The cerebrovascular response to resistance exercise in healthy individuals : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Massey University, Wellington, New Zealand

Abstract

The brain is a small organ that is sensitive to chemical and pressure changes, which is why it requires tightly regulated blood flow to function optimally, particularly under varying stressors. Resistance exercise (RE) is a unique stressor that produces sinusoidal fluctuations in blood pressure, which alters the cerebral blood flow (CBF) profile to mirror that of the blood pressures. However, the brain possesses regulators that work to achieve stable CBF. These regulators include 1) partial pressure of carbon dioxide (PaCO₂), 2) cerebral autoregulation (CA), 3) neurovascular coupling (NVC), 4) the autonomic nervous system (ANS), and 5) cardiac output (Q̇). CA is a known mechanism that defends against fluctuations in blood pressure, its efficiency is not as robust as previously thought, as the proposed autoregulatory range from earlier studies has been found to be much narrower. Therefore, stressors that challenge CA outside of the autoregulatory range requires investigation since the autoregulatory range being narrower, could expose the cerebral circulation to larger blood pressure perturbations and challenge stable CBF. This thesis investigated the relationship between RE and CA, focusing on cerebrovascular and cardiovascular haemodynamics during and immediately after dynamic RE in RE-trained and untrained individuals. The comparisons between RE-trained and untrained individuals would shed light on if the constant exposure to the sinusoidal fluctuations in blood pressure elicit functional adaptations that assists with maintaining stable CBF during RE. Furthermore, if functional adaptations do exist, whether that is specific to just increases in blood pressure or decreases in blood pressure or in both directions was also investigated. Additionally, this thesis explored the role of NVC in CBF regulation during RE. Transcranial Doppler (TCD) ultrasound was used to measure middle cerebral artery blood velocity (MCAv) during and after RE as a proxy for CBF. Three experimental chapters examined different aspects of RE-induced fluctuations in blood pressure and their effects on CBF regulation. Chapter Five: During unilateral lower body dynamic RE, using a leg extension exercise, RE-trained individuals experienced greater increases and fluctuations in mean arterial pressure (MAP) compared to untrained individuals. However, mean MCAv (MCAvₘₑₐₙ) did not differ between the two groups. This finding suggests that RE-trained individuals may possess cerebrovascular adaptations that help maintain stable MCAvₘₑₐₙ despite greater blood pressure variability. Chapter Six: After completing dynamic RE, participants immediately stood up to investigate the effects of a hypotensive stressor on MAP and MCAvₘₑₐₙ. The RE-trained group exhibited a greater reduction in MAP than the untrained group, yet MCAvₘₑₐₙ remained similar between groups. Additionally, the rate of regulation (RoR), a metric of CA, was higher in the RE-trained group, although the time taken for MCAvₘₑₐₙ to return to baseline was identical. These findings imply that RE-trained individuals may have enhanced CA responses during post-exercise hypotension. Chapter Seven: The role of NVC was explored by comparing MCAv in the ipsilateral and contralateral middle cerebral arteries during unilateral upper body dynamic RE, using a biceps curl exercise. The results indicated that MCAvₘₑₐₙ was similar on both sides during RE, demonstrating that MCAv remains bilaterally homogeneous during dynamic RE. This suggests that, during dynamic upper body RE, NVC does not differentially modulate flow between hemispheres and that MAP has more of an influence on CBF. This thesis demonstrated that habitual RE may lead to cerebrovascular adaptations that help maintain CBF during both dynamic RE and post-exercise hypotensive recovery. It also revealed that during dynamic RE, MAP exerts a dominant influence on MCAv, potentially overriding local regulatory mechanisms such as NVC. These results have important implications for understanding how regular RE influences cerebrovascular health and the brain’s ability to manage fluctuations in blood pressure during physical stress. Furthermore, the findings from this research can be applied to enhance future exercise prescription recommendations.