Alterations in the blood velocity profile influence the blood flow response during muscle contractions and relaxations
The present study examined the influences of the muscle contraction (MCP) and relaxation (MRP) phases, as well as systole and diastole, on the blood velocity profile and flow in the conduit artery at different dynamic muscle contraction forces. Eight healthy volunteers performed one-legged dynamic knee-extensor exercise at work rates of 5, 10, 20, 30, and 40 W at 60 contractions per minute. The time- and space-averaged, amplitude-weighted, mean (V mean) and maximum (V max) blood flow velocities were continuously measured in the common femoral artery during the cardiosystolic (CSP) and cardiodiastolic (CDP) phases during MCP and MRP, respectively. The V max/V mean ratio was used as a flow profile index where a ratio of approximately (∼) 1 indicates a "flat" velocity profile, and a ratio significantly greater than (≫) 1 indicates a " parabolic" velocity profile. At rest, a "steeper" parabolic velocity profile was found during the CDP (ratio: 1.75 ± 0.06) than during the CSP (ratio: 1.31 ± 0.02). During the MRP of exercise, the V max/V mean ratio shifted to be less steep (p < 0.05) than at rest during the CDP (ratio: 1.41-1.54) at 5, 10, 20, 30, and 40 W; whereas it was slightly higher (p < 0.05) at 30 and 40 W than at rest during the CSP (ratio: 1.43-1.46). During the MCP, the parabolic blood velocity profile was enhanced (p < 0.05) at higher contraction forces, ≥20 W during the CDP (ratio: 2.15-2.52) and ≥30 W during the CSP (ratio: 1.49-1.77), potentially because of a greater retrograde flow component. A higher blood flow furthermore appeared during the MRP compared to during the MCP, coinciding with a greater uniformity of the red blood cells moving at higher blood velocities during the MRP. Thus part of the difference in the magnitude of blood flow during the MRP vs. MCP may be due to the alterations of the blood velocity flow profile.
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