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The purpose of this study was to compare the cardiovascular control mechanisms that defend arterial blood pressure against blood pooling between rest and moderate dynamic exercise. We studied ten physically active men during rest and five 12-min graded supine cycle ergometer exercise bouts with and without application of LBNP in 25 degrees C and 35 degrees C. Exercise intensities were 10, 50, and 100 watts (W), each for 4 min, and LBNP was applied at 0 (control), -20, -40, and -60 mm Hg in 25 degrees C and -40 mm Hg in 35 degrees C. At rest, cardiac stroke volume (SV) decreased from 120 +/- 5 ml during control to 94 +/- 6, 67 +/- 5 and 49 +/- 3 ml during -20, -40, and -60 mm Hg LBNP, respectively, and to 55 +/- 3 ml during -40 mm Hg at 35 degrees C. Exercise elevated SV toward the control level during LBNP due to muscle pumping action. Heart rate (HR) did not increase significantly during application of LBNP until SV decreased by 20-25 ml during LBNP, both during rest and exercise. The magnitude of HR increase per decrease in SV, once an increase in HR occurred, was similar between rest and exercise, regardless of exercise intensity. The change in total peripheral resistance (TPR) with respect to SV was linear, confirming that peripheral vascular adjustments were proportional to changes in the heart's preload. The slopes of the TPR-SV relation were similar during rest and exercise, although shifted to the left with increasing exercise intensity.(ABSTRACT TRUNCATED AT 250 WORDS)