The effect of exercise intensity and cardiorespiratory fitness on the kinetic response of middle cerebral artery blood velocity during exercise in healthy 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 (V̇o_2max) and MCAv kinetics. Seventeen healthy adults (23.8 ± 2.4 yr, 9 females) completed a ramp incremental test to exhaustion on a cycle ergometer to determine V̇o_2max and the gas exchange threshold (GET). Across six separate visits, participants completed three 6-min transitions at a moderate intensity (90% GET) and three at a heavy intensity (40% of the difference between GET and V̇o_2max). Bilateral MCAv was measured using transcranial Doppler (TCD) ultrasonography and analyzed using a monoexponential 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-intensity cycling (23.9 ± 10.0 cm·s^-1, 34.1 ± 14.4%) compared with moderate-intensity cycling (12.7 ± 4.4 cm·s^-1, 18.7 ± 7.5%; P < 0.01). Following the exponential increase, a greater fall in MCAv was observed during heavy-intensity exercise compared with moderate-intensity exercise (9.5 ± 6.9 vs. 2.8 ± 3.8 cm·s^-1, P < 0.01). MCAv after 6 min of exercise remained elevated during heavy-intensity exercise compared with moderate-intensity exercise (85.2 ± 9.6 vs. 79.3 ± 7.7 cm·s^-1, P ≤ 0.01). V̇o_2max 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.NEW & NOTEWORTHY This is the first study to model the MCAv kinetic response to moderate- and heavy-intensity cycling in healthy adults. This study found that the amplitude of the exponential rise in MCAv at exercise onset was greater during heavy-intensity exercise (∼34%) compared with moderate-intensity exercise (∼19%), but the time-based characteristics of the responses were similar between intensities. Higher cardiorespiratory fitness was not associated with a greater or faster MCAv response to moderate- or heavy-intensity exercise.

Keywords: cerebral blood flow; exercise; kinetics.

Graphical abstract

Figure 1.

A representative trace for one participant for moderate (A) and heavy (B) intensity cycling. Data shown as an average of left and right MCAv responses across three repeat transitions, with the residual plot included below each figure. Time 0 s indicates onset of exercise. Data show a clear delay from exercise onset before MCAv increased exponentially. In A, MCAv attained a steady state, and the monoexponential was fit to the end of exercise (360 s). In B, a clear decrease in MCAv seen from t = 141 s, representing the end of the model fit. MCAv, middle cerebral artery blood velocity; τ, time constant; TD, time delay.

Figure 2.

Group-averaged absolute (A) and relative (B) middle cerebral artery blood velocity (MCAv) responses to moderate (gray line) and heavy (black line) intensity cycling in healthy adults (n = 17, 9 females). Dashed line indicates exercise onset. Δ%, relative change from baseline.

Figure 3.

Correlations between V̇o_2max and MCAv amplitude (A: absolute terms; B: relative change from baseline) and MCAv time constant, τ (C). Data shown for moderate-intensity exercise (circles, dashed line) and heavy-intensity exercise (black triangles, solid line) (n = 17, 9 females). MCAv, middle cerebral artery blood velocity; V̇o_2max, maximal oxygen uptake.

Figure 4.

Group-averaged oxygen uptake (V̇o₂, A), minute ventilation (V̇e, B), and heart rate responses (C) to moderate (gray line) and heavy (black line) intensity cycling in healthy adults (n = 17, 9 females).