Increases in skin perfusion and blood oxygen in the non‐exercising human limbs during exercise in the heat: Implications for control of circulation

Blood flow in the inactive limb tissues and skin is widely thought to decline during incremental exercise to exhaustion due to augmented sympathoadrenal vasoconstrictor activity, but direct evidence to support this view is lacking. Here, we investigated the inactive-forearm haemodynamic ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>Q</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mi>forearm</mml:mi></mml:msub> <mml:annotation>${\dot{Q}}_{\mathrm{forearm}}$</mml:annotation></mml:semantics> </mml:math> ) and oxygenation responses to a range of two-leg exercise intensities and durations in the heat. Blood oxygen and flow were measured in the forearm tissue and skin of endurance-trained males during three incremental cycling exercise tests, with tests 1 and 2 separated by a 2 h bout of moderate constant load cycling exercise, all performed in the heat (35°C, 50% relative humidity, with fan cooling). In incremental exercise tests 1 and 3, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>Q</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mi>forearm</mml:mi></mml:msub> <mml:annotation>${\dot{Q}}_{\mathrm{forearm}}$</mml:annotation></mml:semantics> </mml:math> was stable from rest to ∼40% W_peak, before increasing by ∼118% at 80% W_peak (P <0.001). Correspondingly, forearm skin arterio-venous oxygen difference (a-vO₂ diff) decreased by ∼62% at 80% W_peak (P = 0.043), remaining reduced through to W_peak. Concomitantly, forearm skin blood flow more than doubled, while forearm deep tissue O₂ saturation decreased. When incremental exercise started shortly after constant load exercise (test 2), <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>Q</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mi>forearm</mml:mi></mml:msub> <mml:annotation>${\dot{Q}}_{\mathrm{forearm}}$</mml:annotation></mml:semantics> </mml:math> was 2- to 3-fold higher than during tests 1 and 3, whereas skin a-vO₂ diff was suppressed to a low level. Similar changes were observed during constant load exercise. In conclusion, skin perfusion increases during incremental exercise in the heat, concomitant to proportional reductions in oxygen extraction from the cutaneous circulation. Hence, contrary to the generally held view, skin perfusion remains elevated during maximal exercise and heat stress despite profound increases in sympathoadrenal activity.