Excess VO₂ during ramp exercise is positively correlated to intercostal muscles deoxyhemoglobin levels above the gas exchange threshold in young trained cyclists

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 VO_2max observed and VO_2max 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.