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Abstract Purpose Portable near-infrared spectroscopy devices allow measurements of muscle oxygen saturation (SmO 2 ) in real time and non-invasively. To use NIRS for typical applications including intensity control and load monitoring, the day-to-day variability needs to be known to interpret changes confidently. This study investigates the absolute and relative test–retest reliability of the Moxy Monitor and investigates side differences of SmO 2 at the vastus lateralis muscle of both legs in cyclists. Methods Twelve trained cyclists and triathletes completed 3 incremental step tests with 5 min step duration starting at 1.0 W/kg with an increase of 0.5 W/kg separated by 2–7 days. SmO 2 was averaged over the last minute of each stage. For all power outputs, the intra-class coefficient (ICC), the standard error of measurement (SEM) and the minimal detectable change (MDC) were calculated. Dominant and non-dominant leg SmO 2 were compared using a three-factor ANOVA and limits of agreement (LoA). Results ANOVA showed no significant systematic differences between trials and side. For both legs and all intensities, the ICC ranged from 0.79 to 0.92, the SEM from 5 to 9% SmO 2 and the MDC from 14 to 18% SmO 2 . The bias and LoA between both legs were −2.0% ± 19.9% SmO 2 . Conclusion Relative reliability of SmO 2 was numerically good to excellent according to current standards. However, it depends on the specific analytical goal whether the test–retest reliability is deemed sufficient. Wide LoA indicate side differences in muscle oxygenation during exercise unexplained by leg dominance.
Skotzke et al. (Wed,) studied this question.