Reliability of NIRS portable device for measuring intercostal muscles oxygenation during exercise.

This study assessed the intra-individual reliability of oxygen saturation in intercostal muscles (SmO ₂ - m.intercostales ) during an incremental maximal treadmill exercise by using portable NIRS devices in a test-retest study. Fifteen marathon runners (age, 24.9 ± 2.0 years; body mass index, 21.6 ± 2.3 kg·m ^-2 ; V̇O ₂ -peak, 63.7 ± 5.9 mL·kg ^-1 ·min ^-1 ) were tested on two separate days, with a 7-day interval between the two measurements. Oxygen consumption (V̇O ₂ ) was assessed using the breath-by-breath method during the V̇O ₂ -test, while SmO ₂ was determined using a portable commercial device, based in the near-infrared spectroscopy (NIRS) principle. The minute ventilation (VE), respiratory rate (RR), and tidal volume (Vt) were also monitored during the cardiopulmonary exercise test. For the SmO ₂ - m.intercostales , the intraclass correlation coefficient (ICC) at rest, first (VT1) and second ventilatory (VT2) thresholds, and maximal stages were 0.90, 0.84, 0.92, and 0.93, respectively; the confidence intervals ranged from -10.8% - +9.5% to -15.3% - +12.5%. The reliability was good at low intensity (rest and VT1) and excellent at high intensity (VT2 and max). The Spearman correlation test revealed ( p ≤ 0.001) an inverse association of SmO ₂ - m.intercostales with V̇O ₂ ( ρ  = -0.64), VE ( ρ  = -0.73), RR ( ρ  = -0.70), and Vt ( ρ  = -0.63). The relationship with the ventilatory variables showed that increased breathing effort during exercise could be registered adequately using a NIRS portable device.