Adaptation of blood flow during the rest to work transition in humans

Beat-by-beat measurements show that limb blood flow rises rapidly and in a biphasic manner at the onset of rhythmic exercise in humans. In this review the time course of change in limb flow with the onset of exercise is descr ibed and the mechanisms that may or may not contribute to its regulation ar e discussed. The pumpings action of contracting skeletal muscle appears to form an important regulator of increasing flow with the first contraction. However, evidence from human studies suggests that vasodilation begins with the first contraction. Whether this early dilation is regulated by neural recruitment of motor fibers and/or muscle contraction per se is discussed, but the mechanism(s) remains unclear. Finally, the contribution of endothel ial-derived relaxation factors to the exponential increase in flow at the e xercise onset is examined. Based on studies in humans with intra-arterial i nfusion of blocking drugs, neither acetylcholine, nitric oxide, nor prostag landins appear to be essential for a normal dynamic flow response on going from rest to exercise. Overall, evidence from human studies supports the hy pothesis that the rate of increase in blood flow during rhythmic voluntary exercise is closely coupled to motor unit recruitment with dilation beginni ng at the first contraction.