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.
The majority of bodily oxygen consumption occurs in the mitochondria of the skeletal muscle. As oxygen is the last electron acceptor in the electron transport system, oxygen levels within the cell create the electron ‘pull’ necessary to establish a large proton motive force resulting in the synthesis of ATP and are therefore integral to sustained muscle contraction during exercise. In fact, the ability of skeletal muscle mitochondria to uptake and utilize oxygen has been shown to be the single best predictor of endurance performance in a group of elite level cyclists. However, measuring mitochondrial function is an invasive, technical procedure, requiring participants to get a muscle biopsy and researchers to have a high level of technical acumen to get valid measurements.
NIRS devices measure the balance of oxygen delivery and utilization within the skeletal muscle. And skeletal muscle mitochondria are the primary consumer of oxygen, so it’s no surprise that researchers have developed and validated non-invasive procedures to measure mitochondrial oxidative capacity simply by using NIRS. Measuring dynamic, real-time, oxygenation levels within the capillary beds of the muscle is very useful for elucidating a participants’ ability to deliver and utilize oxygen. Apart from the evaluation of skeletal muscle oxidative capacity, NIRS has also been used to evaluate changes in forearm blood flow, phosphocreatine recovery kinetics, and changes in microvascular reactivity in health and disease, amongst others.
Limitations. Using NIRS in research is a cost effective way of evaluating changes in blood volume, and oxygenation in real-time, however, this technology does come with some limitations:
- Most continuous wave NIRS devices are only able to measure relative changes for the intensity of light making it back to the device, which means that an ischemic calibration step is required to get values that can be compared between participants but these values are only reported as percentages.
- NIRS devices can only penetrate tissue up to ½ the distance between the emitter and detectors and most NIRS devices assume that the light from the emitter is travelling through homogenous tissue. Different tissues (i.e. skin, adipose, muscle) have different properties, which means that light travels differently through each of the tissues. This will affect the signal that is being detected. In order to mitigate the chances of adipose tissue (low oxygen changes) affecting the signal, measuring adipose tissue thickness (ATT) via skin fold caliper or, more preferably, ultrasound is highly recommended. Then, if the NIRS device can, space the emitter and detector to AT LEAST double the (ATT) so that the light is penetrating into muscle.
- Skin-blood flow can during exercise in a hot environment can affect the oxygenated hemoglobin (O2HB) signal. It is highly recommended to use the deoxygenated hemo/myo-globin (HHB) signal as it is more robust.
These limitations can be controlled for with properly planned research protocols, for a more in-depth look at proper practices for NIRS implementation see (Barstow 2019). In the next blog post, I will walk through the protocol for skeletal muscle oxidative capacity from the papers above.