Muscle Oxygenation Is a Reflection Of Exercise Intensity, Not A Measure of It.
SmO2 reflects changes in exercise intensity, but is not itself a measure of exercise intensity. That difference matters. Learn why.
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People often think of muscle oxygenation (SmO2) as a measure of exercise intensity. While SmO2 is strongly correlated with exercise intensity, they are not the same.
A better way to think about muscle oxygenation is that it measures our body's response to exercise intensity. This delineation is critical and is why devices like NNOXX can be used to identify physiological limitations (see footnote)1. To understand why, we must appreciate that muscle oxygenation reflects the balance of oxygen supply and demand in exercising muscles.
Under most circumstances, increasing exercise intensity disproportionately increases the muscle's demand for oxygen relative to supply, and thus, muscle oxygenation decreases. However, this isn't always the case — there are times when we increase exercise intensity and muscle oxygenation remains stable.
Understanding that muscle oxygenation represents our body's response to exercise intensity, not intensity itself, opens up a new way of interpreting our SmO2 measurements. By comparing the changes in SmO2 during different exercises, we can gain insights into how our body copes with each activity's demands.
In this workout, we have muscle oxygenation and acceleration data recorded with the NNOXX wearable by an elite Crossfit athlete performing three rounds for time of 300m Run, 20 cal Echo Bike, and 250m row2. Interestingly, even though this athlete is exerting themselves equally across the three different movements in this workout, we can see that their body responds differently to each modality.
Notably, this athlete is deoxygenating their muscles to the greatest degree while running and has a continual decline in SmO2, indicating that their muscles are utilizing oxygen faster than it can be supplied. However, we see more stable SmO2 levels while echo biking and rowing, telling us that the body can better cope with the demands imposed by these activities. Interestingly, we've seen this same pattern across multiple workouts, indicating that running may be a weakness for this athlete that should be prioritized leading into the CrossFit Games, where it is heavily tested.
Do you have any questions about NNOXX’s technology, measurements, or use cases? If you, ask me directly on the NNOXX Connect Forum.
Earlier, I mentioned that SmO2 measures our body's response to exercise intensity and that this is the basis for how technologies like NNOXX can be used to identify physiological limitations. If SmO2 were to be perfectly [inverse] linearly correlated with intensity (i.e., power) under all circumstances, then anytime an athlete increases their intensity, we should see a proportional decrease in SmO2. However, this is not the case. There are athletes for whom SmO2 does not change, or changes minimally, when transitioning from a sustainable to an unsustainable exercise intensity, which is a sign that their performance may be limited by their muscle's ability to utilize oxygen. Thus, combining muscle oxygenation data with external load data can give us unique insights into an athlete's physiological limiters and performance.
To learn more about the functionalities of the NNOXX wearable, check out my previous article titled, A Lab In Your Pocket - The Future Of High Performance.