Distinguishing Muscle Oxygen Saturation from Tissue Oxygen Saturation: Accuracy, Limitations, and Practical Utility in Muscle Oximetry.

Muscle near-infrared spectroscopy (mNIRS) is widely used to measure skeletal muscle oxygen saturation, named as muscle-specific oxygen saturation (SmO₂) or tissue oxygen saturation (StO₂). The distinction between SmO₂ and the less specific abbreviation StO₂ is often unclear and frequently conflated. This ambiguity persists in part because these commonly used terms do not explicitly reflect the layered, heterogeneous nature of the tissue beneath the probe, nor do they account for the tissue-optics assumptions required to derive absolute values. Two key factors differentiate SmO₂ from StO₂: the anatomical source of the optical signal and whether the value represents a true oxygen saturation or a derived index. To improve clarity, we propose the term regional SmO₂ (rSmO₂) to emphasize that mNIRS captures a spatially localized, muscle-weighted estimate influenced by instrumental factors, probe geometry, superficial tissues attenuation, and photon migration model, rather than whole-muscle oxygenation. A practical approach for determining whether a device captures a primarily muscle-specific rSmO₂ signal or a more mixed-tissue StO₂ signal is the arterial occlusion method. Muscle-dominant signals display a broad dynamic range, with nadirs of 20% and hyperemic peaks near 80%, whereas signals more strongly influenced by skin pigmentation and adipose tissue thickness show markedly attenuated responses. Although no commercial system can completely isolate muscle tissue, mNIRS remains a valuable tool for evaluating regional muscle oxygenation. Validated instrumentations and detailed protocols should be utilized, always recognizing the potential attenuation from superficial tissues.