Control of muscle blood flow during dynamic exercise: muscle contraction / blood flow interactions

Abstract

The interaction between dynamic muscle contractions and the associated muscle blood flow is very intriguing leading to questions regarding the net effect of these contractions on oxygen delivery and utilization by the working muscle. Study 1 examined the impact of contractions on muscle blood flow at the level of the femoral artery. We demonstrated that muscle contractions had either a facilitory, neutral, or net impedance effect during upright knee extension exercise as intensity increased from very light to ~70% peak work rate. This led to the question of what impact a change in contraction frequency might have on the coupling of blood flow to metabolic rate during cycling exercise. The blood flow/VO2 relationship has been shown to be linear and robust at both the central (i.e., cardiac output/pulmonary VO2) and peripheral (leg blood flow/leg VO2) levels. However, an increase in contraction frequency has been reported to either decrease, have no effect, or increase the blood flow response during exercise. Study 2 determined if the steady state coupling between muscle blood flow and metabolic rate (centrally and/or peripherally) would be altered by varying contraction frequency. Our results indicate that both central and peripheral blood flow/VO2 relationships are robust and remain tightly coupled regardless of changes in contraction frequency. Study 3 examined muscle microvascular hemoglobin concentration and oxygenation within the contraction/relaxation cycle to determine if microvascular RBC volume was preserved and if oxygen extraction occurred during contractions. We concluded that microvascular RBC volume was preserved during muscle contractions (i.e., RBCs remained in the capillaries), which could facilitate continued oxygen delivery. Further, there was a cyclic pattern of deoxygenation/oxygenation that corresponded with the contraction/relaxation phases of the contraction cycle, with deoxyhemoglobin increasing significantly during the contractile phase. These data suggest that oxygen extraction continues to occur during muscle contractions. Significant insight has been gained on the impact of muscle contractions on oxygen delivery to and exchange in active skeletal muscle. This series of studies forms a base of knowledge that furthers our understanding of the mechanisms which govern the control of skeletal muscle blood flow and its coupling to muscle metabolic rate.