Why Threshold Detection Matters More Than You Think
In endurance sports and performance testing, thresholds are key checkpoints that reveal how the body transitions between energy systems. Detecting these thresholds allows coaches and athletes to individualize training intensity, monitor adaptation, and predict performance without needing to rely on maximal tests. Traditionally, ventilatory thresholds (via VO₂ and gas exchange) and lactate thresholds (via repeated blood sampling) have been the gold standards. Both methods are valid but also come with practical downsides: VO₂ testing requires expensive lab equipment, specialized expertise, and controlled environments, while lactate testing is invasive, time-consuming, and less suited to repeated field use. This is where near-infrared spectroscopy (NIRS) offers a game-changing alternative: a non-invasive, real-time method of tracking muscle oxygenation during exercise.
The Challenge of Measuring the First Threshold (BP1)
However, while NIRS has shown strong potential, recent scientific work has made it clear that measuring the first breakpoint (BP1, corresponding roughly to the aerobic threshold/VT1/LT1) is more difficult than identifying the second threshold (BP2). For example, a 2025 study by Arnet et al. compared NIRS-derived thresholds with ventilatory thresholds in endurance and CrossFit athletes. Their results showed only weak-to-moderate agreement at the first threshold, while BP2 aligned much more consistently. Similarly, Reinpõld et al. (2024) demonstrated that SmO₂ breakpoints in trained cyclists tended to occur at lower intensities than VT1, again underlining the challenges of early threshold detection. A 2023 meta-analysis (Sendra-Pérez et al.) confirmed this pattern across the literature, reporting moderate reliability for BP1 (ICC ≈ 0.53) compared to higher reliability for BP2 (ICC ≈ 0.80).
Why BP1 Is Harder to See with Muscle Oxygen
Why is BP1 so elusive? First, the physiological changes at VT1 are subtle: oxygen extraction rises only slightly, making SmO₂ signals shallow and harder to interpret. Second, muscles recruit fibers heterogeneously; even left vs. right legs can desaturate differently, meaning a single-site sensor may not fully represent systemic transitions. Third, local tissue factors like adipose thickness can attenuate the optical signal, masking small shifts at low intensity. Finally, the algorithms used to detect breakpoints often struggle when the curve inflection is minimal, increasing variability in BP1 detection.
Where NIRS Excels
Despite these challenges, the picture is far from discouraging. NIRS consistently shows stronger accuracy and agreement for BP2—arguably the more important threshold for structuring high-performance training, pacing strategies, and race preparation. Moreover, unlike VO₂ or lactate testing, NIRS can be deployed repeatedly in the field, providing immediate insights during real training sessions. This ability to capture dynamic muscle oxygen trends in real time is a breakthrough for athletes who want actionable data without the limitations of lab tests.
How Train.Red Bridges the Gap
At Train.Red, we recognize both the challenges and the opportunities in threshold detection. Our approach focuses on:
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Wearable NIRS hardware that integrates seamlessly into training and competition.
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Affordable, repeatable testing that saves athletes time and resources.
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Advanced data processing that turns complex SmO₂ signals into actionable feedback.
While VO₂ and lactate testing remain indispensable in controlled lab settings, they are not practical for daily training. NIRS brings threshold science to the field, and with Train.Red, athletes and coaches gain a tool that balances scientific accuracy, usability, and accessibility.
In short: VO₂ and lactate testing remain powerful, but they belong mostly in specialized labs. NIRS is the tool that brings threshold science into the everyday training environment. And with Train.Red, you can turn that science into performance.
-Arnold JI, Yogev A, Nelson H, van Hooff M, Koehle MS. Muscle reoxygenation is slower after higher cycling intensity, and is faster and more reliable in locomotor than in accessory muscle sites. Front Physiol. 2024 Aug 14;15:1449384. doi: 10.3389/fphys.2024.1449384. PMID: 39206382; PMCID: PMC11349675.
-Sendra-Pérez C, Sanchez-Jimenez JL, Marzano-Felisatti JM, Encarnación-Martínez A, Salvador-Palmer R, Priego-Quesada JI. Reliability of threshold determination using portable muscle oxygenation monitors during exercise testing: a systematic review and meta-analysis. Sci Rep. 2023 Aug 4;13(1):12649. doi: 10.1038/s41598-023-39651-z. PMID: 37542055; PMCID: PMC10403529.
