What is muscle near-infrared spectroscopy (mNIRS)?
Muscle near-infrared spectroscopy (mNIRS) is a non-invasive technique that can be used to measure the changes in blood volume and oxygenation responses within the smaller diameter blood vessels in skeletal muscle. This is possible because near infrared light (~700-900 nm) emitted into biological tissues can help determine the oxygenation status of heme-containing compounds such as hemoglobin and myoglobin. Due to the different near infrared light absorption characteristics of hemoglobin and myoglobin, mNIRS has the ability to differentiate between oxygenated heme (oxy(heme)) and deoxygenated heme (deoxy(heme)) within the smaller diameter blood vessels. This is important as gas exchange (e.g., oxygen utilization) occurs in the capillaries, so mNIRS can provide a non-invasive assessment of local oxygen demand within a skeletal muscle during a variety of states (e.g., rest, exercise, recovery).
Two common mNIRS variables that are used by practitioners and discussed in the research are the total heme concentration (total(heme)) and tissue oxygenation saturation (StO2). Total(heme) can be calculated as the sum of oxy(heme) and deoxy(heme) (total(heme) = oxy(heme) + deoxy(heme)). The total(heme) response is often used as an indicator of changes in blood volume within the small diameter blood vessels in skeletal muscle. StO2 can be calculated as the amount of oxy(heme) in relation to the amount of total(heme) (StO2 = {oxy(heme) / total(heme)} x 100%). The StO2 response indicates the oxygenation levels within the small diameter blood vessels and is thought of as a balance between oxygen delivery and oxygen utilization. The total(heme) and StO2 responses during exercise are commonly expressed as a change from resting baseline measurements, especially when using continuous wave mNIRS systems.
How can muscle near-infrared spectroscopy (mNIRS) be used?
Exercise Threshold Detection Assessments
mNIRS has been used to non-invasively detect certain aerobic thresholds and these mNIRS-derived thresholds have been compared to other ventilatory and lactate threshold measurement techniques. A recent systematic review and meta-analysis showed that mNIRS-derived thresholds had higher reliability when detecting the second ventilatory/lactate threshold compared to the first ventilatory/lactate threshold. These results might allow practitioners to use mNIRS in assessing the second ventilatory/lactate threshold with more sport- or activity-specific testing methodologies and be able to monitor potential threshold changes in response to a training program.
Occlusion Tests
There are two types of occlusion tests that have been used with mNIRS; the vascular occlusion test (VOT) to non-invasively assess microvascular function and an intermittent occlusion protocol to non-invasively assess mitochondrial function. A recent investigation used mNIRS during the VOT and observed that the microvascular function measurement had good between-session reliability and that microvascular function was maintained subsequent to an acute bout of blood flow restriction walking. Additionally, mNIRS has been used during an intermittent occlusion protocol to assess the oxy(heme) and deoxy(heme) recovery time constants after muscle contraction which represents mitochondrial function. This mNIRS intermittent occlusion protocol has been shown to produce moderate reliability in assessing the oxy(heme) and deoxy(heme) recovery time constants (e.g., mitochondrial function). With all results taken together, practitioners could use the mNIRS to non-invasively evaluate microvascular function or mitochondrial function in different populations (e.g., clinical populations versus healthy populations) or to test for potential changes in response to a training program.
Conclusion
mNIRS provides practitioners with a non-invasive and reliable measurement technique to evaluate various physiological attributes (thresholds, microvascular function, mitochondrial function). The portability and non-invasive qualities of mNIRS technology make these assessments more convenient for practitioners to perform and could allow more sport- or activity-specific measurements to be collected.
Trent E. Cayot, PhD, CSCS, EPis an associate professor and the Human Performance Laboratory director within the Department of Kinesiology Health and Sport Sciences at the University of Indianapolis. He is currently a member of ACSM’s Communications and Public Information (CPI) committee. He previously served as the President of the Midwest Chapter of ACSM. Dr. Cayot has used near-infrared spectroscopy techniques to investigate the impacts of exercise on skeletal muscle microvascular oxygenation responses in athletic, general population, and clinical populations.
