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 O fraction (FO) 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 delivery (DO ). 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 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 was estimated by MBF times saturation. LPO in hypoxia was associated with no changes in VC and MBF leading to reduced DO In LPO under hypoxia, despite no apparent functional limitation in the VC recruitment, rise in MBF to maintain DO was associated with marked increase in muscle electromyographic activity, indicating greater metabolic stress. In HPO under hypoxia, a functional limitation for the recruitment of VC constrained MBF and DO Elevated muscle electromyographic signal in HPO 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.