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1 School of Physical and Health Education, Queen's University, Kingston, Canada
* To whom correspondence should be addressed. E-mail: mt29{at}post.queensu.ca.
We tested the hypothesis that vasoregulatory mechanisms exist in humans which can rapidly adjust muscle blood flow to repeated increases and decreases in exercise intensity. 6 males and 7 females (24.4 ±1.3 yrs) performed continuous dynamic forearm handgrip contractions (1:2 s contraction:relaxation duty cycle) during repeated step increases and decreases in contraction intensity. Three step change oscillation protocols were examined: Slow - 7 contractions per contraction intensity x 10 steps. Fast - 2 contractions per contraction intensity x 15 steps. Very Fast - 1 contraction per contraction intensity x 15 steps. Forearm blood flow (Doppler and echo ultrasonography), heart rate (HR;ECG) and mean arterial pressure (MAP; arterial tonometry) were examined for the equivalent of a cardiac cycle during each relaxation phase (FBFrelax). MAP and HR did not change during repeated step changes (P=0.352 and P=0.190). For both Slow and Fast, relaxation phase FBFrelax adjusted immediately and repeatedly to both increases and decreases in contraction intensity and the magnitude and time course of FBFrelax changes were virtually identical. For the Very Fast condition, FBFrelax increased with the first contraction, and thereafter slowly increased over the course of repeated contraction intensity oscillations. We conclude that vasoregulatory mechanisms exist in human skeletal muscle which are capable of rapidly and repeatedly adjusting muscle blood flow with ongoing step changes in contraction intensity. Importantly, they demonstrate symmetry in their response magnitude and time course with increasing vs. decreasing contraction intensity, but cannot adjust to very fast exercise intensity oscillations.
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