Not All NIRS Devices are Created Equal - Here’s What You Need to Know!
When it comes to optimizing endurance training, precision matters. Many athletes and coaches now use near-infrared spectroscopy (NIRS) devices, like the Moxy Monitor, to monitor oxygen levels during exercise, but not all measurements are created equal. Understanding the difference between muscle oxygen saturation (SmO2) and tissue oxygen saturation (StO2) or tissue saturation index (TSI) can significantly impact how you interpret and apply this data to your training.
Muscle vs. Tissue Oxygenation - Why Different NIRS Devices Get Different Numbers
The most fundamental difference lies in what's actually being measured. SmO2 specifically targets the skeletal muscle layer, providing data on oxygen saturation within the working muscles themselves. In contrast, StO2/TSI measures oxygen across multiple tissue layers, including skin, fat, and muscle. This distinction is critical because different tissues respond differently to exercise and environmental factors.
For example, during intense exercise, your skin blood flow increases to help dissipate heat. A device measuring StO2/TSI might detect this change in skin perfusion, potentially masking what's happening in the muscle itself. Moxy Monitor is specifically designed to isolate the muscle layer, filtering out these confounding signals from other tissues.
The Full Physiological Range: 0-100% vs. Narrower Scales
Another key difference is the measurement scale. True muscle oxygen saturation exists on a physiological scale from 0% (completely desaturated) to 100% (fully saturated). During research-validated protocols like arterial occlusions, we can observe that Moxy provides readings consistent with this full physiological range – dropping to around 5-10% during maximum extraction and rising to 80-90% during hyperemic responses.
Many other NIRS devices operate on much narrower scales, often showing minimum values of 40-50% during identical testing protocols. This compressed range isn't simply a difference in calibration – it suggests these devices are measuring a mixed tissue signal where the contribution from non-muscle tissues prevents readings from reaching physiologically valid minimum values.
Why You Should Measure SmO2 NOT StO2/TSI
These differences have real implications for how you use NIRS data to guide your training. When measuring true SmO2 with Moxy, you can:
- Establish accurate physiological thresholds based on oxygenation responses.
- Compare workouts over time with consistent, physiologically valid measurements.
- Make training decisions based on absolute values of SmO2, for example with Moxy you can determine if someone has a muscular limitation if SmO2 doesn’t drop below 60% during all-out exercise, with ther devices their numbers may never drop by 60%, this isn’t a physiological limitation in the person, it’s a device limitation!
- Make training adjustments based on muscle-specific responses, not confounded by skin blood flow changes during hot or cold conditions.
- Identify true muscle oxygen recovery patterns without interference from other tissues.
If you're using a device that measures StO2/TSI rather than SmO2, you may need to focus more on relative changes rather than absolute values, and be cautious about comparing readings across different environmental conditions. And definitely do not compare numbers to different devices!.
How to Spot a StO2 Device Masquerading as SmO2
Even when devices report their measurements as percentages this doesn’t mean that the device is actually measure SmO2. Here are telltale signs that can help you identify whether you're getting true SmO2 or just StO2/TSI:
- Perform an occlusion test: Apply a blood pressure cuff above the muscle being monitored and inflate your cuff above systolic pressure (~280mmHg) for 3-5 minutes. A true SmO2 device will show values dropping to around 5-15% during this test. If your device bottoms out at 40-50%, it's likely measuring StO2 rather than isolated muscle oxygen.
- Look at the range during intense exercise: During maximal effort intervals, true SmO2 measurements can drop into the teens or single digits in well-trained athletes. If your device rarely shows readings below 40% even during exhaustive exercise, it's could be measuring a mixed tissue signal.
- Check the response to environmental changes: If your readings change dramatically when moving from cool to hot environments without a corresponding change in exercise intensity, your device may be picking up skin blood flow changes—a clear sign of StO2/TSI measurement.
These simple tests can help you determine what your device is actually measuring, allowing you to interpret the data appropriately and make informed training decisions.
The Bottom Line - What Makes Moxy Different
Not all oxygen measurements are the same. When choosing a NIRS device for training guidance, understanding what you're actually measuring is crucial. Moxy Monitor's focus on isolating the skeletal muscle layer and providing readings on a true physiological scale allows for more accurate, consistent, and actionable insights into your muscle's oxygen utilization – the key factor that ultimately determines endurance performance interested in learning how to use SmO2 to optimize your training?
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