1. An indicator-dilution technique was used to determine human leg blood flow at rest and during exercise. The method is based on the infusion of Indocyanine Green into the femoral artery with blood sampling from the femoral vein at the level of the inguinal ligament. Evidence for mixing of dye and blood is presented, based on the finding of equal dye concentrations at two different sampling levels in the femoral vein. The minimum time of infusion required for equilibration at rest is 3 min and during exercise 1 min 20 s.

2. Leg blood flow was measured in eight healthy athletic subjects at rest and during upright exercise on a bicycle ergometer at 400, 800 and 1200 kpm/min. Linear relationships were found between blood flow on the one hand and work intensity and pulmonary oxygen uptake on the other.

3. Leg oxygen uptake was measured as the product of blood flow and femoral arterio-venous oxygen difference. Linear regressions were found for leg oxygen uptake in relation to both work intensity and pulmonary oxygen uptake. Leg mechanical efficiency during exercise averaged 34%.

4. A formula for the approximate calculation of leg blood flow is suggested, based on the pulmonary oxygen uptake and the femoral arterio-venous oxygen difference.

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