Effects of muscle contraction on skeletal muscle blood flow: when is there a muscle pump?

PURPOSE: The purpose of this study was to determine the effects of rhythmic muscle contraction on the dynamics of venous outflow in rat skeletal muscle.
METHODS: The effects of frequency and duration of tetanic contraction on venous blood flow (BF) were examined with transonic flow probes placed on the femoral artery and vein.
RESULTS: Results reveal that instrumentation of the venous system with cannulas or flow probes alters vascular mechanics so that the muscle pump effect is masked. Measurements conducted without instrumentation of the venous vasculature in situ, as well as experiments with conscious exercising animals, indicate that the muscle pump enhances BF during exercise. Also, recent in vivo studies of humans indicate an important role for the muscle pump. In contrast, results reported herein and recent results from in situ experiments, which allow control of more parameters, indicate that there is no measurable muscle pump effect on BF during rhythmic muscle contraction. Review of the literature indicates that many in vitro/in situ experiments used instrumented veins that may have altered venous vascular mechanics and the interactions of muscle contraction and venous vascular mechanics, thus minimizing or abolishing the muscle pump effect.
CONCLUSIONS: The muscle pump contributes to the initial increase in BF at exercise onset and to maintenance of BF during exercise.