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Department of Exercise Science, University of Iowa, Iowa City, Iowa 52242
We sought to clarify the roles of contraction
frequency (speed) and contraction force (grade) in the rise in muscle
blood flow at the onset of locomotion. Shoemaker et al. (Can
J Physiol Pharmacol 76: 418-427, 1998) explored this
relationship in human handgrip exercise and found that the time course
of the rise in muscle vascular conductance was similar when a light
weight was lifted in a fast cadence and a heavy weight was lifted in a
slow cadence (total work constant). This indicates that muscle pumping (contraction frequency) was of limited importance in governing the time
course. Rather, vasodilator substances released in proportion to the
total work performed appeared to determine the pattern and extent of
the rise in conductance. We hypothesized that conductance would rise
faster during locomotion at a high speed (frequency) and low grade
(force) than at a low speed and high grade, despite similar total
increases in conductance, owing to more effective muscle pumping at
faster contraction rates. Seven male rats performed nine 1-min bouts of
treadmill locomotion across a combination of three speeds (5, 10, and
20 m/min) and three grades (
10, 0, and +15°) in random order.
Locomotion at 10 m/min and 0° grade and 20 m/min and 10° grade
led to an equal rise in terminal aortic vascular conductance. However,
the equal rise was achieved more quickly at the higher running speed,
suggestive of more effective muscle pumping. Across the nine
combinations of exercise, speed began to exert a statistically
significant influence on conductance by the 3rd s of locomotion. Grade
did not begin to exert an influence until the 12th s of locomotion
(similar to the delays reported for arteriolar dilation to muscle
contraction). Additional experiments in dogs provided similar results.
Thus the muscle pump appears to initiate the increase in blood flow in
proportion to contraction frequency at locomotion onset.
dog; rat; nitric oxide; muscle blood flow; iliac artery; terminal aorta; arterial pressure; vasodilation; vascular conductance
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