Cardiac output, leg blood flow and oxygen uptake during foot plantar flexions

When studying the adjustment of muscle perfusion during exercise, the influence of central factors (e.g. blood volume, central blood pressure and venous return) can be reduced by choosing small muscle groups. In the present study parallel determinations of cardiac output (CO), leg blood flow (LBF) and pulmonary oxygen (V̇O₂) uptake were performed in 9 healthy male subjects at the onset and cessation of dynamic foot plantar flexions. The volunteers exercised with both feet for 5 minutes at 3 different resistances corresponding to 6%, 18% and 30% of the mean maximal voluntary contraction. Doppler measurements at the aortic root and in the femoral artery were utilized to estimate CO and LBF. Oxygen uptake was analyzed breath-by-breath as the difference between inspired and expired oxygen volumes. Within the first 10 s of exercise LBF increased from 400 ml x min^-1 to about 1,000 ml x min^-1 at all exercises intensities. During the subsequent 5 minutes of exercise, LBF decreased to about 800 ml x min^-1 at the lowest intensity. By contrast, it increased to about 1,900 ml x min^-1 at the highest intensity. The changes in CO during exercise were quantitatively identical with the changes in LBF. The present results suggest that the fine adjustment of muscle blood flow and muscle metabolism starts only after a fast and uniform circulatory onresponse. The second component may lead to leg perfusion values above, at or below the initial peak perfusion levels. The off-transients of LBF displayed no comparable fast responses. They were slower than the recovery kinetics of any cardiovascular parameter measured in the present study.