Reliability and Repeatability of Determining Power Associated with Maximal Metabolic Steady State Using Changes in NIRS-Derived Muscle Oxygenation

PURPOSE: The heavy-severe domain boundary can be estimated with muscle oxygen saturation (%SmO 2 ) by determining the power at which a %SmO 2 slope of zero occurs. The purpose of this study was to examine the reliability and repeatability of determining power at a %SmO 2 zero-slope. METHODS: Thirty endurance-trained participants (15 women) completed six laboratory visits. Maximal oxygen uptake and gas exchange parameters were determined via a cycling ramp test. Participants completed four weekly cycling protocols consisting of four 4-min stages spanning the exercise intensity domains. During each stage, %SmO 2 was measured using near-infrared spectroscopy (NIRS) on the rectus femoris (Moxy Monitor, Fortiori Design). The %SmO 2 zero-slope power was determined from %SmO 2 responses at each stage, followed by a 4-min confirmation stage at the predicted %SmO 2 zero-slope power. Linear regression between stage power ( X -axis) and the slope of the %SmO 2 response during each stage ( Y -axis) was used to predict the power at a %SmO 2 zero-slope. Differences in %SmO 2 zero-slope power across visits were examined via one-way analysis of variance, and reliability was assessed using the intraclass correlation coefficient (ICC 2,1) as well as the standard error of measurement, expressed as a coefficient of variation. RESULTS: %SmO 2 zero-slope power was not significantly different across visits in both men and women combined ( P = 0.108), nor was it different in men and women separately. The ICC across visits was 0.953 (95% CI: 0.917-0.978; P < 0.001) indicating excellent reliability with a standard error of measurement of 2.6 W or 1.2%, and a minimal difference to detect a real change of 7.3W or 3.4%. CONCLUSIONS: These results demonstrate that the power at maximal metabolic steady state can be reliably determined using a %SmO 2 zero-slope method using NIRS.