ABSTRACTThis 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; V?O
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 (V?O
2) was assessed using the
breath-by-breath method during the V?O
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 V?O
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.
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