| Summary: | We lack reliable methods to continuously assess localized, resting-state muscle activity that are comparable across individuals. Near-infrared spectroscopy (NIRS) provides a low-cost, non-invasive means to assess localized, resting-state muscle oxygen metabolism during venous or arterial occlusions (VO<sub>2VO</sub> and VO<sub>2AO</sub>, respectively). However, this technique is not suitable for continuous monitoring, and its utility is limited to those who can tolerate occlusions. Combining NIRS with diffuse correlated spectroscopy (DCS) enables continuous measurement of an index of muscle oxygen metabolism (VO<sub>2i</sub>). Despite the lack of previous validation, VO<sub>2i</sub> is employed as a measure of oxygen metabolism in the muscle. Here we characterized measurement repeatability and compared VO<sub>2i</sub> with VO<sub>2VO</sub> and VO<sub>2AO</sub> in the medial gastrocnemius (MG) in 9 healthy adults. Intra-participant repeatability of VO<sub>2i</sub>, VO<sub>2VO</sub>, and VO<sub>2AO</sub> were excellent. VO<sub>2i</sub> was not significantly correlated with VO<sub>2AO</sub> (<i>p</i> = 0.15) nor VO<sub>2VO</sub> (<i>p</i> = 0.55). This lack of correlation suggests that the variability in the calibration coefficient between VO<sub>2i</sub> and VO<sub>2AO</sub>/VO<sub>2VO</sub> in the MG is substantial across participants. Thus, it is preferable to calibrate VO<sub>2i</sub> prior to every monitoring session. Important future work is needed to compare VO<sub>2i</sub> against gold standard modalities such as positron emission tomography or magnetic resonance imaging.
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