Monitoring Treadmill Physical Exercise and Recovery in Elite Water Polo Players with Local Muscle Oxygen Saturation Measurements—Regional and Sex Differences

<b>Background</b>: Despite numerous data on whole-body responses, we have less information about local muscular changes during physical exercise in athletes. Oxygen saturation (SmO₂) changes in the working muscles follow phases of load and are useful, as local metabolism could influence physical fitness. <b>Methods</b>: A total of 100 asymptomatic elite water polo players (63% male, age: 17.2 (interquartile range: 16.1-18.9) years) were examined using near-infrared spectroscopy to measure SmO₂ in both vastus lateralis and left deltoid muscles during continuous uphill running treadmill exercise. <b>Results</b>: Differences were observed between upper and averaged lower limb resting SmO₂ (82.1% (77.0-89.0%) vs. 68.3% (59.2-73.6%), <i>p</i> < 0.001). During exercise, the relative decrease in averaged lower limb SmO₂ was greater compared to the upper limb at the anaerobic threshold (-0.371 (-0.539--0.200) vs. -0.224 (-0.340--0.099), <i>p</i> < 0.001) and at maximal exercise (-0.557 (-0.750--0.411) vs. -0.420 (-0.556--0.271), <i>p</i> < 0.001). Higher averaged lower limb relative SmO₂ was recorded compared to the upper limb after 5 min cool-down (+0.081% (-0.046-+0.195%) vs. -0.047% (-0.140-+0.000), <i>p</i> < 0.001). No differences were found between males and females in resting lower limb SmO₂. Both sexes showed a monotonic decrease in SmO₂ during exercise, with differences in the relative values at the anaerobic threshold and at maximal intensity. Females exhibited a rebound in SmO₂ after a 5 min cool-down. <b>Conclusions</b>: We provide insights into SmO₂ alterations during maximal-intensity exercise and recovery through the measurements of elite water polo athletes, also highlighting sex differences in SmO₂. Measuring local SmO₂ changes is a promising additional method in physical fitness follow-up.