Hypothesis: A role of cardiac receptor nerve afferent in reflex control of heart rate during light exercise in upright humans

To examine a decrease in heart rate (HR) following the initial rise associated with increased central blood volume (CBV) due to the muscle pump after onset of light exercise, eight untrained male subjects were studied at rest and during cycle exercise at 20% peak oxygen uptake in upright (sitting) and supine positions. During upright exercise, HR increased to a peak of 86.3 ± 1.4 beats·min^-1 after 25 ± 1 sec, returning to just above the resting level; supine HR had no corresponding peak. HR after 6 min was significantly less in the upright than in the supine position (71.9 ± 1.2 vs. 77.2 ± 1.4 beats·min^-1). From rest to exercise, upright stroke volume (SV, by impedance cardiography) increased significantly by about 20%, whereas supine SV was unchanged with probably already elevated resting CBV. The high-frequency amplitude of R-R interval variability, an index of cardiac vagal modulation, by spectrum analysis was significantly less in the upright than in the supine position at rest, but vice versa during exercise. There was no significant difference in mean arterial pressure between each position. Therefore, the decreased and sustained HR corresponded to the increases in CBV, estimated by SV, and vagus nerve activity during upright exercise, as compared to supine exercise. These results support our hypothesis that the reduction in HR following the initial rise during light-intensity upright exercise is mediated reflexly by the cardiac vagal afferent activation associated with the increase in CBV from increasing venous return by the muscle pump.