This study found reduced oxygen extraction in the thigh muscles of people with ongoing symptoms after Covid-19 infections as compared to healthy controls:
Structural and functional impairments of skeletal muscle in patients with [PASC], 2023, Colosio et al.
The measure is "Δ[HHbMb]peak (% ischemia)" and it is essentially a measure of the level of oxygen in the blood relative to that that occurs when the blood supply to the muscle is stopped for a few minutes. When the blood supply is stopped (the ischemia), the oxygen in the blood in the muscle eventually comes to an equilibrium, where no more oxygen is diffusing out of the blood. That's the baseline, the 100%.
The technique used is Near Infra-Red Spectroscopy. It involves a sensor being placed in a specified part of the muscle, taped securely to ensure there is no movement, and with an opaque barrier placed over it to prevent ambient light affecting the results. The thickness of epidermal fat affects the results, so that needs to be controlled for. I don't know, but perhaps the colour of skin, the density of melanin, might also have a confounding effect?
The specific thigh muscle that seems to be used is the Vastus lateralis.
Here's a chart of how Δ[HHbMb]peak changes over the course of exercise tests that continue on to VO2max (source: Oxygenation Threshold Derived from Near-Infrared Spectroscopy: Reliability and Its Relationship with the First Ventilatory Threshold, 2016)

This was for male cyclists. There's a plateauing at around the 60% mark. If I'm understanding that correctly, that plateauing begins at around ventilatory threshold, with some minor decline through to the point of VO2max when the exercise test is stopped.
The Colosio paper looking at people with Long Covid linked above reported that the healthy controls had a mean value of 61% (SD 13) - that is in line with the chart above - see the plateau on the y axis. However, the people with Long Covid had a mean value of only 45% (SD 9). They didn't get close to extracting more oxygen from the blood during a time of high demand in the way that the healthy controls did. The study was small, but the standard deviations are also relatively small and the separation of the results from each group is substantial. It suggests that the tissue is not receiving enough oxygen for optimal performance.
The 2016 paper noted that the oxygen extraction values as determined by NIRS were reproducible, so, if the measures are done carefully and the confounding of epidermal fat is taken into account, it sounds as though this is a measure that we can have confidence in.
I'd like to know a bit more about what this might means for the physiology. The baseline (under a situation of ischemia for a few minutes) is determined for each person, and the reported peak measure during exercise is relative to that. I wonder, does the baseline vary much from person to person? Could repeated exposure to hypoxic conditions alter that baseline? So, is it the baseline that changes or the actual oxygen extraction during exercise?
I'd also like to know - has oxygen extraction during exercise been measured in people with ME/CFS or related conditions?
Structural and functional impairments of skeletal muscle in patients with [PASC], 2023, Colosio et al.
The measure is "Δ[HHbMb]peak (% ischemia)" and it is essentially a measure of the level of oxygen in the blood relative to that that occurs when the blood supply to the muscle is stopped for a few minutes. When the blood supply is stopped (the ischemia), the oxygen in the blood in the muscle eventually comes to an equilibrium, where no more oxygen is diffusing out of the blood. That's the baseline, the 100%.
The technique used is Near Infra-Red Spectroscopy. It involves a sensor being placed in a specified part of the muscle, taped securely to ensure there is no movement, and with an opaque barrier placed over it to prevent ambient light affecting the results. The thickness of epidermal fat affects the results, so that needs to be controlled for. I don't know, but perhaps the colour of skin, the density of melanin, might also have a confounding effect?
The specific thigh muscle that seems to be used is the Vastus lateralis.
Here's a chart of how Δ[HHbMb]peak changes over the course of exercise tests that continue on to VO2max (source: Oxygenation Threshold Derived from Near-Infrared Spectroscopy: Reliability and Its Relationship with the First Ventilatory Threshold, 2016)

This was for male cyclists. There's a plateauing at around the 60% mark. If I'm understanding that correctly, that plateauing begins at around ventilatory threshold, with some minor decline through to the point of VO2max when the exercise test is stopped.
The Colosio paper looking at people with Long Covid linked above reported that the healthy controls had a mean value of 61% (SD 13) - that is in line with the chart above - see the plateau on the y axis. However, the people with Long Covid had a mean value of only 45% (SD 9). They didn't get close to extracting more oxygen from the blood during a time of high demand in the way that the healthy controls did. The study was small, but the standard deviations are also relatively small and the separation of the results from each group is substantial. It suggests that the tissue is not receiving enough oxygen for optimal performance.
The 2016 paper noted that the oxygen extraction values as determined by NIRS were reproducible, so, if the measures are done carefully and the confounding of epidermal fat is taken into account, it sounds as though this is a measure that we can have confidence in.
I'd like to know a bit more about what this might means for the physiology. The baseline (under a situation of ischemia for a few minutes) is determined for each person, and the reported peak measure during exercise is relative to that. I wonder, does the baseline vary much from person to person? Could repeated exposure to hypoxic conditions alter that baseline? So, is it the baseline that changes or the actual oxygen extraction during exercise?
I'd also like to know - has oxygen extraction during exercise been measured in people with ME/CFS or related conditions?