps femoris, biceps brachii, triceps brachii)? Are there are sex-related differences? - What is the main finding and its importance? BP1 was unreliable, but BP2 closely matched LT2 when measured on biceps femoris and biceps brachii (small heart rate bias, high agreement), while triceps brachii performed poorly. No meaningful sex differences were found, likely due to low adipose tissue thickness in elite athletes. Practically, NIRS offers a non-invasive field alternative to estimate LT2 in Nordic skiing, with recommended sensor placement on biceps femoris and biceps brachii.
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