Investigation into effects of supplementary O-2 on tissue oxygenation during isometric handgrip in the ipsilateral and contralateral limb in young (Y) and older (O) men using Near Infra Red Spectroscopy (NIRS)

We have shown by using venous occlusion plethysmography (VOP) that breathing 40% O₂ attenuates post-contraction hyperaemia evoked by isometric contraction of the forearm in Y and O men. 40% O₂ also attenuates reflex vasoconstriction in the ipsilateral leg in Y men. We attributed these effects to improved tissue oxygenation and impaired release of prostaglandins, which induce local dilatation and evoke reflex vasoconstriction by stimulating muscle afferents. However, plethysmography precludes blood flow recording during contraction. Further, there is little direct evidence of how ageing affects tissue oxygenation during isometric contraction, or the exercise reflex. We investigated these issues by using NIRS in recreationally active Y (18 – 25 yr; n=10) and recreationally active O (65 – 78 yr; n=11) men. They performed isometric handgrip contraction at 60% Maximum Voluntary Contraction (MVC) using their dominant hand with a NIRS probe over flexor digitorum superficialis on ipsilateral and contralateral arms. Tissue oxygenation index (TOI), which indicates microvascular O₂ saturation, was higher at rest in Y than O men (70±2 vs 66±2 %), but fell to 58±4* vs 58±3* % 30 s into contraction and increased gradually to 59±3* vs 60±3%* at 120 s (*:P <0.01 vs baseline, 2-way RM ANOVA). Meanwhile, Total haemoglobin index (THI) which reflects muscle blood flow, was 1.00±0.01 vs 1.00±0.00 arbitrary units (au) at rest in Y and O men, fell to 0.96±0.04 and 0.97±0.03 au respectively 5 s into contraction, but increased gradually to 1.11±0.09* vs 1.09±0.09* au respectively at 120 s of handgrip. Breathing 40% O₂ attenuated the fall in TOI during contraction to 65±3^§ and 60±3^§ % in Y and O men at 30 s and until contraction ceased (^§: P<0.01 vs air). The increase in THI during contraction was also attenuated in Y and O men: to 1.01±0.04^§ and 1.04±0.06^§ au respectively at 120 s into contraction. In resting, contralateral arm of Y men, TOI was 69±2 % before, and increased to 74±3* and 71±3* % at 30 and 120 s into handgrip contraction, while THI fell from 1.00±0.00 to 0.98±0.01* and 0.98±0.01* au at 60 and 120 s. Breathing 40% O₂ attenuated the decrease in THI in Y men: 1.01±0.02 and 1.01±0.03 au at 60 and 120 s into contraction. By contrast, in resting arm of O men, TOI was 65±1% and THI was 1.01±0.01 au before contraction, and neither changed: 65±2 % and 1.01±0.01 au at 120 s of handgrip contraction.

These findings show that during isometric contraction at 60% MVC, tissue oxygenation falls to similar levels in recreationally active Y and O men and muscle blood flow increases throughout contraction. Breathing 40% O₂ does increase tissue oxygenation in the contracting limb in Y and O men as we proposed. Further, in Y men, reflex vasoconstriction evoked in the contralateral arm is greatly attenuated by breathing 40% O₂. However, our results indicate that in recreationally active O men, the reflex vasoconstriction is blunted even during air breathing.