Introduction: Classical estimations suggest that non-exercising limb tissue and skin blood flow is progressively reduced through to volitional exhaustion (Rowell, 1974) associated with the exponential rise in sympathetic nerve activity and circulating catecholamines (Rosenmeier et al., 2004; Ichinose et al., 2008; Trangmar et al., 2014, 2017). However, there is no direct evidence that skin perfusion decreases during maximal aerobic exercise.

Aims and objectives: The aim of the present study was to investigate the non-exercising limb tissue and skin hemodynamic and oxygenation responses to a range of exercise intensities and durations in the heat (35 °C, rH 50%; fan cooling).

Methods: Blood oxygen content, O₂ saturation, and arterio-venous oxygen difference (a-vO_{2 diff}) in the inactive forearm were initially measured in nine endurance-trained males during three incremental cycling exercise tests to volitional exhaustion (W_peak, 322 ± 38 W), with test 1 and 2 separated by a 2 h-bout of constant load cycling (55% W_peak). Forearm (brachial artery) blood flow (FBF), muscle oxygen saturation (mO₂Sat), skin blood flow (SkBF) and a-vO_{2 diff}, and body temperatures were assessed in a further seven endurance-trained males using the same experimental protocol. Data (presented as mean + SD) were assessed using repeated measures ANOVA, with the alpha level for significance set at P<0.05.

Results: In incremental exercise tests 1 & 3, FBF was stable from rest to ~40% W_peak, before increasing to a peak of 285 + 52 ml/min at 80% W_peak (N=7, P<0.001). Concomitantly, skin a-vO_{2 diff}, decreased from a baseline rest value of 56±27 mL/L to a nadir of ~25±27 mL/L at 80% W_peak (N=9, P<0.05), remaining at this level through to W_peak. SkBF increased, whilst mO₂Sat decreased at intensities above 80% W_peak. In incremental exercise test 2, that followed shortly after constant load exercise, baseline FBF was 3-fold higher than tests 1 & 3 (449 + 153 vs. 157 + 59 ml/min; N=9, P<0.01), remaining at this high level throughout, whilst skin a-vO_{2 diff} was suppressed to a low level, and remained constant, compared to tests 1 & 3. Similar changes were observed during constant load exercise, with a rise in FBF mirrored by a fall in a-vO_{2 diff}, concomitant to a high skin blood flow, and elevated core temperature.

Conclusions: Skin perfusion and oxygen delivery remain elevated during incremental lower-limb exercise to volitional exhaustion in the heat. Moreover, differential haemodynamic and oxygenation responses in the tissues of the inactive-forearm occur during strenuous exercise in the heat, where 1) skin blood flow and oxygenation increase and remain high, concurrent to proportional reductions in skin a-vO_{2 diff} and 2) muscle oxygenation declines during high-intensity exercise, indicating that increased forearm blood flow reflects the skin. These findings support observations in cool environmental conditions (Calbet et al., 2007; Kirby et al., 2021), and argue against the idea that increases in sympathoadrenal activity reduce skin perfusion during strenuous exercise.

Ethical standards: All procedures were approved by the Brunel University London Research Ethics Committee and conformed to the ethical principles of the World Medical Association (Declaration of Helsinki).