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

Abstract We tested the hypothesis during the combined challenges of altered inspired O 2 fraction (F I O 2 ) 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 O 2 delivery (DO 2 est ). 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 DO 2 est 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 DO 2 est was estimated by MBF times saturation. LPO HUT in hypoxia was associated with no changes in VC and MBF leading to reduced DO 2 est . In LPO HDT under hypoxia, despite no apparent functional limitation in the VC recruitment, rise in MBF to maintain DO 2 est 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 DO 2 est . 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.