We assessed respiratory muscles oxygenation responses during a ramp exercise to exhaustion and further explored their relationship with the non-linear increase of VO2 (VO2 excess) observed above the gas-exchange threshold. Ten male cyclists completed a ramp exercise to exhaustion on an electromagnetically braked cycle-ergometer with a rate of increment of 30 W min-1 with continuous monitoring of expired gases (breath-by-breath) and oxygenation status of intercostal muscles. Maximal inspiratory and expiratory pressure measurements were taken at rest and at exhaustion. The VO2 excess represents the difference between VO2max observed and VO2max expected using linear equation between the VO2 and the intensity before gas-exchange threshold. The deoxyhemoglobin remained unchanged until 60% of maximal aerobic power (MAP) and thereafter increased significantly by 37 ± 18% and 40 ± 22% at 80% and 100% of MAP, respectively. Additionally, the amplitude of deoxyhemoglobin increase between 60 and 100% of MAP positively correlated with the VO2 excess (r = 0.69, p < 0.05). Compared to exercise start, the oxygen tissue saturation index decreased from 80% of MAP (-4.8 ± 3.2%, p < 0.05) onwards. At exhaustion, maximal inspiratory and expiratory pressures declined by 7.8 ± 16% and 12.6 ± 10% (both p < 0.05), respectively. In summary, our results suggest a significant contribution of respiratory muscles to the VO2 excess phenomenon.