A recent article (1) has demonstrated that maximal lactate steady state can be accurately determined in healthy subjects in response to an incremental cycling test up to exhaustion based on measures by near-infrared spectroscopy (NIRS) of vastus lateralis deoxygenated hemoglobin concentration (HHb) (considered as an index of O2 extraction). Although the proposed method is very interesting in sports science, the presentation of the NIRS technique and the NIRS muscle oxygenation data are incomplete and raise questions about some of the conclusions drawn by the authors.
For measuring muscle oxygenation, Bellotti et al. (1) used a multidistance frequency-domain oximeter (rather than a continuous-wave instrument as stated by the authors), commercially introduced in 1998, and equipped with a rigid muscle probe in which four source-detector distances ranging from 2 to 3.5 cm implement a multidistance data collection (2). Given typical values of the near-infrared optical properties in muscle tissue (absorption coefficient approximately 0.08 cm−1, reduced scattering coefficient approximately 6 cm−1), the mean and maximal penetration depths can be estimated at about 1 and 2–2.5 cm, respectively (5). Accordingly, the continuous-wave technique adopted in the most recent portable/wearable/wireless devices provides a typical depth sensitivity of approximately 1.5 cm (3) and not 3 cm as stated by Bellotti et al. (1). Although NIRS undoubtedly offers several advantages for muscle studies (4), some questions (other than its applicability to subjects with a thick fat layer) are still open: for instance, 1) the contribution of myoglobin desaturation to the NIRS signal during exercise; 2) the effect of scattering changes during exercise; 3) the effect of changes in skin perfusion, particularly during prolonged exercise; and 4) the lack of standardization for NIRS instrumentation (3).
Bellotti et al. (1) as well as other researchers in the past have evaluated the kinetics of muscle oxygenation using continuous-wave or frequency-domain oximeters and have drawn the physiological conclusions of their studies on the basis of the interpretation of the changes in HHb only. HHb changes (reported erroneously by the authors in their Fig. 1 as millimolar instead of micromolar) might represent tissue oxygenation changes only when tHb (tHb = O2Hb + HHb) is stable (3). Unfortunately, Bellotti et al. (1) did not report or comment on changes in tHb that are necessary for a correct interpretation of muscle oxygenation changes. NIRS oximeters offer the great advantage to also provide measurements of muscle hemoglobin saturation (SmO2), expressed in percent (%). This parameter reflects the dynamic balance between O2 supply in the muscle microcirculation and O2 consumption demand by the muscle. In addition, SmO2 measurements are unaffected by a systematic bias in absorption measurements and, compared with HHb measurements, are not as sensitive to the optical coupling to tissue and to the superficial skin/fat layers. As recently suggested (6), the combined measurement of changes in HHb, O2Hb, and SmO2 allows for novel comparisons of these variables during exercise. Considering the significant physiological information content of NIRS measurements, we recommend that NIRS studies report HHb, O2Hb, and SmO2 data for a comprehensive characterization and thorough discussion of the results.
Department of Life, Health, and Environmental Sciences
University of L’Aquila
Department of Biomedical Engineering
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