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{{DISPLAYTITLE: Muscle Oxygen Saturation (SmO2)}}
A new type of [[running sensor| Running Sensors]] has come onto the market in the last few years that will determine the oxygen saturation of blood flowing through the muscles, called [[SmO<sub>2</sub>]]. You may be familiar with [[Pulse Oximeter]]s that measure the oxygen saturation of blood flowing through your extremities, called [[SpO2|SpO<sub>2</sub>]]. In healthy individuals at low altitude, finger [[SpO2|SpO<sub>2</sub>]] should remain high, typically above 95%, and this doesn't typically vary with exercise. Also, pulse oximeters are very sensitive to movement, and can't really be used during exercise. Muscle oxygen sensors use similar principles of light absorption, but measure the blood flow deeper in the tissues, and are relatively unaffected by movement. [[SmO<sub>2</sub>]] varies substantially with exercise, though this pattern is not a simple relationship between saturation and exercise intensity. Instead, [[SmO<sub>2</sub>]] is often lower at rest than during moderate exercise, falling again as intensity increases. Using a muscle oxygen sensor to evaluate exercise intensity is tricky at best, though it can provide some fascinating insight into how the body is responding. I don't think these sensors are of much value to the average runner, and I'd urge caution before buying one to ensure that you fully understand how they work and their limitations. The current SmO<sub>2</sub> sensor is [[Moxy]], as [[BSX]] is discontinued, and Humon has not been released yet.
=Other Metrics=
Muscle oxygen sensors can also estimate blood flow (total hemoglobin, tHb), which also changes with exercise intensity. This is the amount of hemoglobin is measured in grams per deciliter of tissue. Given tHb and [[SmO<sub>2</sub>]] it's possible to calculate the relative volume of oxygenated hemoglobin (O<sub>2</sub>Hb) which is [[SmO<sub>2</sub>]]% * tHb. Then there's deoxygenated hemoglobin (HHb), which is tHb - O<sub>2</sub>Hb. Golden Cheetah does these calculations automatically. It's worth remembering that tHb and the derived metrics are influenced by the fat layer between the sensor and the muscle which are in the light path, so more fat will produce a lower value. This means that the tHb values should be considered relative, and positioning the sensor in a slightly different place can result in a different range of values. If you look at the chart below, at the top you'll see the [[Stryd]] power estimate in yellow jumps up as I go from walking to running at the start of my warmup. Next is Heart Rate in red, climbing as expected. SmO<sub>2</sub> in gray-green drops, then fluctuates slightly, showing the increased oxygen demand from the muscles. Surprisingly (to me at least), the brown tHb rises initially, then drops off. The calculated values show there's a lot less O<sub>2</sub>Hb and a lot more HHb. That's what you'd expect, as the muscles are pulling more O<sub>2</sub> from the blood.
