The effect of exercise intensity and cardiorespiratory fitness on the kinetic response of middle cerebral artery blood velocity during exercise in adults

Abstract

The aim of this study was to compare the kinetic response of middle cerebral artery blood velocity (MCAv) to moderate and heavy-intensity cycling in adults, and explore the relationship between maximal oxygen uptake (VO2max) and MCAv kinetics. Seventeen healthy adults (23.8±2.4 years, 9 females) completed a ramp incremental test to exhaustion on a cycle ergometer to determine VO2max and the gas exchange threshold (GET). Across six separate visits, participants completed three 6-minute transitions at a moderate-intensity (90% GET) and three at a heavy-intensity (40% of the difference between GET and VO2max). Bilateral MCAv was measured using transcranial Doppler ultrasonography and analysed using a mono-exponential model with a time delay. The time constant (τ) of the MCAv response was not different between moderate- and heavy-intensity cycling (25±10 vs. 26±8 s, P=0.82), as was the time delay (29±11 vs. 29±10 s, P=0.95). The amplitude of the exponential increase in MCAv from baseline was greater during heavy (23.9±10.0 cm.s-1, 34.1±14.4%) compared to moderate (12.7±4.4 cm.s-1, 18.7±7.5%) intensity cycling (P<0.01). Following the exponential increase, a greater fall in MCAv was observed during heavy compared to moderate-intensity exercise (9.5±6.9 vs 2.8±3.8 cm.s-1, P<0.01). MCAv after 6 minutes of exercise remained elevated during heavy compared to moderate-intensity exercise (85.2±9.6 vs. 79.3±7.7cm.s-1, P≤0.01). VO2max was not correlated with MCAv τ or amplitude (r=0.11-0.26, P>0.05). These data suggest that the intensity of constant-work rate exercise influences the amplitude, but not time-based, response parameters of MCAv in healthy adults, and found no relationship between cardiorespiratory fitness and MCAv kinetics.