Oxygen Extraction Reserve Immediately After Ramp Incremental Maximal Exercise

Abstract

Towards the end of a ramp incremental (RI) test to exhaustion, the near-infrared spectroscopy (NIRS)-derive deoxygenated hemoglobin ([HHb]) signal plateaus, suggesting an upper limit in oxygen (O2) extraction. However, it is unknown whether this plateau is the highest level of O2 extraction, or if a “reserve” in O2 extraction exists, such that the plateau in the [HHb] in the presence of a still raising O2 utilization (VO2) towards the end of a RI test is indicative of a local increase in blood flow. PURPOSE: To assess the existence of a “reserve” in O2 extraction immediately at the end of a RI cycling test to exhaustion. METHODS: Nine male participants (27 ± 4.6 yrs; 79.9 ± 8.6 kg) performed a RI (30W[BULLET OPERATOR]min -1) test to exhaustion on a cycle ergometer (Velotron Dynafit Pro, Seattle, WA, USA) to determine the VO2 (Quark CPET, Cosmed, Rome, Italy) and the [HHb] (Oxiplex TS, ISS, Champaign, USA) responses. The [HHb] signal was measured on the Vastus Lateralis (VL) muscle. An automatic rapid inflation cuff was used to occlude blood flow to the leg (300 mmHg), at the upper portion of the thigh, for two minutes immediately at test failure. A paired samples t-test was used to compare the VO2 (onset of the plateau in the [HHb] signal and end-exercise) and the normalized (0-100% of the response during RI test) [HHb] signal (plateau and peak value obtained during occlusion). RESULTS: The end-exercise VO2 (VO2max; 4.35 ± 0.57 L[BULLET OPERATOR]min-1) was larger than that observed at the onset of the plateau in the [HHb] response (3.77 ± 0.52 L[BULLET OPERATOR]min-1; p <0.05). Post-exercise peak [HHb] (following occlusion) was higher compared to its plateau value (p <0.05), with a mean difference of 38.1 ± 18.9 %. CONCLUSIONS: This study demonstrated the existence of a “reserve’ in O2 extraction, despite a continuous increase in the VO2 response towards the end of a RI cycling test. These data suggest that the observed plateau in the [HHb] response is not related to O2 extraction reaching its upper limit, but likely due to increased local blood flow.