Capacity of blood flow delivery to exercising skeletal muscle in humans.

Several studies using different techniques to estimate muscle blood flow during exercise in humans support the concept that peak muscle perfusion is at least 150 ml/100 g/min. Such high blood flows are achieved when only part of the muscle mass is recruited during exercise. With 2 or more limbs exercising, the pump capacity of the heart will limit the blood flow available for the muscles. Norepinephrine spillover from an exercising limb is increased when a small muscle group performs the work, but it appears to have no functional effect on blood flow, and alpha-blocking drugs do not significantly elevate peak blood flow. During whole-body exercise, the norepinephrine spillover becomes further elevated, but the functional significance is not apparent until the oxygen uptake reaches more than 80% of maximal oxygen uptake. At these high work rates the sympathetic discharge overrides the local vasodilator factors and causes vasoconstriction. Thus, blood pressure can be maintained.