Background and Purpose        The impact of high intensity interval exercise (HIIE) on cerebrovascular function continues to gain interest, particularly surrounding the safety of such exercise in at-risk and ageing populations. HIIE is characterised by intermittent increases in hemodynamic and ventilatory responses during exercise, and may pose a substantial challenge to dynamic cerebral autoregulation (dCA) in the acute post-exercise recovery period. Furthermore, whether the intensity of an acute bout of HIIE impacts dCA recovery post-exercise has not been investigated. Therefore, this study aimed to investigate the acute effect of two popular, time-matched HIIE protocols on dCA in inactive, middle-aged adults.

Methods            Eleven inactive middle-aged adults (5 females, mean±SD age: 53.8±6.8 years, BMI: 27.5±4.3kg/m², maximal oxygen uptake (VO_2max): 29.9±5.4 ml/kg/min) participated in this study, which involved four visits. Participants completed a ramp incremental and verification test to exhaustion on a cycle ergometer in the first visit, to determine VO_2max, the gas exchange threshold (GET) and the difference between VO_2max and GET (Δ). All participants then completed three time-matched visits in a randomised and counterbalanced order: 1) a 30min seated control condition (CON), 2) a 30-min HIIE protocol involving 10x 1 min intervals at 70%Δ interspersed with 1 min recovery (HIIE10), and 3) a 30-min HIIE protocol involving 4x 4 min intervals at 20%Δ interspersed with 3 min recovery (HIIE4). dCA was assessed before, 10 min and 60 min after each condition using 5 min of repeated sit-stands performed at a frequency of 0.05 Hz. Transcranial Doppler ultrasound continuously measured middle cerebral artery blood velocity (MCAv), and beat-to-beat blood pressure was measured using finger plethysmography. dCA was analysed using transfer function analysis of MCAv and mean arterial pressure, with the effect of time and condition on dCA explored using two-way repeated measures ANOVAs.

Results               There was no significant main effect of time, condition or interaction effect for dCA gain (P≥0.21, η_p² ≤0.14) or normalised gain (P≥0.40, η_p² ≤0.09). Furthermore, dCA coherence and phase also did not differ across condition (P≥0.48, η_p² ≤0.07) or time (P≥0.27, η_p² ≤0.12), and there were no significant condition*time interactions (P≥0.11, η_p² ≤0.19).

Conclusions   These findings indicate that the ability of the cerebrovasculature to buffer forced oscillations in blood pressure are maintained following two different types of HIIE in inactive middle-aged adults. dCA recovery during the one hour following exercise was not impacted by the intensity of the HIIE bout, and suggests that, from a cerebral autoregulatory perspective, these bouts are safely tolerated in this population.