BLOOD-FLOW TO CONTRACTING HUMAN MUSCLES - INFLUENCE OF INCREASED SYMPATHETIC ACTIVITY

The purpose of this study was to examine the effects of the increased sympathetic activity elicited by the upright posture on blood flow to exercising human forearm muscles. Six subjects performed light and heavy rhythmic forearm exercise. Trials were conducted with the subjects supine and standing. Forearm blood flow (FBF, plethysmography) and skin blood flow (laser Doppler) were measured during brief pauses in the contractions. Arterial blood pressure and heart rate were also measured. During the first 6 min of light exercise, blood flow was similar in the supine and standing positions (~ 15 ml·min-1·100 ml-1); from minutes 7 to 20 FBF was ~ 3-7 ml·min-1·100 ml-1 less in the standing position (P < 0.05). When 5 min of heavy exercise immediately followed the light exercise, FBF was ~ 30-35 ml·min-1·100 ml-1 in the supine position. These values were ~ 8-12 ml·min-1·100 ml-1 greater than those observed in the upright position (P < 0.05). When light exercise did not precede 8 min of heavy exercise, the blood flow at the end of minute 1 was similar in the supine and standing positions but was ~ 6-9 ml·min-1·100 ml-1 lower in the standing position during minutes 2-8. Heart rate was always ~ 10-20 beats higher in the upright position (P < 0.05). Forearm skin blood flow and mean arterial pressure were similar in the two positions, indicating that the changes in FBF resulted from differences in the caliber of the resistance vessels in the forearm muscles. These observations indicate that the changes in sympathetic outflow that normally accompany muscular exercise in the upright position do not decrease the local metabolic dilation of the resistance vessels in active muscles at the beginning of exercise but do so as exercise continues.