Abstract
The time courses of leg blood flow, systolic peak velocity, heart rate and blood pressure
have been studied in nine healthy volunteers during dynamic exercise in upright sitting
and in a tilted sitting body position. In both positions the subjects performed single
and repeated foot plantarflexions against light and moderate forces corresponding
to 5%-10% and 25%-30% of maximal voluntary contraction. The following measurement
techniques were used: Doppler ultrasound method (blood flow parameters), FINAPRES™
device (arterial blood pressure) and standard ECG chest leads (heart rate). At rest
the supine blood flow parameters measured in the arteria femoralis were significantly
higher than in the upright sitting position. In both positions, even one single plantarflexion
at the light exercise intensity caused significant increases in blood flow for almost
20 s. The major part of the blood flow response to repeated contractions always occurred
within the first 10 s at virtually unchanged blood pressures. During this initial
phase upright leg blood flow increased by factors of 2.5 (light exercise) and 3.1
(moderate exercise). The corresponding values in the tilted sitting posture were 1.7
and 1.9, respectively. The initial increases in the upright position were too large
to be attributed only to the increase of the perfusion pressure caused by the withdrawal
of the hydrostatic pressure on the venous side (“muscle pump”). Additional, fast decreases
in local resistance have to be considered. In the supine posture effects on local
resistance have to be taken into account for the early increases in blood flow since
hydrostatic effects on arterio-venous pressure differences are too small. The present
findings indicate that the effects of repeated contraction-relaxation cycles on the
early adjustment of muscle blood flow are not sufficiently described by a “muscle
pump” that induces only venous volume shifts and hydrostatic pressure changes. Additional
fast effects on local resistance have to be taken into account.
Key words
Leg blood flow - muscle pump - body position - exercise - humans
