An article published in the Journal of Applied Physiology on April 29th shed light on some critical applications for the use of near-infrared spectroscopy (NIRS) in endurance sport – The Balance of Muscle Oxygen Supply and Demand Reveals Critical Metabolic Rate and Predicts Time to Exhaustion
VO2max, thresholds, and efficiency are thought to be and certainly do have considerable predictive power for endurance performance. They are great global variables to measure and monitor how an athlete is coping with the stress of long endurance athletics. However, most metabolic devices are bulky, require uncomfortable mouth pieces, backpacks, or other restrictive equipment which can interfere with an athlete’s performance. They also fail to directly measure the stress of arguably the most important organ during exercise, the skeletal muscle. Recently, the ability for skeletal muscle to utilize oxygen measured by near-infrared spectroscopy (NIRS) has been shown to the best predictor of endurance performance in trained cyclists. Therefore in order to gain the best picture of an athlete’s physiology, these global measures should be accompanied by devices that can measure local stress.
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.
NIRS in Research. Near-infrared spectroscopy (NIRS) is a relatively new technology that has shown utility for many different non-invasive protocols for physiologic experimentation. Essentially, NIRS technology monitors changes in capillary oxygenation values by measuring the intensity of light (600-1000nm) after it passes through biological tissue, (i.e. skeletal muscle). The amount of light that is absorbed by a tissue depends mainly on the amount of oxygen that is bound to the chromophores, hemoglobin and myoglobin, underneath the sensor. Therefore, by measuring dynamic changes in the amount of light that passes through a tissue, researchers are able to get an idea of oxygen consumption within the tissue underneath the sensor.