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We sought to investigate the role of group III/IV muscle afferents in limiting endurance exercise performance, independently of their role in optimizing locomotor muscle O₂ delivery. While breathing 100% O₂ to ensure a similar arterial O₂ content () in both trials, eight male cyclists performed 5-km time trials under control conditions (H_CTRL) and with lumbar intrathecal fentanyl (H_FENT) impairing neural feedback from the lower limbs. After each time trial, common femoral artery blood flow (FBF) was quantified (Doppler ultrasound) during constant-load cycling performed at the average power of the preceding time trial. The assessment of end-tidal gases, hemoglobin content and saturation, and FBF facilitated the calculation of leg O₂ delivery. Locomotor muscle activation during cycling was estimated from vastus lateralis EMG. With electrical femoral nerve stimulation, peripheral and central fatigue were quantified by …