Reliability of Muscle Oxygen Saturation for Evaluating Exercise Intensity and Knee Joint Load Indicators

<b>Objectives:</b> This study aimed to evaluate the reliability of muscle oxygen saturation (SmO₂) and its correlation with variables from an inertial measurement unit (IMU) sensor placed on the knee at different exercise intensities. <b>Methods:</b> Fourteen university athletes participated in the study. Incremental ergospirometry was performed to exhaustion to calculate V'O₂max, determine training zones, heart rate, and workload using the IMU, and analyze muscle metabolism by SmO₂. <b>Results:</b> The analysis revealed significant differences between moderate-to-high-intensity zones (80-89% vs. 50-69%, Δ = 27% of SmO₂; <i>p</i> < 0.001) and high-intensity zones (90-100% vs. 50-79%, Δ = 35% of SmO₂; <i>p</i> < 0.001). SmO₂ values showed moderate reliability at moderate exercise intensities (e.g., ICC 0.744 at 50%) and high variability at higher intensities, with ICC values around 0.577-0.594, and CV% increasing up to 77.7% at 100% intensity, indicating decreasing consistency as exercise intensity increases. SmO₂ significantly decreases with increasing angular velocity (β = -13.9, <i>p</i> < 0.001), while knee joint load only shows significant correlations with SmO₂ in the moderate-to-high-intensity zones (r = 0.569, <i>p</i> = 0.004) and high-intensity zones (r = 0.455, <i>p</i> = 0.012). <b>Conclusions:</b> SmO₂ is a key predictor of performance during maximal incremental exercise, particularly in high-intensity zones. Moreover, SmO₂ has the potential to serve as a physiological marker of the internal load on the muscles surrounding the knee during exercise. The SmO₂ decrease could depend on the angular velocity and impact of the exposed knee during running.