Purpose This study analysed the relationship between haematological indices and near-infrared spectroscopy (NIRS) parameters, as they are rarely explored. Methods Nineteen elite male speed skaters performed a 2-min cycling time trial (TT) during which muscle oxygen saturation (SmO2) and local haemoglobin concentration (tHbmuscle) were continuously monitored using NIRS. Haematological parameters was evaluated through (1) total haemoglobin mass (tHbmass), plasma volume (PV), blood volume (BV), and red cell volume (RCV), using the carbon monoxide (CO) rebreathing method, and (2) complete blood count from venous blood samples. Exploratory correlation and regression analyses were conducted to investigate potential associations between the variables. Results Baseline SmO2 was not associated with any parameters, whereas tHbmuscle was positively correlated with RCV, BV, and PV (p < 0.05). SmO2 values exhibited three distinct phases: a descent and plateau during the TT, followed by an ascent during recovery. None of the haematological variables explained SmO2 variability in the entire cohort. After dividing the participants based on the median of haematological variables into low and high groups, regression analyses showed that dichotomised tHbmass (p < 0.01), BV (p < 0.01), and PV (p < 0.05) explained the variability of SmO2 only in the linear part of the descent phase. During recovery, half-time of local muscle reoxygenation correlated negatively with relative values of PV, BV (p < 0.05) and tHbmass (p < 0.06). Conclusions Haematological status, irrespective of the assessment method, did not appear to influence either resting SmO2 or SmO2 kinetics during the time trial. However, muscle reoxygenation dynamics may be associated with haematological parameters obtained via the CO rebreathing method.
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