AbstractIntroduction: The increasing number of older adults has seen a corresponding growth in those affected by neurovascular diseases, including stroke and dementia. Since cures are currently unavailable, major efforts in improving brain health need to focus on prevention, with emphasis on modifiable risk factors such as physical activity. Moderate-intensity continuous training (MICT) paradigms have been shown to confer vascular benefits translating into improved musculoskeletal, cardiopulmonary and cerebrovascular function. However, the time commitment associated with MICT is a common barrier to participation. High-intensity interval training (HIIT) has since emerged as a more time-efficient mode of exercise that can promote similar, if not superior improvements in cardiorespiratory fitness (CRF) and further promote vascular adaptation. However, randomised controlled trials investigating the impact of HIIT on the brain, specifically cerebrovascular and cognitive function, are surprisingly limited, thus justifying the work submitted in the current thesis.
Aims: Two functionally integrated studies were designed to i) compare the acute cerebrovascular responses to both HIIT and MICT exercise and ii) determine to what extent long-term HIIT impacts cerebrovascular and cognitive function.
Hypothesis: It was hypothesised that i) a high-volume HIIT (HVHIIT) protocol would result in greater acute increases in cerebral perfusion during exercise compared to MICT and a low-volume HIIT (LVHIIT) protocol and ii) long-term HVHIIT would result in increased cerebrovascular function and corresponding improvements in cognitive function relative to a stretching and toning (i.e., non-HIIT) control group.
Methods: Study 1 – Nine healthy young adult males (24 ± 2 years of age) underwent a maximal CRF assessment to determine maximal oxygen consumption (V̇O2MAX) and maximum heart rate (HRMAX). Following this, they completed a single bout of HVHIIT (4 x 4 min intervals of high-intensity exercise at 85-95% HRMAX, separated by 3 min of cycling recovery), LVHIIT (10 x 1 min intervals of high-intensity exercise at 90% HRMAX, separated by 1 min of cycling recovery) and MICT (40 min of continuous exercise at 65% HRMAX) in a randomised order on separate days. Cerebrovascular (middle cerebral artery blood flow velocity (MCAv)) and cardiopulmonary function (including volume of oxygen consumed (V̇O2) and end-tidal carbon dioxide (PETCO2)) were recorded continuously and an average of the final 30 s of data at rest, the protocol mid-point (HVHIIT, high-intensity interval 2; LVHIIT, high-intensity interval 5; MICT, 20-min) and end of exercise was calculated. Study 2 – Eighteen physically inactive but otherwise healthy older males (9 experimental vs. 9 control) were randomly assigned into a supervised HVHIIT programme (62 ± 6 years of age) or a control (61 ± 9 years of age) group that completed home-based stretching and toning exercise. Both groups completed their respective training programmes three times a week over a 12-week period. All participants underwent a maximal CRF assessment (to record V̇O2MAX), tests to determine cerebrovascular (MCAv and cerebrovascular reactivity to hyper/hypocapnia (CVRCO2HYPER and CVRCO2HYPO, respectively)) and cardiorespiratory function (PETCO2), and a battery of cognitive assessments, prior to and following completion of the interventions.
Results: Study 1 –V̇O2 was greater at the mid-point and end of exercise during HVHIIT and LVHIIT compared to MICT when expressed as a percentage of V̇O2MAX (P < 0.05). The relative increase in MCAv from rest to the mid-point and end of exercise was greater in HVHIIT than in MICT (P < 0.05). This occurred despite a lower PETCO2 at the end of exercise during HVHIIT compared to MICT (P < 0.05). Study 2 – V̇O2MAX increased following HVHIIT compared to the pre-intervention stage (P < 0.05). Although there was no change in MCAv following HVHIIT (P > 0.05), an increase in CVRCO2HYPER was observed (P < 0.05). There was a collective increase in learning and memory, as well as attention and information processing following both training programmes (P < 0.05), with more marked improvements in visuomotor coordination confined to the controls (P < 0.05).
Discussion: The findings of the studies incorporated in this thesis demonstrate that HIIT, specifically HVHIIT, stimulates acute increases in MCAv compared to MICT and long-term improvements in CVRCO2HYPER, which is significant given that CVRCO2HYPER is a marker of endothelial function. However, improvements in cerebrovascular function did not translate into direct improvements in cognitive function. In fact, stretching and toning exercise had a selective effect on visuomotor coordination, suggesting that enhanced cerebrovascular endothelial function does not necessarily determine cognitive function. Furthermore, it highlights the importance of a control group.
Conclusion: These findings demonstrate that HVHIIT is a suitable intervention to increase cerebrovascular function with increasing age, but further research is required to identify the optimal exercise regime to improve cognition.
|Date of Award||2023|
|Supervisor||Damian Bailey (Supervisor) & Christopher Marley (Supervisor)|