Continuous Monitoring of Muscle Oxygenation in Endurance Athletes During Incremental Cycling: Experimental Validation of a Wearable Continuous-Wave NIRS Sensor Using Frequency-Domain Near-Infrared Spectroscopy

Individuals often lack field-based tools to monitor exercise effectiveness. New sensing methods may allow for an improved measurement of the individualized response to exercise by monitoring oxygen kinetics directly in muscle tissue. This study aimed to validate a non-invasive wearable sensor capable of measuring muscle oxygen saturation (SmO₂) using continuous-wave near-infrared spectroscopy (CW-NIRS) against a laboratory-validated frequency-domain NIRS (FDNIRS) device. Ten physically fit adults performed an incremental cycling test until voluntary exhaustion. Devices were placed on contralateral rectus femoris muscles. SmO₂ was simultaneously measured continuously for the duration of the protocol. Time series alignment was performed using linear interpolation to enable direct comparison between devices at matched time points. Z-score normalization accounted for inter-individual differences in a group-level analysis. Individual subject validation showed strong correlations between the two devices (r = 0.792, range: 0.69-0.88, <i>p</i> < 0.001) with an RMSD < 5% for most subjects, a mean bias of 0.005 and low proportional bias (-0.199) between all paired measurements. Group-level analysis demonstrated a correlation of r = 0.788. Bland-Altman analysis revealed that 95% of all measurements fell between -8.1% and 7.6% SmO₂. The CW-NIRS device delivered reliable performance compared to the FDNIRS device, offering potential applications for real-time physiological monitoring during exercise and performance assessment.