Cerebrovascular responses during rowing: Do circadian rhythms explain morning and afternoon performance differences?

The purpose of this study was to characterize cerebrovascular responses to rowing exercise, investigating whether their diurnal variation might explain performance differences across a day. Twelve male rowers completed incremental rowing exercise and a 2000‐m ergometer time trial at 07:00 h and 16:00 h, 1 week apart, while middle cerebral artery velocity ( MCA v), cerebral (prefrontal), and muscular (vastus lateralis) tissue oxygenation and hemoglobin volume (via near‐infrared spectroscopy), heart rate, and pressure of end‐tidal CO 2 ( P ET CO 2 ) were recorded. MCA v was 20–25% above resting levels (68 ± 12 cm/s) during submaximal and maximal exercise intensities, despite P ET CO 2 being reduced during maximal efforts (down ∼ 0.5–0.8 k P a); thus revealing a different perfusion profile to the inverted‐ U observed in other exercise modes. The afternoon time trial was 3.4 s faster (95% confidence interval 0.9–5.8 s) and mean power output 3.2% higher (337 vs 347  W ; P  = 0.04), in conjunction with similar exercise‐induced elevations in MCA v ( P  = 0.60) and reductions in cerebral oxygenation ( TOI ) ( P  = 0.12). At the muscle, afternoon trials involved similar oxygen extraction ( HH b volume and TOI ) albeit from a relatively lower total H b volume ( P  < 0.01). In conclusion, rowing performance was better in the afternoon, but not in conjunction with differences in MCA v or exercise‐induced differences in cerebral oxygenation.