Near-infrared spectroscopy (NIRS) devices have seen growing popularity in research and sporting application over the last decade because of their ability to non-invasively determine muscle oxygen saturation changes during real-time activities. These devices have the potential to change the way exercise is prescribed. However, most NIRS devices are too expensive for consumer use and/or require large power sources and cords, relegating athletes and coaches to only using these devices in a laboratory setting. NIRS devices use a few different methods to determine changes in muscle oxygenation, which I won’t go into detail in this post, but the least cost prohibitive is a method called continuous-wave NIRS. This involves emitting 2 to 4 different wavelengths of light into the tissue of interest and measuring changes in the intensity of light to determine how tissue oxygenation is changing. One major drawback of using most CW-NIRS devices is that they use 2 wavelengths of light while assuming that the tissue the light is passing through remains constant which limits these devices to ONLY reporting changes in muscle oxygenation. Indeed, these devices can estimate percent changes in oxygenation, but only after a calibration step is completed and applied to the data after tests are finished.
Moxy Monitor is a wireless, CW-NIRS device that uses four wavelengths of light and assumes that light passes through multiple layers of tissue, to estimate, in real-time, the percent muscle oxygenation from 0 to 100%. A paper that was recently published in the Journal of Biomedical Optics entitled “Near-infrared spectroscopy-derived muscle oxygen saturation on a 0% to 100% scale: reliability and validity of the Moxy Monitor” tested the validity and reliability of the Moxy Monitor to measure oxygen saturation from a 0 – 100% scale. A summary of these results is given below.
In the paper linked above, the authors wanted to test three different things. 1) Repeatability: the closeness of repeated measurements made on the same person under the same conditions. 2) Reproducibility: the closeness of agreement between measures made under changing conditions. 3) Face validity: the reasonable expectation of measurements taken based on certain criteria. Are SmO2 percentages measured by the Moxy close to what would be physiologically expected. The researchers tested the three above objectives in 22 participants (11 males, 11 females). Each participant was fitted with four different Moxy monitors on different portions of their legs. One was placed on the vastus lateralis, rectus femoris, vastus medialis, and the gastrocnemius. Next the researchers did a series of arterial occlusions (AOMs). AOMs involve placing a pneumatic cuff high on the thigh, above the sensors, then inflating it to suprasystolic blood pressure (300 mmHg). The cuff was inflated for 5-6 minutes, until an SmO2 minimum was reached, then the pressure was released and SmO2 was monitored for another 3 minutes. When the cuff is released, an influx of oxygenated blood reaches the region being monitored cause a hyperemic response in SmO2. Two types of AOMs were completed, the first is a passive trial where participants rested quietly on a bed, the second was an active trial where participants completed leg raises until fatigue. Each cuffing procedure was repeated in two different session, 1 week a part.
For passive trials, there were no differences in SmO2 min or max when comparing between the same site during trails 1 and 2. When comparing between sites only the gastric and vastus lateralis can be considered statistically equivalent. When active trials were compared to passive trials there was no difference detected in SmO2 min or max in the muscle of the thigh. Furthermore, when the max and min values were compared to the selected thresholds of expected max and min values based on previous literature no, the SmO2 values were within the predicted limits. Overall, this study illustrates that Moxy is a valid and reliable device for measuring skeletal muscle oxygenation levels within the parameters tested in the paper. While there are still many limitations to NIRS device usage, adipose tissue thickness being a major one, this study shows the potential utility of retail NIRS devices, specifically, the usability of Moxy monitor and the vast potential these devices have for research and athletics.