Respiratory and muscle oxygenation responses to two constant-load exercise intensities

Objective: We investigated the respiratory and vastus lateralis oxygenation responses to exhaustive, constant-load cycling at two exercise intensities. Methods: Eight moderately-trained male randomly cycled to exhaustion at 75% and 85% of maximal work-load (CL75 and CL85, respectively) measured during a maximal incremental test. From continuous recordings of respiratory variables and near-infrared spectroscopy signals of the vastus lateralis muscle, VO2 slow component (SC) and deoxyhemoglobin (HHb) SC were calculated. Results: At exhaustion, VO2 (+19 ± 25%), VO2 SC (+59 ± 24%) and minute ventilation (+14 ± 14%) were significantly higher during CL85 vs. CL75, whereas oxyhemoglobin (-67 ± 22%) and total hemoglobin (-36 ± 3%) responses were lower (all p<0.05). Additionally, VO2 SC during CL75 and CL85 were strongly correlated (r=0.88, p<0.001). The HHb SC did not differ between CL75 and CL85 (3.10 ± 0.75 vs. 3.44 ± 1.1 A.U, respectively; p=0.60). HHb SC and VO2 SC during CL85 (r=0.94, p<0.001), but not during CL75 (r=-0.08, p=0.90), were correlated. Finally, HHb SC during CL75 and CL85 were unrelated (r=-0.20, p=0.70). Conclusion: Our results highlight that only the contribution of locomotor muscles to the VO2 SC magnitude is affected by the exercise intensity and not that of ventilation.