Near‐infrared spectroscopy measures of sternocleidomastoid blood flow during exercise and hyperpnoea

New Findings What is the central question of this study? How does sternocleidomastoid blood flow change in response to increasing ventilation and whole‐body exercise intensity? What is the main finding and its importance? Sternocleidomastoid blood flow increased with increasing ventilation. For a given ventilation, sternocleidomastoid blood flow was lower during whole‐body exercise compared to resting hyperpnoea. These findings suggest that locomotor muscle work exerts an effect on respiratory muscle blood flow that can be observed in the sternocleidomastoid. Respiratory muscle work influences the distribution of blood flow during exercise. Most studies have focused on blood flow to the locomotor musculature rather than the respiratory muscles, owing to the complex anatomical arrangement of respiratory muscles. The purpose of this study was to examine how accessory respiratory (i.e. sternocleidomastoid, and muscles in the intercostal space) muscle blood flow changes in response to locomotor muscle work. Seven men performed 5 min bouts of constant load cycling exercise trials at 30%, 60% and 90% of peak work rate in a randomized order, followed by 5 min bouts of voluntary hyperpnoea (VH) matching the ventilation achieved during each exercise (EX) trial. Blood‐flow index (BFI) of the vastus lateralis, sternocleidomastoid (SCM) and seventh intercostal space (IC) were estimated using near‐infrared spectroscopy and indocyanine green and expressed relative to resting levels. BFISCM was greater during VH compared to EX (P = 0.002) and increased with increasing exercise intensity (P = 0.036). BFISCM reached 493 ± 219% and 301 ± 215% rest during VH and EX at 90% peak work rate, respectively. BFIIC increased to 242 ± 178% and 210 ± 117% rest at 30% peak work rate during VH and EX, respectively. No statistically significant differences in BFIIC were observed with increased work rate during VH or EX (both P > 0.05). Moreover, there was no observed difference in BFIIC between conditions (P > 0.05). BFISCM was lower for a given minute ventilation during EX compared to VH, suggesting that accessory respiratory muscle blood flow is influenced by whole‐body exercise.