Countdown before voluntary exercise induces muscle vasodilation with baroreflex-mediated decrease in muscle sympathetic nerve activity in humans.

We examined whether a countdown (CD) before voluntary cycling exercise induced prospective vascular adjustment for the exercise and, if so, whether and how muscle sympathetic nerve activity (MSNA) was involved in the responses. Young men performed voluntary cycling in a semi-recumbent position (n=14) while middle cerebral artery blood flow velocity (VMCA ; Doppler ultrasonography), heart rate (HR), arterial pressure (AP; finger photoplethysmography), oxygen consumption rate (VO2 ), oxygen saturation in the thigh muscle (StO2 ; near-infrared spectrometry), cardiac output (CO; Modelflow method) and total peripheral resistance (TPR) were measured (Exp1). Another group underwent the same exercise protocol but used only the right leg (n=10) while MSNA (microneurography) was measured in the peroneal nerve of the left leg (Exp2). All subjects performed 8 trials with a ≥5-min rest between trials. In 4 trials randomly selected from the 8 trials, exercise onset was signaled by a 30-sec CD, whereas in the remaining 4 trials, exercise was started without CD. We found that CD first increased VMCA , HR, CO, and mean AP, then decreased TPR and increased StO2 and VO2 (Exp1; all, P<0.021). Furthermore, the CD-induced increase in mean AP decreased total MSNA and burst frequency (Exp2; both, P<0.048) through the baroreflex, with decreased TPR and increased StO2 (Exp2; both, P<0.001). The vasodilation and increased VO2 continued after the start of exercise. Thus, CD before starting exercise induced the muscle vasodilatory response with a concomitant reduction in MSNA through the baroreflex to accelerate aerobic energy production after the start of exercise.