About half the athletes who try a NIRS sensor on a treadmill see no breakpoint at all. Not a blurry one. Not an ambiguous one. Nothing. That’s the finding most NIRS marketing skips over — and it’s the most important thing to understand before you spend $700 on a Moxy.
The answer to the title question is: partly. For roughly 50–65% of cyclists doing a step test, a NIRS sensor can detect the second threshold with good reliability. For the first threshold, running, and a large subset of athletes, it can’t.
What SmO2 Actually Measures
SmO2 (skeletal muscle oxygen saturation) is the percentage of hemoglobin and myoglobin in your muscle that is carrying oxygen at any given moment. The technology is called NIRS (near-infrared spectroscopy) — the sensor shines near-infrared light through your skin into the muscle beneath, and reads how much is absorbed by oxygenated vs deoxygenated blood.
Think of it like a fuel gauge for your muscle’s oxygen supply. When demand outpaces delivery, the needle drops. A sharp drop at a specific intensity is the signal coaches look for.
That signal sounds clean. In practice, it isn’t always.
How Two SmO2 Breakpoints Map to LT1 and LT2
In a step test on trained cyclists and triathletes (N=40), SmO2 drops in three distinct stages as intensity climbs. From baseline to Fatmax, it falls about 16%. From Fatmax to VT1, it falls another 16%. Then from VT1 to VT2, it plunges 45% — the biggest drop of all (Guerrero-Calderón et al., 2023).
That 45% cliff is the signal. When you see it on a graph, you’ve found your second threshold.
ROC analysis on the same cohort put the cut-offs at:
- VT1 (aerobic threshold / Zone 2 ceiling): ≤34% SmO2
- VT2 (anaerobic threshold / FTP zone): ≤26% SmO2
Those are population averages for trained cyclists. Your numbers may differ. Body composition, muscle fiber type, and sensor placement all shift the absolute value.
What the Meta-Analysis Numbers Really Mean for SmO2
A 2023 systematic review pooled 15 studies and 344 athletes (Sendra-Pérez et al.). The headline numbers:
- Second threshold (LT2/VT2): pooled ICC = 0.80 — good reliability.
- First threshold (LT1/VT1): pooled ICC = 0.53 — moderate at best.
ICC of 0.80 sounds strong. Here’s where it gets honest.
That meta-analysis carries 86% heterogeneity — meaning the results vary enormously depending on sport, device, and method. An ICC from cycling studies alone runs 0.91–0.97. For running, it falls as low as 0.23. Average the two and you get 0.80. That average hides the sport-specific split.
The individual limits of agreement matter more for training prescription. In NCAA Division I female rowers (Eserhaut et al., 2025), SmO2 breakpoint 2 vs. lactate threshold 2 showed a group bias of just −5.76 W — which looks fine. But individual limits of agreement spanned −38.52 to +22.25 W. That’s a total spread of about 61 W.
For zone prescription, 61 W is the difference between a Zone 3 tempo and a VO2max interval.
This doesn’t make NIRS useless. It means you can’t skip the one blood lactate co-validation.
The Sport-Specific Split: Cycling vs Running
NIRS works in cycling because your legs stay in roughly the same position throughout the effort. The sensor sits on your vastus lateralis, motion artifact is low, and the signal is clean.
Running is different. Every footstrike moves the sensor. Sweat loosens the strap. The gastrocnemius fires explosively instead of smoothly. These factors drive SmO2 variability at high intensity to a coefficient of variation of 43.8% — versus just 11% at low intensity (Feldmann et al., 2023). At exactly the moment you need a clean signal (near threshold), the noise is worst.
NIRS is a cycling tool that also works in rowing and skiing. For running, treat the data as directional, not prescriptive.
The Athlete Who Sees Nothing
Here’s the finding almost no review article covers: 10 of 21 subjects in a treadmill study (Baiget et al., 2023) showed no identifiable SmO2 breakpoint at all. Not at VT1. Not at VT2. The curve just drifted downward with no inflection.
Zero.
That’s 48% of participants with completely uninterpretable data.
In a separate cycling study, 3 of 10 athletes also showed no clear second-threshold inflection. The technology is blind for a meaningful minority of athletes.
Why? Two main causes. First, adipose tissue thickness. The layer of subcutaneous fat between the sensor and the working muscle absorbs the NIRS signal before it reaches muscle. Fat doesn’t desaturate during exercise the way muscle does. A thicker layer means the sensor reads a mixture of fat and muscle — blurring the breakpoint. Adipose tissue explains about 80% of SmO2 variance at peak exercise (Feldmann et al., 2016). If you have more than 10–12 mm of fat at the sensor site, your readings are significantly compromised.
Second, individual physiology. Some athletes’ respiratory and cardiac systems become the limiting factor at threshold before the quadriceps desaturate sharply. The breakpoint exists physiologically but doesn’t show up at the vastus lateralis.
A multi-muscle approach helps. World-class Nordic skiers (N=52) achieved 96.1% breakpoint detection rates when sensors were placed on both biceps femoris and biceps brachii simultaneously (Forot et al., 2026).
Moxy vs Train.Red: What’s Actually Different
The two popular consumer NIRS devices aren’t reporting the same thing.
| Feature | Moxy Monitor | Train.Red FYER 2.0 | Train.Red PLUS |
|---|---|---|---|
| Price (approx.) | $650–$850 | ~€699 | ~€1,799 |
| Wavelengths | 4 (680/720/760/800 nm) | 1 (850 nm) | 1 (850 nm) |
| Output type | Absolute SmO2 (0–100%) | TSI (relative index) | TSI (relative index) |
| Reporting rate | Standard | 10 Hz | 100 Hz (6 spatial channels) |
| Peer-reviewed studies | Many (most sport science NIRS studies) | Few | Few |
| Key strength | Validated absolute scale; most studied | Deeper hardware pedigree (Artinis) | High-resolution spatial data |
The unit difference matters. Moxy reports absolute SmO2 on a validated 0–100% scale. Train.Red reports a Tissue Saturation Index derived from a single 850 nm wavelength. They trend together but the absolute numbers don’t match. You can’t apply Moxy’s 26% VT2 cut-off to Train.Red data. Each device needs its own calibration.
How to Run the Step Test
The 5-1 cycling step test is the validated protocol for NIRS threshold detection.
Setup:
- Place the sensor on the vastus lateralis, 12 cm above the kneecap, under a dark elastic strap.
- Start at 1.0 W/kg.
- Increase by 0.5 W/kg every 5 minutes.
- Rest for 1 minute between stages.
The rest periods allow partial resaturation. This “refresh” between stages makes the SmO2 curve step-shaped rather than one smooth descent — and clean steps make breakpoints easier to identify visually.
Analyze SmO2 averaged over the final 60–90 seconds of each stage. Apply double linear regression or look for the inflection visually.
Avoid ramp tests for NIRS threshold detection. SmO2 lags behind power output. On a 20 W/min ramp, the curve never has time to stabilize, and the breakpoint shifts earlier than the true threshold.
A Mini Case Study: What the Data Looks Like in Practice
James is 41, a Category 4 cyclist with five years of structured training. He did a blood lactate step test in March: LT1 at 215 W, LT2 at 285 W. He bought a Moxy and repeated the protocol in April.
His SmO2 curve showed a clear inflection around 230 W (step 4) and a sharp drop through step 5, bottoming near 26% at 280 W. The NIRS-detected second threshold was 280 W versus 285 W from blood lactate — 5 W off.
Eight weeks of base work later, he ran the same step test. Now the sharp drop came at 310 W. SmO2 at the same old threshold power of 285 W had risen to 33%, meaning he was using less oxygen at that intensity. His muscle delivery capacity had improved.
That’s where NIRS earns its keep: not the single snapshot, but the trend over weeks. The watch shows the number. The trend shows the adaptation.
Where SmO2 Genuinely Replaces a Lactate Meter
Use NIRS without co-validation when:
- You’re monitoring Zone 2 compliance in real time. If your SmO2 stays above 34% during a long ride, you’re below VT1. If it dips toward 26%, you’ve drifted into threshold territory.
- You’re tracking training adaptation over weeks. Progressive desaturation at the same power is direct evidence of improved muscular oxygen extraction.
- You’re in a field environment where blood drawing is impossible — open water, mountain terrain.
Don’t rely on NIRS alone when:
- You’re setting zones for the first time. Do one blood lactate test to confirm breakpoints are detectable and positioned where you expect.
- You’re running. Motion artifact makes threshold prescription from SmO2 unreliable.
- You have significant adipose tissue at the sensor site (more than ~10–12 mm subcutaneous).
- You’re trying to detect LT1 precisely. Pooled ICC of 0.53 isn’t good enough for zone prescription.
How AthleteOS Uses Your SmO2 Data
AthleteOS ingests live SmO2 streams from Moxy and Train.Red sensors via its ANT+/BLE pipeline. During session analysis, it overlays SmO2 on concurrent power and heart rate data from the same workout. A two-breakpoint algorithm runs on step-test data to detect SmO2BP1 (aligned with LT1) and SmO2BP2 (aligned with LT2) — no finger prick needed.
Detected thresholds feed directly into your zone model and update upcoming plan workouts. If you’ve validated your NIRS thresholds with blood lactate and the numbers diverge, AthleteOS flags the discrepancy and prompts a manual override.
That’s the model the research supports: NIRS as a high-frequency monitoring layer, with periodic blood lactate as the ground truth. They’re complementary. Neither replaces the other entirely.
For more on threshold methods, read FTP testing protocols and which one matches your event and how Zone 2 builds the aerobic base. If you’re a triathlete, power meter selection for multisport athletes covers the hardware side of threshold tracking. Start your free AthleteOS account to connect your Moxy or Train.Red and see your SmO2 overlaid on power in your next session analysis.