The effect of sex on respiratory-locomotor blood flow during exercise

Evidence suggests that at exercise intensities above 80% maximum, respiratory muscles may “steal” blood from locomotor muscles to supply metabolic demand. In addition, literature indicates that females may allocate a greater proportion of total blood to the respiratory muscles. We hypothesized that, during high-intensity exercise (>80% of maximal oxygen uptake, V̇O2max), females would divert a greater proportion of blood flow from locomotor muscles to respiratory muscles, when compared to males. Twenty healthy adults (10 F, 10 M) completed four cycling exercise bouts of increasing intensity (25–90% V̇O2max), with an injection of indocyanine green (ICG) dye in the last minute of each 3-minute exercise trial, detected by near infrared spectroscopy (NIRS). Blood flow index (BFI) was measured in the sternocleidomastoid (SCM) and vastus lateralis (VL). SCM BFI did not differ between sexes across exercise stages (p = 0.208). In contrast, VL BFI demonstrated a significant sex × stage interaction (p = 0.011). Importantly, females had greater skinfold thickness than males (19.6 ± 2.7 vs. 9.4 ± 2.8 mm, p < 0.01, respectively), and when VL BFI was adjusted for skinfold thickness, there were no sex-differences in VL BFI. Taken together, the results do not support the original hypothesis that females divert a greater proportion of blood flow from the locomotor to the respiratory muscles during high-intensity exercise. Instead, the findings suggest that respiratory and locomotor BFI are shaped by both physiological demand and methodological considerations, including tissue composition and the interpretation of NIRS-ICG signals.