Vascular conductance and muscle blood flow during exercise are altered by inspired oxygen fraction and arterial perfusion pressure

We tested the hypothesis during the combined challenges of altered inspired O2 fraction (FIO2) and posture changes at lower power output regardless of body position that the vascular conductance (VC) recruitment to the exercising muscle would not limit muscle perfusion and estimated O2 delivery (DO2est ). However, in head-down tilt at the higher power output exercise in hypoxia, the recruitment of VC would have a functional limitation which would restrict muscle blood flow (MBF) leading to a limitation in DO2est with consequent increases in metabolic stress. Ten healthy volunteers repeated plantar flexion contractions at 20% (low power output = LPO) and 30% (higher power output = HPO) of their maximal voluntary contraction in horizontal (HOR), 35° head-down-tilt (HDT) and 45° head-up-tilt (HUT). Popliteal diameter and muscle blood flow velocity were measured by ultrasound determining MBF. VC was estimated by dividing MBF flow by MPP, and DO2est was estimated by MBF times saturation. LPO HUT in hypoxia was associated with no changes in VC and MBF leading to reduced DO2est In LPO HDT under hypoxia, despite no apparent functional limitation in the VC recruitment, rise in MBF to maintain DO2est was associated with marked increase in muscle electromyographic activity, indicating greater metabolic stress. In HPO HDT under hypoxia, a functional limitation for the recruitment of VC constrained MBF and DO2est Elevated muscle electromyographic signal in HPO HDT under hypoxia was consistent with challenged aerobic metabolisms which contributed to a greater increase in the relative stress of the exercise challenge and advance the onset of muscle fatigue.