The Spatial Distribution of Absolute Skeletal Muscle Deoxygenation During Ramp-Incremental Exercise Is Not Influenced by Hypoxia

Abstract

Time-resolved near-infrared spectroscopy (TRS-NIRS) allows absolute quantitation of deoxygenated haemoglobin and myoglobin concentration ([HHb]) in skeletal muscle. We recently showed that the spatial distribution of peak [HHb] within the quadriceps during moderate-intensity cycling is reduced with progressive hypoxia and this is associated with impaired aerobic energy provision. We therefore aimed to determine whether reduced spatial distribution of skeletal muscle [HHb] was associated with impaired aerobic energy transfer during exhaustive ramp-incremental exercise in hypoxia. Seven healthy men performed ramp-incremental cycle exercise (20 W/min) to exhaustion at 3 fractional inspired O₂ concentrations (F_IO₂): 0.21, 0.16, 0.12. Pulmonary O₂ uptake ($\dot{\mathrm{V}}{\mathrm{O}}_2$) was measured using a flow meter and gas analyser system. Lactate threshold (LT) was estimated non-invasively. Absolute muscle deoxygenation was quantified by multichannel TRS-NIRS from the rectus femoris and vastus lateralis (proximal and distal regions). ${{\dot{\mathrm{V}}\mathrm{O}}_2}_{\mathrm{peak}}$ and LT were progressively reduced (p < 0.05) with hypoxia. There was a significant effect (p < 0.05) of F_IO₂ on [HHb] at baseline, LT, and peak. However the spatial variance of [HHb] was not different between F_IO₂ conditions. Peak total Hb ([Hb_tot]) was significantly reduced between F_IO₂ conditions (p < 0.001). There was no association between reductions in the spatial distribution of skeletal muscle [HHb] and indices of aerobic energy transfer during ramp-incremental exercise in hypoxia. While regional [HHb] quantified by TRS-NIRS at exhaustion was greater in hypoxia, the spatial distribution of [HHb] was unaffected. Interestingly, peak [Hb_tot] was reduced at the tolerable limit in hypoxia implying a vasodilatory reserve may exist in conditions with reduced F_IO₂.