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

When studying the adjustment of muscle perfusion during exercise, the influ ence of central factors (e.g, blood volume, central blood pressure and veno us return) can be reduced by choosing small muscle groups. In the present s tudy parallel determinations of cardiac output (CO), leg blood flow (LBF) a nd pulmonary oxygen ((V) over dot O-2) uptake were performed in 9 healthy m ale subjects at the onset and cessation of dynamic foot plantar flexions. T he volunteers exercised with both feet for 5 minutes at 3 different resista nces corresponding to 6%, 18% and 30% of the mean maximal voluntary contrac tion, Doppler measurements at the aortic root and in the femoral artery wer e utilized to estimate CO and LBF. Oxygen uptake was analyzed breath-by-bre ath as the difference between inspired and expired oxygen volumes. Within t he first 10 s of exercise LBF increased from 400 ml x min(-1) to about 1,00 0 ml x min(-1) at all exercises intensities. During the subsequent 5 minute s of exercise, LBF decreased to about 800 ml x min(-1) at the lowest intens ity. By contrast, it increased to about 1,900 ml x min(-1) at the highest i ntensity. The changes in CO during exercise were quantitatively identical w ith the changes in LBF. The present results suggest that the fine adjustmen t of muscle blood flow and muscle metabolism starts only after a fast and u niform circulatory onresponse, The second component may lead to leg perfusi on values above, at or below the initial peak perfusion levels, The off-tra nsients of LBF displayed no comparable fast responses; They were slower tha n the recovery kinetics of any cardiovascular parameter measured in the pre sent study.